Insects as Pests
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Old pictures and texts show that man has been defending himself against fleas, lice, mosquitoes, wasps, and other insects since 4000 years. The damage to humans caused by insects is diverse. It ranges from slight impairments such as unpleasant odors caused by cockroaches or sounds caused by mosquitoes and flies to painful wasps stings and serious, potentially fatal diseases. Very few insects, e.g., head and body lice, are even dependent on human or animal hosts as a «constant parasite». Insects are a serious threat not only to humans, but also to animals. These include animals in the wild, e.g., deer, hares, birds, livestock such as cows, sheep or cattle, and pets such as dogs and cats. They are all tormented, bitten and sometimes also infected by parasites. The diseases can lead to animal epidemics, reductions in entire populations, or also cases of zoonosis, where the disease is also transmitted to humans. Insects have always fed on plants and, therefore, impeded their growth or even caused their death. Stories thousands of years old tell about menacing locusts, caterpillars, and beetles. The Bible also mentions examples of how insects interfere with field agriculture and destroy reserves.
KeywordsDengue Fever Head Louse Rickettsial Disease Rocky Mountain Spotted Fever West Nile Fever
Old pictures and texts show that man has been defending himself against fleas, lice, mosquitoes, wasps, and other insects since 4000 years. In China, rice weevils as well as tobacco and bread beetles were found in burial sites more than 3500 years old. Already more than 2500 years ago, the Greek reported epidemics caused by flies and the Romans reported painful mosquito bites. Pea beetles destroyed beans and peas, grain maggots destroyed the grains, and clothes moths were already eating holes into pieces of clothing.2
Pests are defined as organisms that affect the well-being, performance or the health of humans, their homes or domestic animals, disrupt the normal development of crops, reduce the value of animal and plant products, harvested produce and supplies as well as materials and make them unusable.1
3.1 Insects as a Danger to Humans
The damage to humans caused by insects is diverse. It ranges from slight impairments such as unpleasant odors caused by cockroaches or sounds caused by mosquitoes and flies to painful wasp stings and serious, potentially fatal diseases. Very few insects, e.g., head and body lice, are even dependent on human or animal hosts as a “constant parasite.”3
Infectious diseases are caused by the transmission of viruses, i.e., infectious particles. Bloodsucking insects such as mosquitoes and ticks consume blood from humans or animals by biting them. If the human or animal is carrying the virus, it is transmitted to the insect, which is not harmed itself. The next time a human or animal is bitten, the insect transmits the virus with its infected saliva to the healthy organism, which is then infected with the disease.
The insects themselves are therefore the carriers (vectors), and the human or animal is the host. Insects can transmit the viruses among humans and between humans and animals.
The well-known febrile diseases yellow fever, dengue fever, and malaria are most harmful to humans. They are transmitted by mosquitoes that are endemic to tropical and subtropical regions. Particularly in Africa, but also in Asia and South America, more than 250 million people are infected with these viral diseases each year, and more than 700,000 of them die.
Most of the virus infections transmitted by insect bites were described at the end of the nineteenth and beginning of the twentieth centuries. Researchers strove to develop vaccines and insecticides to control the disease-transmitting animals. Through international efforts, they succeeded in pushing back the diseases in the 1950s and 1960s, especially in Africa. In the last 30 years, the number of infections has been rising again, despite education and the use of vaccines and insect control measures. The most rapidly spreading disease is dengue fever, for which there is still no vaccine today. Reported cases have increased by more than 25% in recent years. The World Health Organization (WHO) of the United Nations estimates that about 100 million people are infected annually. West Nile fever and Japanese encephalitis are also spreading more rapidly. Progress was made in the fight against malaria. Fatalities have dropped from the year 2000 to 2014 by 47%. In the year 2013, however, there are still about 200 million people sick with malaria, of which more than 550,000 have died.4
New unknown infectious diseases must also be considered. For example, the Alkhurma virus was discovered in Saudi Arabia in 1994, as a man was fatally infected while slaughtering a sheep. The sheep was carrying the virus, which is transmitted by the tick Ornithodoros savignyi. The ticks prefer camels and sheep, but also sometimes bite humans. Up to the year 2009, around 100 people have been infected, and 25 have died. Although the tick has already been known for a long time, researchers did not know about the virus and the threat it poses.5
In fact, one million species are already known and described today. However, biologists estimate that most insects have not been discovered yet because they live in poorly accessible tropical forests and are difficult to observe. With every meter of untouched nature we discover, we encounter new species and therefore new chances or dangers. In the future, we are likely to face new viruses and vectors as well as new cures.
Infectious diseases have never stayed in one region, but rather they enlarge their range over land routes, later through ships and today increasingly through air travel. Since travel activities within individual countries have also increased, the disease vectors and insects always spread rapidly after arriving in a country.
In 1999, for example, the tiger mosquito infected with the Nile virus, originally only found in Southern Europe and Africa, was suddenly found in New York City. The insects extended their habitat rapidly throughout all of North America and still represent a serious threat today. In the year 2014 alone, more than 2000 people were infected with West Nile fever, of which 97 died. In total, there were 41,762 infections with 1765 fatalities.6
Dangerous febrile diseases are generally transmitted by insects that previously only felt at home in tropical rainforests. However, because of rising temperatures, they can now also settle in more northern regions. In 1975, for example, Asian tiger mosquitoes (Aedes albopictus) were introduced from East Asia to Romania with deliveries of used car tires and have spread to Italy, southern France, and Spain in recent decades, and also to Switzerland since 2003.7
The threat posed by the mosquitoes depends on whether they are carrying a viral disease. In 2010, the first autochthonous cases of dengue fever were recorded in Western Europe. Two persons in southern France, one in Italy, and one in Croatia were directly infected by mosquitoes.8 In the years 2009–2011, more than 1000 European residents returned from trips to Asia infected with dengue fever every year.9 Since the disease cannot be transmitted from person to person without mosquitoes, the infection was not able to spread. The direct transmissions now observed in 2010 represent a serious problem: Dengue fever has arrived in Europe and is here to stay. For this reason, the WHO also considers Europe as a potential region for dengue fever outbreaks and ranks the disease as a serious threat for 2.5 billion people worldwide.10
The WHO is very concerned about the development of this disease caused by insects and describes it as a global threat. Accordingly, the WHO dedicated the World Health Day in 2014 to “vector-borne diseases.”
3.1.1 Virus-Transmitting Insects
There are numerous dangerous and sometimes life-threatening febrile diseases, which are transmitted by a few insect species.
Mosquitoes are the most dangerous insects. For example, the yellow fever mosquitoes (Aedes aegypti) and the Asian tiger mosquitoes (Aedes albopictus) transmit dengue, chikungunya, West Nile, and yellow fever. Mosquitoes from the genus Anopheles cause malaria, sand flies (Phlebotominae) trigger leishmaniasis, and rice paddy mosquitoes (Culex tritaeniorhynchus) infect people with Japanese encephalitis.
Flies like the tsetse fly (Glossina) transmit sleeping sickness. Ticks, e.g., the castor bean tick (Ixodes ricinus), transmit Lyme disease and ticks like Dermacentor reticulatus cause Omsk hemorrhagic fever.
While mosquitoes mainly populate tropical zones and practically the entire southern hemisphere, ticks are mostly found in the northern hemisphere. But the traditional habitats are mixing rapidly: Tropical insects are spreading to the north and Asian insects are spreading to the west.
For example, the precursor virus of the well-known tick-borne encephalitis, which is transmitted by hard ticks, has spread from Eastern Russia and Japan through Central and Western Europe all the way to England.11
The problem with vector-borne diseases has become a global topic in recent decades. There is no country in the world that can reliably protect itself from the introduction of infections, and unfortunately also not from autochthonous outbreaks.
3.1.2 Causes and Trends of Virus Transmission
Why have the infections and the dangerous insects continued to spread? A few causes and trends for virus transmission are explained in the following:
The extent of the insect population and the associated frequency of infections correlate with the state of development of the region. The poorer the people, the more they are exposed to dangerous mosquitoes, flies, and ticks. In tropical and subtropical “regions of origin,” the poor living and working conditions promote the development of the insects. Virus transmission from person to person by a vector is favored, e.g., by the fact that many small huts or lodgings are standing close to each other and many people are living in close quarters. Due to the rapid expansion of cities, more and more such poor settlements are established, offering ideal breeding grounds for dangerous insects.
The insufficient sewer systems attract flies that can transmit febrile diseases just like mosquitoes. In turn, sparse waste removal leaves lots of small objects behind, such as plastic bags or cans, which can then fill with water and therefore provide ideal breeding grounds for more mosquitoes. In most cases, waste is disposed of directly beside the living area. Mosquitoes that used to develop in faraway watercourses are thus unintentionally “bred” directly in human surroundings.
Humans have influenced natural waterways all over the world, whether it be by building water reservoirs, protecting themselves with dams, straightening rivers to allow navigation, or supporting agriculture by creating small watercourses. In areas where flowing water is slowed down or stopped and shallow waters are created where they used to be deep, mosquitoes find better breeding sites than otherwise in nature.
Global warming generally favors the development of tropical mosquitoes and flies. Also, ticks that normally only reside in lowland areas are now able to spread into more elevated regions.
Intense population growth in tropical and subtropical areas in recent decades has favored the development of parasites that are dependent on humans.
With increasing mobility, diseases and viruses as well as their vectors have spread nationally and internationally. Parasites are therefore always able to find healthy people to transmit the virus.
Growing global trade with animals increases the presence of infected animals, which represent hosts for transmission to humans. Infected migratory birds that had previously not advanced into temperate regions now can, thanks to climate change, become indirect vectors for disease in new areas.12
The points mentioned show why vector insects were able to thrive in tropical and subtropical regions in recent decades. Although international efforts were made to counter this, they were not able to stop them from spreading. Since insects are capable of spreading across borders, the measures performed by individual countries on a continent are not sufficient. However, continental activities require the consent of all countries, and this can only be achieved when imminent danger is recognized.
Only consistent and especially permanent programs, independent of the respective state of the insect population, are effective on the long term. This is demonstrated by the eventful history of dengue fever in USA, which will be explained further below.
How will virus transmission develop in the northern hemisphere?
Dangerous diseases and their vector insects are being introduced more often outside of tropical regions. International tourism as well as global movement of goods will not decrease. Virus-transmitting insects are already found in Europe today, as demonstrated by tiger mosquitoes. The bigger problem is certainly the possible introduction of insects that carry the virus. Because of the small size and mobility of insects, it will not be possible to solve this problem.
Climate adaptation is the prerequisite for permanent colonization by the insects. The tiger mosquitoes already living in Europe show that the general rise in temperatures is already sufficient to allow the tropical mosquito to establish itself in the northern hemisphere on the long term.
Every expansion of civilization represents an intervention in nature and therefore in natural cycles and regimes. Balances established over the course of centuries are being destroyed. An example from North America shows the possible consequences: North of New York City, large forests were cleared in the 1990s to build small residential areas. Predators such as wolves and birds of prey, which need large continuous areas of forest, migrated away. This allowed small rodents and deer to increase their populations, since they now had access to more habitats. They acted as suitable hosts for deer ticks (Ixodes scapularis), so that they were able to develop rapidly. For years, the region around Dutchess city has recorded the highest numbers of tick bites in the USA: Every year, 400 from 100,000 residents are infected with Lyme disease.13
To demonstrate the spectrum of damage that insects can directly cause to humans, important diseases spread by insects through the direct transmission of bacteria and viruses will be listed in the following.
3.1.3 Diseases that Are Caused by Mosquitoes
Mosquitoes bother us again and again. Almost everyone has been bit before and is accordingly sensitized when they hear the common house mosquito (Culex pipiens) buzzing around the room. Actually, only very few mosquitoes bite humans. Many insects prefer the blood of animals. The males do not bite at all, since they do not need the blood protein to produce eggs but rather feed on nectar.
The fever is transmitted by the mosquitoes A. aegypti and A. albopictus and is mainly found in Asia and Africa. The disease is spreading very rapidly. For example, about 1.2 million people were infected with the virus in India in 2006. There were regions where up to 45% of the population was infected.14 On the two islands near Madagascar, Mauritius, and La Réunion, more than 210,000 people were infected that same year.15
More and more tourists are bringing this disease into Europe. In this way, the fever suddenly broke out in Italy in 2007, as around 200 people in a small region were infected.16 In Germany, between 17 and 53 new cases were reported annually between 2006 and 2013.17
Dengue fever probably broke out for the first time in 1635 in Martinique. However, reliable documents are available for the year 1780, as the disease broke out in Philadelphia and concrete symptoms were described. The dengue virus itself was only isolated and analyzed in 1944.
Starting in Africa, the disease spread all over the world. After a big epidemic in Greece in the 1930s, during which one million people were infected, the mosquito was driven out of Europe. In recent years, however, numerous cases are being reported again in Europe.
In the nineteenth century, dengue fever caused several regional epidemics in South and North America. Local measures taken by individual countries at the beginning of the twentieth century were not enough to drive out the virus-transmitting tiger mosquito. During the “Pan American Health Conferences” in the 1940s, leaders agreed to control A. aegypti systematically with the newly invented synthetic insecticide DDT (dichlorodiphenyltrichloroethane) in all countries of both continents. Already by the 1950s, only one single infection was reported, and in 1962, 18 countries were declared to be free of the tiger mosquito.18
This success led the states to the conclusion that joint and targeted interventions were no longer required. Since the late 1960s, cases of illness were being reported again, which are considered to be the result of the reduced measures and increasing resistance of the tiger mosquitoes to DDT and other insecticides. In the 1970s, around 120,000 severe cases were recorded annually, almost 300,000 in the 1980s, and more than 500,000 since the 1990s.19
The WHO estimates that A. aegypti infects approx. 100 million people worldwide with the dengue fever virus every year.20 Among the 500,000 people who develop the disease, about 22,000 do not survive. There is no vaccine against the disease.
The yellow fever mosquito is mainly native to tropical and subtropical regions. It has already been known in Africa for thousands of years. Thanks to the slave trade taking place 500 ago, the mosquito spread all across the world, mainly in South America, but also in North America and Europe.21 In the eighteenth and nineteenth centuries, more than 10,000 people died in over 100 epidemics.
Although reliable vaccination methods have already existed since 50 years ago, about 200,000 people are infected every year, of which 30,000 die of yellow fever, 90% of which are in Africa.22 In Europe, there were no autochthonous cases in recent years, and in Germany, also no cases of introduced yellow fever.23
The disease occurs almost exclusively in rural areas, far away from urban agglomerations. For this reason, it is estimated that many cases are not even reported and that the number of infections is higher than this.25 No cases have been reported in Europe.
The insects are mainly found in the tropics and subtropics. The WHO has calculated that approx. twelve million people are infected and that there are one to two million new infections every year, of which approx. 200,000–400,000 take a visceral course. In total, 20,000–40,000 people die of leishmaniasis every year. More than 90% of the cases are reported from the following countries: India, Bangladesh, Sudan, South Sudan, Ethiopia, and Brazil.26
From Africa, the sand fly spread to the entire Mediterranean region, where an average of 85,000 cases of leishmaniasis were reported yearly from 2004 to 2008.27 On average in the period from 2005 to 2013, there were around 100 potentially fatal cases of visceral leishmaniasis in Italy and around 200 cases in Spain.28
In Germany, two autochthonous cases were reported at the beginning of the 2000s: The disease was diagnosed in a child and a dog, although they had never been out of the country. At the same time, a potential vector was observed in South Germany: the sand fly Phlebotomus mascittii. However, the fly did not spread and further cases were not reported.29
Transmitted by the Anopheles mosquito, malaria is the most common vector-borne disease: In the year 2013, 200 million people were affected. More than 550,000 of them died.30 Over 90% of the fatalities were recorded in Africa, the rest particularly in Southeast Asia and more than two percent in Iran and Saudi Arabia.31 Malaria is not endemic to Europe. In the year 2011, 69 autochthonous infections were registered in Southeast Europe alone: 65 in Tajikistan, four in Azerbaijan and Turkey,32 and nine cases in 2012 in Greece.33 Otherwise, the European cases of malaria were all observed in tourists who had mainly visited Africa. In recent years, the Robert Koch Institute recorded an average of 500 cases.34
St. Louis encephalitis
West Nile fever
After its discovery, only isolated cases were initially observed until, in 1997, several people were suddenly infected with West Nile fever in Israel. From there, insects carrying the virus spread to North America and also to Southern and Eastern Europe, where numerous cases were also reported. In 2010, a total of 200 autochthonous cases were confirmed in the European Economic Area, of which 121 were in Greece, 52 in Romania 52, 19 in Hungary, three in Italy, and two in Spain. 40 people died overall.39 In 2012, 232 people were infected, of which 22 died. In 2013, 226 people fell ill in the EU, of which 42 were in Italy alone. There were also increased reports of West Nile fever in neighboring countries with 557 recorded cases.40
3.1.4 Diseases that Are Caused by Ticks
Ticks belong to the class of the mites and therefore to the spiders, which do not belong to the insects, but like insects, are arthropods. The animals are bloodsucking ectoparasites, i.e., animals that only live on their hosts for the purpose of feeding. Ticks are mainly found in the northern hemisphere: Asia, Russia, Europe, and North America. Ticks can cause the following dangerous and sometime fatal diseases.
Tick-borne encephalitis (TBE)
The disease is associated with flu-like symptoms. In approx. ten percent of the infections, the course of the disease is severe, associated with meningoencephalitis (meningitis) and is fatal in 0.5 to two percent of the cases.42
Meningoencephalitis has been widespread in Southern and Central Europe and Russia for many years. Since the 1990s, there are increasing reports of cases in Northern Europe—and the trend is rising: While in the period from 1976 to 1989, Europe including Russia reported about 38,000 cases (yearly average: 2000), it was already 170,000 cases from 1990 to 2009, with a yearly average of 8500. In Western Europe, the average number of yearly new infections was more than 2500.43 In Germany, in the period from 2002 to 2012, between 195 and almost 550 cases were registered on an annual basis.44
The relatively low number for Germany is mainly attributed to the widespread acceptance of immunization. In 2012, for example, more than 30% of the residents in South German states at risk, Baden-Württemberg and Bavaria, had themselves immunized.45
The rapid spread of the disease in recent decades was mainly attributed to climate change: The rise in temperatures in the second half of the last century as well as increased atmospheric humidity expanded the ticks’ habitats. They were able to spread further to the north and also populate more elevated areas. For example, increased cases of meningoencephalitis have only been reported in Scandinavian countries since the 1990s.46 Meanwhile, the tick is already found at elevations between 900 and 1300 m.47
Crimean-Congo hemorrhagic fever
The disease causes fever, chills, headaches, joint, and muscle aches. In approx. 20% of the cases, there are severe hemorrhages (bleeding) that are often fatal. The lethality lies between two and 50%. The fever is highly contagious, and there is no vaccine.
Since the end of the 1990s, the fever has been observed more frequently in Southeast Europe. Cases are being reported from Albania, Bulgaria, and Kosovo. Most of the infections occur in Turkey: From 2002 to 2008, the number of yearly infections rose from ten to more than 1100; a total of 113 people have died from the hemorrhagic fever.49 In 2010, the number of deaths was 61.50 In Greece in the year 2008, a fatal autochthonous infection was reported.51 In Germany, there were two cases of introduced Crimean-Congo hemorrhagic fever in 2008, and one case ended fatally.52
Borreliosis can affect multiple body systems. Following initial flu-like symptoms, joint aches, and chronic symptoms of fatigue develop. Ultimately, neural symptoms can remain for years.
The disease has spread extensively throughout Europe. More than 30,000 cases are registered annually, of which about 5000 are recorded in Germany.55 In the USA, the number of infections has been increasing for years: While almost 10,000 cases were registered in 1992, it was already almost 20,000 in 2006 and around 25,000 in 2012.56
The disease is very contagious. Less than ten bacteria are sufficient for infection, and ticks can carry more than one billion per gram of body weight.59 Due to their resistance to the weather and environmental conditions, the bacteria remain viable for weeks. They are carried with the wind into the surrounding area and can trigger infections within a radius of more than ten kilometers.60
The disease can lead to fever, joint aches, headaches, and muscular pain as well as to inflammation of the liver or lungs, which is associated with long periods of invalidity. There are no known cases of fatality.
The infections spread suddenly. There was an epidemic in Holland in 2007, during which more than 4000 people were infected up to the year 2010. To prevent further spreading, more than 60,000 animals were killed.61 In Germany, 299 people were suddenly infected in one city in 2003, and in 2005, more than 330 people in another city.62 In all three cases, the trigger was herds of sheep carrying the virus. From 2006 to 2010, approx. 7000 people were infected in Europe, more than 80% of whom were in Holland, Germany, and France.63 In the same time period in the USA, around 650 people were infected.64 As a matter of principle, a greater number of infections are assumed, since the disease only leads to persistent symptoms in 50% of the cases, which are then associated with hospitalization and therefore recorded in the statistics.
For Europe, mainly the Mediterranean spotted fever is relevant, the symptoms of which were described for this first time in 1909 in Tunisia. In 1925, transmission through ticks was described in Marseilles.66 The vector for the transmission of Rickettsia conorii is the tick Rhipicephalus sauguineus, which mainly feeds on dogs. However, transmission to humans is also possible. The disease is associated with fever and exanthema (rash) as well as headaches and limb pain. The lethality lies at under three percent.67 Other forms of Rickettsial disease are transmitted by, e.g., I. ricinus and run a milder course. In Europe, only isolated cases were recorded in humans until now. In recent years, however, there were increased observations of ticks carrying dangerous Rickettsia bacteria in Italy, England, and Sweden.68
The Rickettsial disease Rocky Mountain spotted fever, already known since 1906, is significant for USA and particularly North America. It is mainly transmitted by Ixodes dermacentor.69 The disease is associated with severe flu-like symptoms and extensive exanthema. In serious cases, it causes drops in blood pressure, kidney failure, and states of shock, which can be lethal without treatment in 20–80% of cases.70 The lethality with treatment is under one percent. Rickettsial disease is spreading fast in the USA: Until the end of the 1990s, less than 500 people were infected annually. In the time period from 2000 to 2005, around 1000 people were infected, and from 2005 to 2010, it was about 2000 people.71
3.1.5 Other Dangers Caused by Insects
Wasps and bees
Oak processionary caterpillar
The caterpillar’s hair contains a toxin that can trigger dermatitis and asthma. Broken-off hairs can be carried by wind into the surrounding areas and can thus pose an immediate threat to humans. Each caterpillar has more than 500,000 toxic hairs, and their effect can persist for up to twelve years.83
3.2 Insects as a Danger to Animals
Insects are not only a serious threat to humans, but also to animals. These include animals in the wild, e.g., deer, hares, birds; livestock such as cows, sheep, or cattle; and pets such as dogs and cats. They are all tormented, bitten, and sometimes also infected by parasites. The diseases can lead to animal epidemics, reductions in entire populations or also cases of zoonosis, where the disease is also transmitted to humans.
The World Organization for Animal Health (OIE) ranks a total of 89 diseases that must be observed and reported, of which 29 are caused or transmitted by insects.84 Several million animals die every year because of insect bites. In Africa, for example, approx. three million cows and heifers die of sleeping sickness (Nagana), which is caused by the tsetse fly.85
The insect bites are generally associated with skin irritations and fever, in severe cases, however, also with paralysis and miscarriage. For people in the regions most strongly affected, e.g., Central Africa, this represents high additional costs and a reduction or even loss of income.
Animal diseases caused by insects occur mainly in Africa, followed by Asia and Russia. However, they are also coming more often to North America and Europe, as was the case with the bluetongue disease, which has been spreading in all of Europe since 2006.
Migratory birds can play a special role in the spreading of disease. In the years 2010 and 2011, virus-carrying ticks were found on migratory birds in Italy. The infected birds carry the ticks for thousands of kilometers far into foreign areas, where they infect other animals.86
New unknown viruses are also more frequently observed. For example, the Schmallenberg virus, which affected more than 8000 livestock herds in Europe within two years, was only described in 2011.87 In the following, examples of important diseases will be briefly described.
Tsetse flies (Glossina) not only transmit sleeping sickness to humans, but also bite animals, in which case the disease is called Nagana (African animal trypanosomiasis). Infected animals already exhibit fever and paralysis after a few days and generally die within three months. The disease is observed in the so-called tsetse belt in Africa. With an area of ten million square kilometers, it ranges from 14° north (directly below the Sahel Desert) to 29° south (Johannesburg).
Nagana is considered the most dangerous animal disease, since it strongly restricts livestock husbandry in Africa. Today, 80% of soil cultivation is still performed manually.88 Due to concern about insect bites, many areas are not even accessible, and accordingly, are not used for agriculture. The economic damage caused by the tsetse fly is equally high.
3.2.2 Bluetongue Disease
The first cases of bluetongue disease were already registered in 1880 in South Africa.89 The animals, especially adult sheep, suffered from fever and inflammation in the nose and mouth area, associated with a blue coloring of the tongue. The disease spread rapidly all over the world and can still be found on all continents today, however, only in particularly warm regions between the latitudes 40° north and 35° south.
It is transmitted by a small midge, only one to three millimeters in size, from the genus Culicoides (biting midges). There are more than 1400 known Culicoides species, but only 17 of them transmit the bluetongue virus.90 The mortality rate varies greatly, ranging between two and 30% for current outbreaks.91
The disease plays an important role especially in South Africa. Studies have shown that today, more than 50% of sheep are infected with the virus.92 Worldwide, the annual damage caused by the disease is estimated at three billion US dollars.93
After two epizootics already in the 1950s in Spain and Portugal with more than 175,000 deaths,94 the virus has been spreading from Africa toward the north since the end of the 1990s. After Sicily, Greece, and Turkey, the first cases were reported in Holland in 2006. The Food and Agriculture Organization (FAO) has calculated that in Europe, more than 1.5 million sheep have fallen victim to the disease since 1998.95
The spread rate is menacing: It only took four days after the first infection of a sheep in Holland in August of 2006 until eleven sheep herds were already infected in Belgium and only three more days until the disease affected seven herds in Germany. Thanks to vaccinations, the outbreak was rapidly suppressed and has been eliminated since 2009.96
It is assumed that Culicoides midge species known to be responsible for transmission have spread from Africa further north into Europe due to warmer temperatures. For reasons unknown to this day, the virus is now also appearing in Culicoides species endemic to Europe. In affected regions, mainly Culicoides obsoletus and Culicoides pulicaris were found, which have settled into Europe years ago but had never transmitted the virus until now.97
3.2.3 Schmallenberg Virus
The virus was discovered in November 2011 in the German city of Schmallenberg and then spread rapidly to almost all countries of Northern and Central Europe. Infected heifers, cows, sheep, and goats exhibit fever and reduced performance, causing, e.g., significant milk yield reductions in cows. Pregnant animals give birth to deformed babies or stillborns.
Until the middle of 2013, more than 8000 farms in Europe (of which about 2500 in Germany) were affected by Schmallenberg disease.98 Here also, the vectors are the tiny midges from the genus Culicoides. The virus is also transmitted by the midges to other animal groups, e.g., deer or dogs. The disease is spreading very rapidly. In England, a deer population studied in 2010 had absolutely no virus infections, and one year later, more than 43% of the animals were infected.99
Since the virus was only discovered at the end of 2011, there is still no vaccine available today. In addition, it is not yet possible to determine how many of the 120 species of Culicoides endemic to Western and Northern Europe actually transmit the virus.100
The zoonosis Louping ill, known since 1934,101 demonstrates that an animal disease can also be transmitted to humans without direct contact. A case from 2011 is particularly interesting, where a young woman fell seriously ill with the Louping Ill virus. The virus was transmitted during a stroll with open shoes over a meadow on which infected sheep were grazing. Feces and other deposits from the sheep were still on the grass and therefore came into contact with the woman’s feet.102
The Louping Ill virus is transmitted by the castor bean tick (I. ricinus), endemic to Central and Northern Europe, mainly to sheep but also to other animals such as dogs or birds. The infection is associated with uncoordinated movement and is therefore called Louping ill. The lethality ranges between 20 and 50%.103
The disease is very rare. In addition to a suddenly occurring outbreak in 2011 in a goat herd in Spain,104 during which all 70 animals died, the disease is practically only endemic to England, with between 25 and 35 reported cases annually in recent years.105
3.3 Insects as a Danger to Plants
Insects have always fed on plants and therefore impeded their growth or even caused their death. Stories thousands of years old tell about menacing locusts, caterpillars, and beetles. The Bible also mentions examples of how insects interfere with field agriculture and destroy reserves.106
Despite years of experience in pest control and despite annual expenses for modern crop protection products of more than 40 billion US dollars annually, insects continue to strongly interfere with agriculture, fruit and vine cultivation, and the forests.107 Just for agriculture, the damage that can be attributed to insects amounts to 20% of the overall production. This corresponds to more than 50 billion US dollars every year.108
3.3.1 Agricultural Damage in Developing and Threshold Nations
The greatest plant damage caused by insects worldwide occurs in developing and threshold countries in tropical and subtropical regions. The reasons are obvious109:
Great fluctuations in the weather, from rainfall to drought, weaken the plants’ resistance and promote insect growth.
The short-term orientation toward maximizing agricultural yields is associated with unhealthy growth and monocultures, which in turn weakens the crops’ resistance.
Crop protection measures are not performed sufficiently due to a lack of expertise and money.
Here, intensified international migration of insects due to globalization also plays an important role. In these regions, introduced insects find particularly attractive habitats and accordingly, they spread more rapidly than usual.
The FAO speaks of a “dramatic” increase in cross-border plant pests in recent years. It is estimated that insects cut possible yields in developing countries by more than half: One-third is lost during the flowering period and approx. ten to 35% is lost during storage.110
According to FAO’s Emergency Program, the greatest threats to crop plants include the following problems caused by insects111:
Threat to the cassava plant due to insects,
Spreading of fruit flies, and
Spreading of fruit flies
Around 30% of the 4000 known species of fruit flies from the Tephritidae family cause damage to approx. 200 fruit plant species around the world. Their annual damage is estimated at more than one billion US dollars.115 The damage is particularly high in Central America, Africa, and Asia, where the insects regularly destroy between 30 and 80% of the cultivation.116
Two developments are making the situation worse:
New species of fruit flies are being introduced
Fruit as a natural host
These insects prefer very arid areas with less than 200 mm of annual rainfall. They are native to 30 African and Asian countries covering an area of more than 15 million km2. Isolated occurrences of locust infestations even spread to 60 countries over an area of almost 30 million km2. The animals therefore endanger 20% of the entire worldwide land area and the income of ten percent of the population.119 Even modern satellites are not capable of identifying growing locust swarms.
Normally, locusts only come together to mate. The animals have a life span of up to five months and lay approx. 100 eggs up to three times within this period. The more humid the climate, the more animals can hatch and develop. If there is increased moisture in arid regions, a disproportionate amount of insects are able to develop. At the same time, their color changes from brown to pink and their behavior changes: The previously solitary insects assemble in small swarms over a period of several months and then leave their region to look for more food together. Since particularly locust-friendly weather conditions can be very specifically pronounced in an entire region, multiple swarms develop at the same time. They join to form a large unit consisting of several billion animals. They let themselves be carried by the wind and can cover more than 100 km per day.
Locusts require lots of food: The phytophagous insects weigh two grams and require their own weight in food every day, mainly from plants such as flowers, leaves, tree bark as well as cereals, corn, and fruit.
Threatening gatherings of locusts occur every year in Africa and in the Middle East. The swarms rapidly accumulate more than 1000 insects per square meter and cover and area of 1000 km2.
In 2004, a swarm developed in Mauritania that was 230 km long and 150 m wide, comprising almost 70 billion insects.
The locusts spread over practically all of North Africa as well as over parts of Portugal and Crete. The FAO estimates the resulting economic damage at 2.5 billion US dollars, and 400 million US dollars were spent just on control measures.
Together with several offices, the FAO constantly coordinates a comprehensive monitoring system as well as control of the insects.120 More than 1000 km2 is sprayed by farmers annually with insecticides to prevent the spreading of growing swarms.
Despite all of the efforts, larger locust infestations occur time and again, consisting of approx. 60 million insects per square kilometer and covering areas of more than 100 km2. The quantity of food that they eat on a daily basis would be able to feed 2500 people for about four months.
In the last 100 years, there were six large infestations in Africa and in Asia, which sometimes lasted for several years. In the last large infestation at the end of the 1980s, a density of several billion insects per square kilometer was estimated. The locusts spread over all of North Africa and finally reached the open Atlantic with the wind. Instead of dying, they crossed a distance of 5000 km and after ten days, finally arrived in the Caribbean and South America.121 In 1954, locusts even flew once without food intake over the open ocean from West Africa to England.122 The examples demonstrate the potential danger posed by these insects due to their range.
The late harvest prolongs the time period for insect infestations. Several beetles only appear at the end of the flowering period and are then taken along with the harvest unnoticed. Unprotected storage enables easy access for other insects.
The two biggest orders of pests are beetles and butterflies, particularly food moths.123 Beetles infest cereals during the flowering period as well as during storage. They have a life span of up to one year and lay up to 500 eggs. Weevils (Curculionidae) are considered as the most damaging insects for stored cereals worldwide. If they infest only one to two percent of a harvest, 80% of the entire reserves are affected six months later. The damage to infested crops and reserves in Africa ranges from 30 to 100%.
Food moths are widespread all around the world. Because they like warm temperatures, they are more likely to be found in tropical and subtropical regions. Moths can lay up to 400 eggs in stored produce. The larvae eat the reserves, spinning webs, and contaminating them. The extent of the damage varies and can range from ten to 50% of a harvest.
3.3.2 Agricultural Damage in Europe
Insects also damage crops in highly developed countries. In particular, the agricultural sector suffers from new challenges, which lead to high losses time and again:
Aggressive pests are introduced with foreign crops and reproduce rapidly.
Rising temperatures allow the insects to develop more rapidly in general and enable natural growth from neighboring and southern regions.
New cereal varieties can cause new insect problems.
Practically all cereal and fruit plants today are affected by pests. The adult insects generally feed on the plants and can transmit viruses. However, the greatest damage is caused by the larvae, as the adult animals often dig long tunnels into the plants to lay their eggs. The larvae feed on the plants over a period of weeks, which then develop much slower and can even die. The most important pests are as follows:
Codling moth (Cydia pomonella): Fruit plant such as apple, peach, nuts.
Aphids (Aphidoidea): Potatoes, sugar crops, and citrus fruit.
Whiteflies (Aleyrodidae): Cereals, tomatoes, beans, cotton, and potatoes.
Thrips (Thysanoptera): Onions, potatoes, and melons.
Leafhoppers (Cicadellidae): Potatoes and apples.
European corn borer (Ostrinia nubilalis): Corn.
European corn borer
In the 27 EU States, an area of about 95,000 km2 is under corn cultivation, producing approx. 65 million tons of grain corn.124 The extent of the affected areas ranges between 20 and 60%, leading to yield losses of five to 30% of the total harvest.125
However, because of the large trade volume, sufficient controls are not possible. This is why every year, new insect species are introduced to Europe. The animals cause significant damage, since they do not have any natural enemies, their presence is noticed too late and only then can control measures be developed. In the following, current invasive species will be described.
Diabrotica virgifera virgifera
Due to the complexity of the flow of goods, there are no reliable numbers available on the storage damages in developed countries. Worldwide, the losses caused by pests in the storage area are estimated at ten to 20%. Approx. 80% of this can be attributed to insects, the remaining damage is caused by fungi, rodents, and birds.
In contrast to developing and threshold nations, developed regions like Europe have professional storage technologies, hygienic measures, and conservation methods for storage protection. The FAO estimates that damages in the storage area in developing countries are around 30% and are accordingly lower in developed countries.133
Agricultural production in Germany already suffers “post-harvest losses” caused by insects of about two percent.134 Additional damage is caused by insects already infesting the products or incoming insects during the subsequent transport and processing stages. Finally, damage also occurs with the consumers: Every year, at least two million households suffer from food moth infestations, which contaminate purchased food and make it unfit for consumption.135 For this reason, total damages caused by insects of more than five percent of the entire agricultural basic production can be assumed.
In Europe, more and more pests are being introduced and the resistance of endemic species against control measures is on the rise, so that a general increase in storage pests can be expected.136
Confused flour beetle (Tribolium confusum),
Drugstore beetle (Stegobium paniceum),
Indian mealmoth, synonym weevil moth (Plodia interpunctella),
Sawtoothed grain beetle (Oryzaephilus surinamensis),
Wheat weevil (Sitophilus granarius),
European grain worm (Nemapogon granellus),
Flour mite (Acarus siro),
Rusty grain beetle (Cryptolestes ferrugineus),
Warehouse moth, synonym cacao or tobacco moth (Ephestia elutella), and
Book lice, synonym barklice (Psocoptera).
3.3.3 Forest Damage
30% of the world’s land area is covered by forest.138 Their distribution varies greatly. The biggest forested areas are found in North America, Russia, South Africa, and South America, and therefore, the majority is in tropical areas where the number of native insects is particularly high.
Insects that are originally endemic to forests do not cause much damage.139 Apparently, a balance has been established over the course of the centuries in which insects and plants coexist peacefully. Up to 40% of current damage is attributed to the increasing intrusion of foreign insects, which can spread very rapidly due to the lack of natural enemies.
For example, the mountain pine beetle (Dendroctonus ponderosae) was first discovered on the American continent in British Columbia in 1994. In the first ten years, the insect destroyed 240 million m3 of forest over an area of more than 110,000 km2, therefore causing annual damages of 1.7 million US dollars. The beetle then spread rapidly across all of Canada and has meanwhile reached the USA. Up until 2004, the Canadian government spent a total of more than 80 million US dollars to keep the beetle under control.140
Originally native to Asia and discovered in Ontario in 2002, the emerald ash borer (Agrilus planipennis) will cause even more damage in North America. After it spread to nine other US states in 2009, scientists calculated that without control measures, the beetle could destroy a total of 38 million ash trees in the next ten years. Simply the required removal and the reforestation measures for 17 million trees would cost 10.7 billion US dollars. However, the costs are much higher. To cultivate the large infested areas, a total of 30 million trees must be cleared and new cultivated land must be established. A study estimates costs of more than 20 billion US dollars.141
In the 1980s in East and South Africa, three insects that damage conifers were introduced at the same time: Pineus boerneri, Eulachnus rileyi, and Cinara cupressivora. The last-mentioned beetle alone destroyed a forest area valued at 44 million US dollars, which represents an annual economic loss of 14.5 million US dollars. Together, the three beetles are damaging African countries with losses of 17 million US dollars every year.142
Also in Europe, there are reports of aggressive, suddenly occurring forest pests such as the Asian long-horned beetle (Anoplophora glabripennis) and the oak processionary caterpillar (T. processionea), which have caused high costs for the clearing of entire forests as well as for monitoring and control measures.143
At the beginning of the 2000s, at least 370,000 km2 of forest were destroyed by insects annually (Europe: 6.3 million), which corresponds to 1.4% of the total area. Hardly any numbers are available for African countries, so it can be assumed that overall, a much larger area has been affected by the insects than measured until now.144
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