Habitat Management for the Establishment of Bemisia Natural Enemies
Challenges to the establishment of newly introduced parasitoids of Bemisia tabaci biotype B in the desert southwestern USA were presented by the severe climate of the region, the short-lived presence of most of its host plants, and the high mobility of the pest. To increase the chances of establishing new parasitoids capable of surviving in this region, three strategies were employed: (1) multiple releases of large numbers of exotic parasitoid species throughout the growing season, (2) creation of short-term and permanent refuges using a mixture of annual plants, and (3) managed releases in home gardens and backyard vegetation in urban settings. Long-term insectary habitat provided the opportunity to observe parasitoids over a wide range of seasonally related changes, such as weather and host plant. Continuous plantings of annual whitefly hosts were grown in field plots over successive years to provide a year-round habitat for newly introduced parasitoid species in the desert environment of Imperial Valley, California. Collard plants served as a bridge host during the winter season. Two introduced species, Eretmocerus emiratus and Eret. sp. nr. emiratus, represented over 50% of the Eretmocerus attacking Bemisia in the field insectaries in cultivated field insectaries from 1997 to 2000. Encarsia sophia from Pakistan also became established after its release in 1997. In contrast, summer–fall field insectaries promoted the mass production of several introduced parasitoids. Short-term plots of okra and basil, selected for high rates of parasitism sustained by B. tabaci, were planted along the edges of commercial fields to produce large numbers of parasitoids from April to November. Although this conservation approach only functioned for a portion of the year, millions of parasitoids were produced at several locations in Imperial Valley and Palo Verde Valley in California. Home sites containing ornamental host plants of B. tabaci also provided perennial habitats for establishment of exotic parasitoids. Selected commercial crops were also utilized as field insectaries. From 1994 to 1999, large-scale releases of exotic parasitoids were made in spring melons. The fields effectively served as insectaries where large populations of exotic parasitoids developed, greatly enhancing the probability of new species establishing regionally. In southwestern desert valleys alfalfa comprises a large proportion of irrigated acreage and represents a significant percentage of host crops impacted by B. tabaci and a potentially large reservoir for natural enemies of B. tabaci. Frequent cutting removes a large part of whitefly populations but substantial amounts of alfalfa along section borders, in furrows and along field edges escape cutting, and parasitism of B. tabaci in this alfalfa was significant. Releases were also made in alfalfa fields located on a National Wildlife Refuge. In the San Joaquin Valley of central California, citrus foliage serves as an overwintering refuge for B. tabaci, which colonizes during the fall months; imported parasitoids were colonized in citrus to take advantage of this fall–winter habitat.
KeywordsHome Garden Hibiscus Cannabinus Parasitoid Species Habitat Management Alfalfa Field
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