Integrating Parasitoid Releases with Traditional Control Methodologies: Experience in the Spring Melon Production System in the Southwestern USA

During the outbreak of Bemisia tabaci biotype B during the 1990s in the southwestern USA efforts were made to integrate parasitoid releases with existing chemical control strategies in concert with a classical biological control program. Spring melons grown in the desert vegetable producing areas of the southwestern USA were among those crops most severely affected by whitefly infestations. High rates of parasitism of B. tabaci by native parasitoids were observed in melons near the end of the melon production cycle, therefore demonstration projects were conducted in the Imperial Valley of California during 1994–1998 to inoculate these crops with introduced parasitoids earlier in the season in order to increase rates of parasitism. Since spring melon fields would act as field insectaries this had the added benefit of increasing the chances of permanently establishing newly imported species of parasitoids. The project took advantage of mass production systems developed for the importation and establishment of new species of Eretmocerus to provide the numbers needed for large, replicated field releases. Initial tests compared the native Eret. eremicus with two populations of introduced Eret. mundus and showed that Eret. mundus was more effective and increased parasitism five to sixfold. Subsequent field trials utilized Eret. hayati, introduced from Pakistan, Eret. emiratus from the United Arab Emirates, and Eret. sp. nr. emiratus from Ethiopia. Various parasitoid release strategies were tested in melons grown using conventional pest control methods relying on broad spectrum insecticides, in imidacloprid treated fields, and in fields where both imidacloprid and broad spectrum pesticides were used. Releases of Eretmocerus were shown to be compatible with the use of imidacloprid. By comparing whitefly densities in nearby untreated fields to imidacloprid treated fields, these studies showed that it was possible to reduce the number of whitefly by a factor ranging from one half to 300-fold. Estimates of the number of parasitoids needed to increase parasitism by 50% or greater and to reduce whitefly densities by as much as one half over several seasons ranged from 49,400 to 197,600/ha. Releasing parasitoids in organic fields early in the season when whitefly density was low also provided significant reductions in whitefly numbers. Emergence rates using pupae released in cups placed among foliage vs. pupae mixed with vermiculite and spread mechanically by tractor with a drop-box were similar and ranged from 55 to 82%.