Augmentative releases of parasitoids and/or predators within an IPM program may be an option to control the silverleaf whitefly (WF or sweetpotato whitefly strain B) in the Imperial Valley, CA. Cotton was chosen as a model system, because controls of other key pests on cotton are compatible with biological control, cotton is a bridge crop between spring and fall vegetables, and cotton has a relatively long season. We chose to start releases with the aphelinid wasp Er. nr. californicus (herein referred to as Eretmocerus) because records in the literature indicate it is a dominant parasitoid of WF (strain A and B) in the Southwest. Our primary goal for the 1993 season was to determine what numbers of parasitoids needed to be released to achieve effective control (release rate cage experiment) and whether this parasitic wasp could suppress WF populations on cotton in the field (open field experiment). In addition, we started a mass-rearing of Eretmocerus in Tucson with expertise and technology transferred from CIBA-Bunting, England.

Work begun in 1992 to investigate the potential of Eretmocerus as a control agent in an augmentative release program against WF continued in 1993. The work was conducted in cotton fields in the Imperial Valley. Field cage studies with varying release rates of the parasitoid achieved parasitism levels as high as 80% along with WF control (about 0.0 WF nymphs/cm² leaf). The field cage studies of both 1992 and 1993 suggest that augmentative biological control with Eretmocerus is an effective means of WF control, although the required release rate (about 10 parasitoids/plant) is high and not economically feasible (yet).

Open field releases of Eretmocerus were also conducted in 1993. The results from these studies were less promising than the cage studies as overall parasitism was low and there was no significant reduction in WF levels in release fields relative to control plots. The highest level of parasitism achieved was on average 36%, which was similar to the control plots with 30% parasitism. High levels of WF immigration from surrounding fields probably dramatically reduced the effect of the releases of parasitoids.

An IPM approach will be necessary to combat this pest and information about potential strategies such as augmentation or conservation of natural enemies will be required to implement a true IPM program. Our cage studies suggest that Eretmocerus can be an effective parasitoid against WF on cotton though it remains to be seen if these parasitoids can be effective outside of cages. If the major effect of cages is to limit the number of WF that migrate into the cotton, then our results suggest that augmentative biological control with Eretmocerus may be useful as a control strategy in fields distant from source populations of WF immigrants or in years when valley-wide WF levels are low. Next year large scale releases of parasitoids will be conducted in commercial field blocks of cotton to determine if significant levels of WF suppression can be achieved without use of cages.