Effect of parasitoid release pattern on whitefly (Homoptera : Aleyrodidae)control in commercial poinsettia

Under commercial poinsettia production conditions we compared two patterns of parasitoid release for the aphelinid whitefly parasitoid Eretmocerus ere micus Rose and Zolnerowich. We compared the currently used pattern of a fix ed weekly release number (3 females per plant per week) to an experimental pattern in which more parasitoids were released early in the crop (wks 1-8) , followed by a lower number (wks 9-17), with the seasonal release average still being 3 female parasitoids per plant per week. We further compared th e outcome of these two treatments (fixed and variable) to a low release rat e (1 parasitoid per pi per wk) of Encarsia formosa Gahan, an aphelinid para sitoid widely used for whitefly control in greenhouse crops. In control cag es without parasitoid releases, whitefly nymphal densities reached 15-32 li ve nymphs per leaf, which was 7 to 16-fold greater than the acceptable leve l at crop harvest. In cages in which parasitoid releases were made, whitefl y nymphal densities were suppressed 99.8%, 96.8% and 50.9% by fixed-rate E. eremicus, variable-rate E. eremicus, and low-rate E. formosa treatments, r espectively. In greenhouse populations, the final densities of live whitefl y nymphs per leaf were significantly higher in the E. formosa treatment tha n the two E. eremicus treatments. Releases of low numbers off. formosa prov ided commercially acceptable control in only one of two greenhouses. There was no difference between the fixed and variable release rate treatments of f, eremicus, indicating that whitefly suppression was not increased by conc entrating the release of this parasitoid early in the crop.