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 eremicus Rose and Zolnerowich. We compared the currently used pattern of a fixed 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 the outcome of these two treatments (fixed and variable) to a low release rate (1 parasitoid per pl per wk) of Encarsia formosa Gahan, an aphelinid parasitoid widely used for whitefly control in greenhouse crops. In control cages without parasitoid releases, whitefly nymphal densities reached 15-32 live nymphs per leaf, which was 7 to 16-fold greater than the acceptable level at crop harvest. In cages in which parasitoid releases were made, whitefly 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, respectively. In greenhouse populations, the final densities of live whitefly nymphs per leaf were significantly higher in the E. formosa treatment than the two E. eremicus treatments. Releases of low numbers of E. formosa provided commercially acceptable control in only one of two greenhouses. There was no difference between the fixed and variable release rate treatments of E. eremicus, indicating that whitefly suppression was not increased by concentrating the release of this parasitoid early in the crop.