Biological Control ofBemisia argentifolii(Homoptera: Aleyrodidae) on Poinsettia with Inundative Releases ofEncarsia formosa(Hymenoptera: Aphelinidae): Are Higher Release Rates Necessarily Better?

The effectiveness of inundative releases of the parasitoidEncarsia formosafor control ofBemisia argentifoliion poinsettia was determined in replicated experimental greenhouses. We evaluated two release rates ofE. formosa:a low release rate (1 wasp/plant/week, released in two greenhouses, in spring 1995) and a high release rate (3 wasps/plant/week, released in two greenhouses, in fall 1993), each over a 14-week growing season. Each release trial had one or two control greenhouses in whichB. argentifoliideveloped on poinsettia in the absence ofE. formosa.Life-tables were constructed forB. argentifoliiin the presence and absence ofE. formosaby using a photographic technique to follow cohorts of whiteflies on poinsettia leaves. Weekly population counts of the whitefly were also made. In the absence ofE. formosa,egg to adult survivorship ofB. argentifoliion poinsettia was 75–81%. At the low release rate, egg to survivorship ofB. argentifoliiwas 5% and parasitism was 13%. At the high release rate, egg to adult survivorship forB. argentifoliiwas 8% and parasitism was 23%. The net reproductive rates (R₀) forB. argentifoliipopulations in the absence ofE. formosaranged from 18.01–26.12, indicating a rapidly increasing population. Net reproductive rates for whitefly populations subject to wasp releases were 1.54 for the low release rate greenhouses and 2.11 for the high release rate greenhouses, indicating substantially reducedB. argentifoliipopulation growth. The low release rate provided better control ofB. argentifoliithan the high release rate. This difference was attributed to higher levels of mortality of whiteflies at the low release rate in the first 5–6 weeks of the growing period. We suggest that mutual interference may also have affected observed levels of mortality and parasitism.