Studies on dispersal of a native parasitoid Eretmocerus eremicus and augmentative biological control of Bemisia tabaci infesting cotton

Abstract

In the mid-1980s, a new biotype of Bemisia tabaci was introduced into the southern U.S. causing extensive damage to agricultural crops throughout the region. An augmentative biological control project was initiated using a native parasitoid, Eretmocerus eremicus to determine its efficacy against B. tabaci infesting cotton in the desert areas of California and Arizona. A series of experiments were conducted in 1992-1995: release rate studies in cages and open fields; parasitoid dispersal within fields to determine movement rates after point release; and experiments to determine the parasitoid-host spatial relationship. Cage release rate studies, demonstrated that rates of parasitism could be increased 61 to 79% in the highest release treatments, with reductions in whitefly densities of 80 to 100% relative to control treatments. Cotton yields in the high release treatments peaked at 2.5 bales/ac and were 2.6 to 4.2 times greater than in control treatments. Effective release rates were estimated to be equivalent to 770,000 to 1.1 million parasitoids/ha. Field releases equivalent to 7.9 million parasitoids/ha resulted in a peak rate of parasitism of 42% but there were no statistical differences in parasitism, whitefly densities, or cotton yield; relative to no-release plots. High levels of whitefly immigration from surrounding crops, and parasitoid dispersal from release plots, diluted the effects of release. In a second field release rate study, releases equivalent to 2.0 to 3.0 million parasitoids/ha increased levels of the percentage of discovered leaves to greater than 80%. Parasitoid dispersal was analyzed with mark-recapture experiments and data were fit to a diffusion model. One female wasp flew 82 m in one day though the majority of wasps flew a few meters or less. Estimated diffusion rates and median dispersal distances were 0.40 to 0.71 m²/min and 2.4 to 4.4 m/(4 to 8) days respectively. Analysis of dispersal data suggested that releases on 20 m centers would provide effective coverage within a field. Density independent parasitism was common at the spatial scale of leaves and plants. There was positive density dependence for the percentage of discovered leaves suggesting that parasitoids aggregate to high density patches of whitefly but fail to achieve high levels of parasitization possibly due to egg limitation or mutual interference.