Submitted to: Agricultural and Forest Entomology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 7/15/2010
Publication Date: 1/15/2011
Citation: Davies, A.P., Zalucki, M.P., Pufke, U.S. 2011. Spatio-temporal variation in Helicoverpa egg parasitism by Trichogramma in a tropical Bt-transgenic cotton landscape. Agricultural and Forest Entomology. 13:247-258. Interpretive Summary: A USDA-ARS scientist at the Center for Medical, Agricultural, and Veterinary Entomolgoy Gainesville, Florida, published a paper with colleagues on research that investigated the distribution of Trichogramma parasitic wasps that attack the eggs of insect pests. The study found that these wasps are incredibly abundant in tropical cotton crops, but their spatial patterns do not necessarily follow those of the host (pest egg) insect. In addition, the wasp’s activity varies between fields and is generally limited to within the cotton plant canopy before canopy closure. Essentially, spatial variation in Trichogramma activity was evident at the between-field, within-field and within-plant scale in tropical cotton, and sampling regimes targeting their activity should incorporate all life stages, not just parasitized host eggs, to obtain an accurate assessment of their biological control potential and contribution to resistance management strategies.
Technical Abstract: Understanding the spatio-temporal dynamics of insects in agroecosystems is crucial when developing effective management strategies that emphasise biological control of pests. Wild populations of Trichogramma Westwood egg parasitoids are utilised for biological suppression of the potentially resistant pest species, Helicoverpa armigera (Hübner), in Bt-transgenic cotton, Gossypium hirsutum L., crops in the Ord River Irrigation Area (ORIA). Extensive, spatially-stratified sampling during a season of relatively high Trichogramma abundance found spatial patterns of pest egg parasitism in the ORIA tend toward heterogeneity, and do not necessarily coincide with host spatio-temporal dynamics. Both patterns of host egg density and mean rates of parasitism are not good indicators of parasitoid spatio-temporal dynamics in ORIA cotton crops. Parasitism rates can be significantly higher within the middle strata of the cotton plant canopy prior to complete canopy closure despite a similar number of host eggs being available elsewhere in the plant. Spatial variation in egg parasitism by Trichogramma in Bt-transgenic cotton is evident at the between-field, within-field and within-plant scale and is not solely driven by host spatial dynamics. These factors should be considered when estimating Trichogramma impact on pest species during biological control and spatio-temporal studies of host-parasitoid interactions in general.