Submitted to: Environmental Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/2/2006
Publication Date: 2/1/2007
Citation: Sisterson, M.S., Carriere, Y., Dennehy, T.J., Tabashnik, B.E. 2007. Non-target effects of bt crops: implications of source-sink population dynamics. Environmental Entomology. 36:121-127. Interpretive Summary: Crops which have been genetically modified (GM) to produce an insect toxin are planted on millions of hectares in the United States. Considerable effort has been expended to determine if such crops harm non-target insects. To determine if populations of non-target insects are harmed, researchers usually compare the abundance of insects in conventional and GM fields. Using a spatially explicit population dynamics model we show that the large scale planting of GM crops can cause populations to decline in GM and conventional fields. Thus, important effects may be missed by only comparing abundance between the two field types. Instead we suggest comparing the abundance of non-target insects in conventional fields in areas where the use of GM crops is low to areas where it is high. These results will aid researchers in designing experiments that evaluate the effects of GM crops on non-target insects.
Technical Abstract: Widespread planting of crops engineered to produce Bacillus thuringiensis (Bt) toxins for pest control has raised several concerns. In particular, considerable effort has been devoted to examining the effects of Bt crops on non-target arthropods. Field studies typically compare the abundance of selected non-target arthropod species in Bt and non-Bt fields or field plots. However, such analyses may miss important effects. Results from simulation of a spatially explicit, population dynamics model show that large scale planting of Bt crops could cause two types of negative effects: 1) decreased abundance in Bt fields with stable abundance in non-Bt fields, and 2) decreased abundance in both field types, in some cases ultimately leading to regional loss of the arthropod. Loss from both field types occurred only when the adoption of Bt crops was high, arthropod reproduction was low, and arthropod movement was high. Thus, tests conducted before the large-scale adoption of Bt crops can detect decreased arthropod abundance in Bt fields relative to non-Bt fields, but cannot determine if abundance will decrease in both types after Bt crop adoption increases. We present a sensitivity analysis that shows the conditions under which an arthropod could be lost from Bt and non-Bt fields and describe a method to evaluate if such effects are taking place after large-scale adoption of a Bt crop.