Biotechnology and insect management in field crop agroecosystems

Since the introduction of genetically modified crops in 1996, the majority of grain growers in the US have adopted the technology (e.g., currently about 93% of the US soybean acreage is genetically modified for glyphosate resistance). Initially, these transgenic crops were expected to increase yields and simplify farming by reducing pesticide applications. Indeed, the widespread adoption of insect-resistant crops modified to express Bacillus thuringiensis (Bt) toxins has improved yield by reducing injury caused by lepidopteran pest species. In addition to these yield increases, these crops have also reduced insecticide usage, and populations of some target pest species to historical lows. Whereas, the implementation of herbicide resistant crops has not improved yield; instead, widespread adoption of glyphosate-resistant crops and subsequent overuse of glyphosate in these systems has selected for 16 of the 25 glyphosate-resistant weed species worldwide, which has resulted in increased herbicide usage. To combat this resistance epidemic, the agricultural industry has developed crops resistant to dicamba and 2,4-D, two synthetic auxin herbicides that are highly volatile and prone to causing non-target damage.;My research with genetically modified crops had two foci. One involved assessing non-target effects of the herbicide dicamba on insects and their communities. Because herbicides are the dominant pesticide applied in agroecosystems, herbicides may pose significant ecotoxicological risks to non-target plants and insects. To understand these risks, we used a combination of field- and laboratory-based experiments exploring direct and indirect effects of dicamba on aphids, caterpillars, and bees. The other focus evaluated efficacy and value of transgenic, insect-resistant field-corn hybrids for controlling two target caterpillar species, European corn borer (ECB), (Ostrinia nubilalis [Hubner]) and corn earworm (CEW) (Helicoverpa zea [Boddie]. Over three years, we performed large scale field trials with Bt and non-Bt corn hybrids at 29 locations across Pennsylvania. We also worked with moth capture data from the PestWatch pheromone trapping network (pestwatch.psu.edu) and GIS mapping software to determine if the number of moths captured, or land-use variables related to in-field damage from caterpillars.;Despite limited usage, the herbicides dicamba and 2,4-D commonly cause herbicide-drift damage to non-target plants. Now with dicamba and 2,4-D primed to be more widely used in association with pending transgenic crop varieties, there is a pressing need to understand the influence of these synthetic auxin herbicides on insects and their communities. To understand the effects of dicamba on two common lepidopteran species, we applied several sub-lethal, drift-level rates of dicamba to soybean (Glycine max L.) and nodding plumeless thistle (Carduus nutans L.) and evaluated on dosed plants growth and survival of H. zea and Vanessa cardui (L.) larvae, respectively. In the laboratory with several rates of dicamba, we also performed direct toxicity bioassays on the two caterpillar species. Dicamba was not directly toxic to larvae of either species, and H. zea showed no negative effects when feeding upon soybeans dosed with dicamba. I did, however, detect significant negative, indirect effects of higher rates of dicamba on V. cardui larval and pupal mass, thistle biomass, and total plant nitrogen. Notably, in the absence of caterpillars, thistle biomass was not related to dicamba dose, suggesting that the additional herbivore stress was necessary to reveal costs of sub-lethal exposure to dicamba.;We also evaluated whether drift-level doses of dicamba on soybeans can influence soybean aphids and the soybean insect community. We found that when soybean aphid populations were high, population densities started to decrease on plants dosed with rates above 0.56 g/ha (particle drift levels). When aphid population densities were low, however, we did not find any relationship between dicamba dose and aphid populations. During field studies, we found evidence that as dicamba dose increased populations of ants, mirids, nabids, spiders, opiliones, Orius, and lathridids increased; these largely predaceous taxa may have responded to changes in plant structure caused by herbicide injury. (Abstract shortened by UMI.).