Area-wide pest suppression in transgenic landscapes
Abstract
Even though large scale planting of Bt cotton started a decade ago, consequences on a landscape-scale have hardly been studied. Helicoverpa armigera and H. punctigera are highly mobile organisms existing across broad-acre landscapes that are constantly changing, yet little is known about how they respond to crop and non-crop composition and configuration. Using a robust sampling design we collected data on the abundance of Helicoverpa spp (eggs and moths) and egg parasitoids in Bt cotton and sorghum at the spatial scale of fields, groups of fields and landscapes (20 km diameter) across the Darling Downs, QLD over three years. Detailed land-use metrics were generated at various distances from each sampled field in each landscape and season. These data were combined and used in spatially-explicit statistical analyses to identify and predict how crop composition and configuration at the landscape-level and field-level influence Helicoverpa egg and moth density.
Helicoverpa armigera egg and moth density is strongly affected by landscape composition and configuration at the scale of landscapes, or tens of kilometres. The landscape explains most of the variation in H. armigera egg density in Bt cotton, not what immediately surrounds the Bt cotton field. The best-fit model accounts for 50% of the variance among landscapes with large areas of Bt cotton (at a scale of 1.5 km radius) having the biggest affect, resulting in fewer H. armigera eggs laid in Bt cotton. The explanation for this result may be that Bt cotton functions as a population sink and overtime fewer moths are available to lay eggs.
Combinations of crop at various scales could not explain Helicoverpa punctigera egg and moth density. This maybe because H. punctigera interacts with crops at a spatial scale larger than landscapes of 20 km diameter. No H. punctigera were found in sorghum. For H. armigera as sorghum fields increase in size so does the H. armigera egg density. However, sorghum surrounded by sorghum never has as many eggs as sorghum surrounded by Bt cotton. This could be due to either more immigrants to sorghum over time or moths less willing to leave sorghum when surrounded by large areas of Bt cotton.
In Bt cotton, there was a positive relationship between the number of moths (species not separated) and the field-level amount of Bt cotton at 0.5 km r. However this was only found for 2011-12 season. In sorghum, there was a positive relationship between the number of moths and the average landscape-level amount of Bt cotton at 1.5km r. The moth and H. armigera egg result are similar. There are more moths and eggs in sorghum surrounded by Bt cotton than sorghum surrounded by sorghum. Sorghum in a sea of Bt cotton either ‘attracts’ and / or ‘holds’ H. armigera moths.
Interpretation and Implications
These results provide clear demonstration of the importance of landscape composition and configuration. Further, variation was often best explained by landscape-level predictors, and less often by field-level predictors. This demonstrates the importance of area-wide approach to pest and resistance management. Further, the strength of selection for resistance is largely determined by the proportion of the population under selection. Our results allow us to identify and predict some of the factors contributing to eggs deposited by female moths in Bt cotton.
These findings have the potential for positive outcomes in relation to resistance management and area-wide pest control, however the ways in which they might be developed for application will be challenging. Regardless, our approach and results can directly contribute to selection of crop type, amount and location for delaying resistance in Helicoverpa spp.
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- 2012 Final Reports
CRDC Final reports submitted 2012