The Impact of Changes in Cultivation Patterns and VarietySelection on Western Corn Rootworm

Rudy, A. and B. Thomas

Presented at the All Scientist Meeting (2002-10-04 )

We are currently in the initial stage of investigating the ways that social factors and biophysical environmental factors interact in agricultural pest systems – in this case, that of the Western Corn Rootworm (WCR).  For decades, the conventional wisdom has been that two-or three-year corn-soybean (or corn-soy-wheat) rotations act as a low-input, non-resistance inducing, means of holding western corn rootworm populations below economic damage thresholds.  Recently, western corn rootworm “behavioral variants” have been found in areas near the Illinois-Indiana border and in isolated areas in Michigan and Ohio.  These “rotation resistant” WCR appear to have adapted to rotations and now often lays eggs in fields planted with soybeans, which leads to WCR populations in first-year corn near, at or above economic thresholds. Our research follows on published and unpublished research by O’Neal, et al. (2002) – including feeding assays and field studies – that suggest the WCR variant’s behavior may be derived less from the development of a new genotype (though this may be true as well) and more from the expression of already-present, but heretofore unnecessary, behavioral plasticity induced by new and unevenly distributed agroecological conditions.  Recent movement towards earlier planting dates and more cold-hardy varieties may have disrupted what was, in the past, a synchronicity between corn phenology and WCR development.  Following on O’Neal et al., our hypothesis is that the emergence of the WCR behavioral variant is largely caused by regional changes in cultivation practices – early planting and/or planting of new cold-hardy varieties – which have altered the relationship between corn phenology and WCR development.  A second-order hypothesis is that the unevenness of the distribution of the variant is associated with the distribution of rotational and crop-mixture diversity.  For example, in the IL/IN border region corn-soy rotations and few other crops increase the likelihood that beetles leaving corn will oviposit in soy.  In MI or OH, greater cropping diversity and/or three-crop rotations may reduce the likelihood that gravid WCR beetles will find soy upon leaving phonologically-unattractive corn.The purpose of our study is to identify whether or not cultural practices and corn variety selection have changed in the spatial and temporal manner coincident with the arrival of WCR “behavioral variants” and our hypotheses, above.  In the next year we will conduct interviews with extension agents, farmers, and seed company representatives to examine these agroecological changes as they have developed over the past 20 years.  These interviews will focus on crop rotations, pesticide applications, and crops planted.  We will then compare temporal changes in these farming practices to the emergence of WCR behavioral variants in specific areas of the Midwest.  Figure 1.

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