2013 Annual Report
A major thrust of field research during the last 2 years has been to examine the role of volunteer corn on the evolution of resistance of the bollworm. A graduate student is undertaking several studies to examine this aspect of Bt resistance risk. ARS scientists conducted a survey of corn fields after harvest to assess the density and maturity of volunteer corn in the fall as well as the abundance of corn earworm in non-Bt corn at various growth stages. ARS Scientists have also conducted greenhouse and field trials examining the efficacy of various crosses of Bt and non-Bt corn varieties on bollworm under stressed and non-stressed environments. A second graduate student is evaluating the change in performance on cotton of bollworms that developed on Bt corn varieties compared to non-Bt corn varieties. He is also conducting bioassays of various crosses of these bollworm populations to try to determine the genetic basis of differences in performance.
A second method for assessing the risk of resistance development in corn earworm is through computer modeling. ARS scientists used and modify a computer model to address the risk of resistance that comes from changes in ARS scientists agricultural landscape and the Bt technologies available to farmers. In addition to the modeling, ARS scientists are working with a subcontractor to move the population genetics program from a Java/Linux platform to a Windows platform. The new application reduces the need for re-compiling and will make the program available to a broader personal computer community without the need for increased support or maintenance. The goal is to develop a program that is intuitive to the user and accurately models the parameters provided.
Host plant usage and pyrethroid insecticide resistance levels in bollworm continue to be monitored across the cotton belt. Cooperators in 9 states have trapped moths each year since 2007 and tested them for pyrethroid resistance. The moths are then frozen and sent to Southern Insect Management Research Unit, Stoneville, MS, for carbon isotope analysis which indicates the type of host plant on which the larva fed. Data show high levels of pyrethroid resistance in Louisiana and more recently in Virginia. Resistance in other states has been generally low. Corn earworm larval hosts are almost all grasses during mid-summer, with more developing on broadleaves during spring and autumn periods. This trend is consistent in all states being monitored. There has been no indication that pyrethroid-resistant moths are more or less likely to develop on grasses than pyrethroid-susceptible moths.
One of the benefits of Bt crops is reduced insecticide usage. However, the emergence of tarnished plant bug (TPB) as a major cotton pest in the mid-south has reduced this benefit in cotton. ARS Scinetists have initiated some work with TPB to improve ARS Scientists understanding of TPB ecology and physiology. Specifically ARS Scientists have examined the prevelance of microsporidia and viruses in wild populations. Further, ARS Scientist have compared development of TPB populations from different regions within Mississippi on cotton and other crop hosts. ARS Scientists are also examining how TPB alter expression of several polygalacturonases depending on host plant, which could be very important for this highly polyphagous pest. This work will continue in 2013.