2013 Annual Report
1a.Objectives (from AD-416):
Assess the effects of resistance alleles on the frequency, timing, and survival of interplant movement by European corn borer larvae on single- and multiple-toxin Bt hybrids. Evaluate how resistance allele frequencies influence entry into diapause, overwintering survival, and timing of European corn borer adult emergence.
1b.Approach (from AD-416):
European corn borers resistant to Bt corn will be used to measure the impact of Bt resistance alleles on survival, development, and behavior. Experiments will examine how resistance alleles affect the frequency, timing, and survival of interplant movement by larvae on non-Bt, current Bt hybrids, and pending Bt hybrids ("pyramids"). The effects of Bt resistance alleles on corn borers entry into diapause, winter survival, and timing of adult emergence by overwintering generations also will be assessed.
This is the final report of a three-year project that tested whether seed mixtures offer a viable alternative to blocked refuge. Interest in this research is particularly high now that blended seed or “refuge in the bag” products are available to corn growers as an alternative to structured refuges for insect resistance management (IRM) of Bacillus thuringiensis (Bt) corn. Dispersal studies with resistant and susceptible larvae did not raise any red flags. For the type of resistance that was studied, heterozygous first instars (those with one copy of a resistance gene) do not appear to have advantages over susceptible larvae that would expedite selection for resistance. As expected, Cry1F resistant larvae stayed on Cry1F corn longer than the other types of larvae, but all larvae dispersed from pyramid (Cry1F x Cry1Ab) corn. These results suggest that larvae can detect and move from Bt corn without major detrimental impacts. In general, most of the larval dispersal occurs with first instars and appears to be density independent, but larvae will disperse during each of the other instars (2nd, 3rd, 4th, and 5th). High dispersal rates were found particularly for fourth and fifth instars. Such dispersal of late instars appears to be influenced by larval crowding, a density dependent factor. Movement of late instars potentially could compromise the seed mixture IRM strategy, but since such movement is density dependent it may be ameliorated by low populations of European corn borer. A selection experiment shows that larval tendency to disperse or stay on a natal plant is under genetic control, which could be a factor in mixed seed environments. Diapause studies with Cry1F resistant and susceptible larvae suggest that susceptible females may emerge as adults 3-4 days later than resistant females, but there were no differences among resistant and susceptible males. This difference is not likely to have important biological implications.