As you may recall, each year we assess the susceptibility of corn earworm to pyrethroids using the standard AVT (adult vial test) process. This involves collecting live moths using pheromone baited traps from multiple locations across the area. Moths are individually placed into vials pre-treated with cypermethrin (the standard used across the southeastern states) and assessed for mortality after 24 hours. If moths survive this 24 hour exposure, they are considered to be ‘resistant’. The percentage of those that survive provides some indication of what growers might expect to see when they apply pyrethroids to crops (soybean, cotton, peanuts). Although not a perfect system for determining actual resistance levels in the population, this procedure does provide valuable insight. For example, last year we had unusually large numbers of survivors. Whereas in previous years we had seen 5 to 10% survivors, in 2008, survivorship in late June exceeded 20%, and then increased to over 40% by August. Over the course of the summer, we received several calls about spray failures: cases where application of normal rates of commonly used pyrethroids did not provide adequate levels of corn earworm control.
So, what is the situation this year? Things do not look good. We started the season in early June with less than 10% survivors (see the attached line graphs, 2008 on the left side, 2009 on the right side) but are now at over 30% survivorship, even higher than at this time last year. Granted these data are based on a very small subsample of moths (744 moths to date), but all indications are that we may be in for some problems. Of course things could change as we progress through the season. We will continue to provide updates.
Additional information: mean-weekly-cew-moth-survival-jul-2-09l-ppt