Growers in our area have been dependent on pyrethroids for controlling corn earworm (CEW) for many years in many crops including cotton, peanuts, soybeans, sweet corn, tomato and several other vegetable crops.
There is a large selection of pyrethroid insecticides ranging from ‘old-timers’ like Ammo, Ambush and Asana XL to newer ones like Karate, Warrior, Baythroid XL, Mustang Max, Danitol, Brigade, and Prolex.
For the most part, these products have been effective, relatively inexpensive, and provide control of a complex of insect pests. But is this reliance on a single insecticide class of chemistry, the pyrethroid class, leading to resistance issues? Our 2008 data and observations say, ‘maybe’.
For the past several years, we have been involved in region-wide programs for monitoring CEW susceptibility to pyrethroids by employing what we call the AVT system (Adult Vial Testing). We trap live male moths in pheromone-baited traps and place them into small glass vials pre-treated with known amounts of cypermethrin (an ‘old-timer’ pyrethroid insecticide), a standard technique used across the eastern and Southeastern states
Moths are also placed into untreated vials as a control. Moths are kept in the vials for 24 hours, then released and scored as dead or alive (able to fly). Moths that survive the insecticide-treated vials are considered to be ‘resistant’.
The goal of the AVT is to determine and track the percentage of resistant moths over years and locations. We typically begin testing moths as soon in the season as we get decent trap catch numbers, which usually occurs in May, and continue through the end of September.
The number of sample sites where pheromone traps are operated, and the total number of moths tested has depended on the amount of grant funding that was available in that particular year. The number of test sites has varied a lot over the years, from as many as six counties in 2003, to as few as two counties in 2007 and 2008.
The number of moths processed annually has ranged from 1,164 to 2,651. We have placed as much emphasis on this as we could, given our many other program efforts.
The results of these AVTs were very consistent from 2003 to 2007, with just 2 percent to 4 percent of moths surviving the cypermethrin vial tests over a season
We understand from working with entomologists in other states, that until the survival percentage exceeds about 20 percent to 30 percent there is no immediate need for concern.
But in 2008, we saw a very different picture. Moth survival exceeded 20 percent by as early as mid- to late-July We saw two peaks exceeding 40 percent survival during August and a seasonal average of nearly 20 percent survival.
Based on this data, we posted an early warning about this on the Virginia Ag Pest Advisory site (http://www.sripmc.org/Virginia ) suggesting that growers consider switching to non-pyrethroid insecticides like Larvin, Steward, or Tracer, for managing threshold populations of CEW in soybeans.
In August 2008, we visited a late-planted soybean field in King and Queen County where a CEW control failure was noticed by the grower and county agent. The field had been treated twice with pyrethroids (early July and again in mid-August), but still had large numbers of CEW larvae.
Larvae were collected, reared on an artificial diet, and tested in the AVT once they became adults. Of 30 moths tested, 22 survived the cypermethrin vial test (73 percent survival).
In a similar case on the Eastern Shore, Virginia Tech Entomologist Thomas Kuhar collected and reared CEW larvae from sweet corn that had 50 percent moth survival in the cypermethrin AVT. These are the first two documented cases of pyrethroid resistance in Virginia CEW populations.
However, as is the typical pattern, if and when resistance develops in an insect population, it is not uniform across an area, state or region, but happens in spots over both space and time. This erratic spread of resistance is partly due to the genetic diversity of the species.
A case in point is our CEW population which is a mixed bag of both locals that over-winter in Virginia and emerge in the spring, and migrants that can be moved into the area on weather fronts from near (North Carolina) or great (Caribbean) distances — and, currently we have no way to determine the origin of individuals or collections.
One field could be infested by a population with some level of resistance, while another across the county, or across the field path, could be infested by a more typically susceptible population, which we saw in 2008.
We described (above) the resistant population in King and Queen County, but results of a field efficacy trial in a soybean field in Middlesex County, adjacent to King and Queen County, where we compared pyrethroids to seven insecticides in several other insecticide classes showed that one of the pyrethroid insecticides provided the highest level of control.
It is interesting to note that another difference in those two soybean fields was that the one in Middlesex County had not been previously treated with any insecticide prior to when we put in the efficacy trial.
Was the resistance spike in 2008 a fluke? The high survival of CEW moths in the AVTs and in the two field locations raised a red flag and emphasized the need to continue closely monitoring them for development of pyrethroid resistance.
However, since we know of only a few field cases in Virginia in 2008 where pyrethroids failed to provide adequate CEW control, pyrethroids are still considered generally effective against this pest.
It will be important for growers to begin considering rotating insecticide classes, especially when multiple applications are made in a single crop, or when two different, but adjacent, crops are treated (as always, please be sure to follow the pesticide label). We will begin our 2009 corn earworm AVT monitoring program this month and post updates and advisories.