Fall armyworm doesn’t historically cause much problem in cotton, but in some areas of South Carolina this year Clemson Entomologist Jeremy Greene says infestation have been the highest he has ever seen.
Greene says in one sample he counted 360 fall armyworms from 36 row feet of cotton in one field. “I’ve never seen fall armyworm populations in cotton at those levels before,” he says.
In late cotton, insect damage is not typically something a grower pays much attention to, but fall armyworm has the ability to damage cotton in its later stages of growth, Greene adds.
In most cases in late season cotton, fall armyworm damage is limited to small, second stage larvae feeding in the space between bracts and the surface of variously-sized bolls. Without blooms and squares present for early growth, fall armyworms generally do not become established in late August or in September in the Carolinas.
Fall armyworms are occasional pests of cotton grown in the upper Southeast. Although it may cause some suppression of fall armyworm infestations, Bt cotton does not provide complete control of these insect pests.
Studies throughout the Southeast over the past few years have shown a 25-35 percent reduction in fall armyworms with Bollgard cotton. Bollguard II and Widestrike cotton offer better control, but still only 50 percent or so.
Increases in corn acreage in the Southeast in recent years likely contributed to the increased incidence of fall armyworm damage in cotton. Like cotton, Bt-containing corn varieties have been susceptible to armyworm damage. Once corn dries, armyworms typically begin looking for other food sources.
Fall armyworms are likely to feed on both blooms and bolls. Small larvae are difficult to detect, because armyworm is often found throughout the Southeast and as far north as Canada during the late summer and early fall months.
Fall armyworms also have a wide host range that includes cotton, corn, soybeans, sorghum, wheat, bermudagrass and a number of uncultivated plants.
The “cotton strain” of fall armyworms is inherently difficult to control with insecticides, and larvae are often found deep in the canopy in protected areas. Current treatment threshold is based on the number of larvae found in a given number of blooms and bolls or the number of larvae per plant.
Scouting for fall armyworm is relatively easy, with the major problem distinguishing these infrequent pests from bollworms. Most fall armyworm larvae are tan in color with some stripes. Bollworms can be tan also, but are more solid in color and often pink to yellow to black in color, as well. Fall armyworm larvae can grow to sizes similar to bollworms — up to 1.5 inches long.
Fortunately, growers are not likely to have to deal with fall armyworm damage in cotton in coming years. Once the original Bollguard varieties are replaced by the dual gene varieties like Bollguard II and WideStrike, fall armyworms are not likely to be a big problem, because these new Bt varieties do a good job on the pest.
Testing by USDA researchers in Texas indicates WideStrike appears to be more efficacious against fall armyworms than Bollgard II, while Bollgard II is more efficacious against beet armyworms and corn earworms than WideStrike.
University of Georgia Entomologist Phillip Roberts says his observations support the differences in fall armyworm control between WideStrike and Bollguard II. “We have observed improved control of both fall armyworm and corn earworm in WideStrike and Bollgard II cotton compared with Bollgard, but some fields have needed supplemental treatments in some areas.
“WideStrike varieties have tended to be treated for corn earworms, whereas Bollgard II varieties have tended to be treated for fall armyworm,” Roberts says.
For growers with fall armyworm problems in late season cotton, Greene says a combination of a pyrethroid and Diamond, an insect growth regulator, has done a good job of controlling fall armyworms at the Edisto Research Station.