Beet Armyworms: What Do They Really Want?

South Texas farmers face their share of crop pests, with the beet armyworm on their “least- wanted list.” Now, Agricultural Research Service scientists are hot on the pest’s tiny tracks, looking at clues to help control the critter.

One important new clue is that beet armyworms grow bigger when they eat a diet of pigweed--a crop weed--than when they eat cotton plants. The finding suggests pigweed might have a constructive role, by diverting some of the armyworms from cotton.

Shoil Greenberg and Thomas Sappington, ARS entomologists at the Kika de la Garza Subtropical Agricultural Research Center in Weslaco, Texas, cooperating with Tong-Xian Liu at the Texas Agricultural Experiment Station, also at Weslaco, study this cotton pest extensively. Armyworms quickly develop resistance to insecticides; moreover, it is vital to understand the insect’s biology in order to develop effective biological controls or chemicals to prevent crop losses.

At the Integrated Farming and Natural Resources Research Unit, Greenberg tested beet armyworm, Spodoptera exigua, to determine how quickly armyworm larvae consumed five different host plants, and the impact of the feeding on the insect's fertility and life cycle. His findings can be read in the July issue of the Annals of the Entomological Society of America.

Cotton, cabbage, pepper, sunflower and pigweed were tested as host plants, with interesting results. For example, although beet armyworms destroyed the 1995 South Texas cotton crop, pigweed was most nutritional for larvae, as measured by the weight of the pupae that developed from them. Pupae that developed from larvae feeding on pigweed weighed 117 milligrams (mg), compared to 103 mg for those feeding on cotton, even though armyworm larvae feeding on cotton ate more than those dining on pigweed. And larvae reared on pigweed developed into females more than 60 percent of the time.

Greenberg believes an absence of pigweed in South Texas, where it is naturally abundant, could cause damaging outbreaks of armyworm infestation in cotton as the pest moves from its preferred host of pigweed to cotton.

Scientists are now finishing a study to determine where the insects prefer to lay their eggs. With this information, scientists would know where to coat leaves with insecticides that would kill the armyworm’s eggs or larvae.

ARS is the U.S. Department of Agriculture's chief scientific research agency.