Fudd Graham and Michael J. Gaylor

Beet armyworms are major pests of cotton in Alabama, especially in hot, dry weather. Farmers have indicated that armyworms appear to be more numerous in fields that are deficient in potassium (K). AAES research indicates that, under drought conditions, cotton grown in soil containing medium levels of potassium may be more attractive to armyworms.

Soil low in potassium was collected from an established cotton fertility test at the Prattville Experiment Field. This soil was amended to produce soils low, medium, and high in potassium (71, 168, and 318 pounds of K per acre, respectively, based on Auburn University Soil Testing Laboratory recommendations) and placed in two-gallon pots. DPL 90 cotton was planted in the pots. Half the plants in each potassium treatment were supllied with adequate moisture and half were drought-stressed (leaves wilting during the heat of the day and recovering at night). The plants were divided into three tests to determine beet armyworm egg-laying preference, larval development in a growth chamber, and larval development in a greenhouse.

Screen cages were used to segregate groups of three plants, one plant from each of the three potassium treatments. Half the cages contained drough-stressed plants, and the other half contained plants receiving adequate moisture. Fifty male and 50 female moths were placed in each cage late in the afternoon. Egg masses deposited on the plants were counted early the next morning.

Potassium concentration did not effect the egg-laying preference of armyworm moths placed in cages with plants receiving adequate moisture (Figure 1). Potassium concentration did affect egg-laying in the drought-stressed treatment. Moths deposited 74% more egg masses on drought-stressed plants in the medium-potassium treatment than drought-stressed plants in the high-potassium treatment. Statistically, however, the number of egg masses per plant in the medium-potassium cotton was not different from the high-potassium cotton, at either moisture level.

Figure 1. On drought-stressed cotton , armyworm moths preferred to lay eggs on cotton reared in soil with a medium potassium level.

In the growth chamber portion of the study, larvae reared on high- and medium-potassium cotton treatments developed more rapidly than larvae reared on the low-potassium treatment (Figure 2). Armyworms in teh greenhouse study also pupated faster on these two treatments. In both the greenhouse and the growth chamber, larvae developed faster on drought-stressed cotton.

Figure 2. Larvae developed faster in cotton grown in medium- and high-K soil and in the drought-stressed treatment.

Results from this experiment indicate that armyworms develop faster on cotton grown in hot, dry conditions. Larvae developed equally well on high- and medium potassium cotton, but if plants were under drought stress, female moths preferred to deposit eggs on cotton grown in medium potassium soil. This indicates that fields testing medium in potassium could incur more damage during a hot, dry summer due to increased beet armyworm egg deposition and rapid development of larvae. However, if a farmer perceives this to be a problem, a simple preventative would be to apply potassium to all fields according to soil test recommendations. This would promote maximum cotton growth and possibly reduce attractiveness to beet armyworms.

Graham is a Laboratory Supervisor and Gaylor is an Associate Professor of Entomology.