Location of Beet Armyworm (Lepidoptera: Noctuidae) Egg Mass Deposition within Canopies of Cotton and Pigweed

Abstract

The beet armyworm, Spodoptera exigua (Hübner), is a sporadic but devastating secondary pest of cotton. Scouting cotton for egg masses is commonly recommended for identifying potential outbreaks and for proper timing of insecticide applications. However, there is disagreement regarding where to look on the plant for beet armyworm eggs. We investigated and quantified placement of egg masses by laboratory colony females within cotton and pigweed (a preferred wild host plant) canopies of different heights. In cotton, almost all egg masses were deposited on the undersides of leaves, and ≈80% of the egg masses were consistently located in the upper 50% of the cotton canopy, and horizontally within the inner 50% of the canopy around the central axis. Although this trend was consistent among all categories of plant height tested, variation about the means decreased with increasing height. A smaller sample of wild females indicated similar vertical placement, but horizontal placement extending further distally in the canopy. Our results indicate that scouting for egg masses on the underside of leaves in the upper half of the canopy will recover ≈80% of the egg masses present on the plants. In pigweed, egg masses were commonly laid within the inner 50% of the canopy, but along the upper 80% of the vertical axis. As in cotton, variation about the means was less in taller plants. The number of eggs per egg mass was 29% less in cotton than in pigweed. This positional information will aid in further efforts to investigate, predict, and manage beet armyworm populations in cotton and noncotton hosts.