Both beet armyworm (Figs. 45 and 46) and yellow-striped armyworm moths lay eggs on leaf surfaces in masses covered by a whitish, velvety material. Young beet armyworms hatch, “web up,” and feed together on leaves. The damaged leaf or leaves (Fig. 47), called hits, turn tan and are distinctive and easily seen when walking through fields)a quick way to determine if the field has a beet armyworm infestation. After a few days, small beet armyworms disperse and become solitary in their feeding habits.
In early-season infestations, the larvae feed on leaves and terminal areas (Fig. 48), skeletonizing leaves rather than chewing large holes in them. Occasionally, they destroy the terminal, causing extensive lateral branch development and delayed maturity. Early-season insecticide applications may be warranted when plants with undamaged terminals approach lower optimal plant stand limits.
Damaging infestations sometimes develop late in the season. These infestations are more prone to feed on terminals, squares, blooms, and bolls. Several factors can contribute to beet armyworm outbreaks:
- Mild winters (the absence of prolonged freezing temperatures)
- Late planting
- Delayed crop maturity
- Heavy early-season, broad-spectrum insecticide use
- Continued hot, dry weather conditions
- Prebloom presence of beet armyworms
- Conducive weather conditions for long-distance migration
Additional characteristics of high-risk fields are:
- Sandy and droughty fields
- Skip-row planting
- Fields with skippy, open canopies
- Drought-stressed plants and fields infested with pigweed
The likelihood of a substantial outbreak increases as more of these factors occur in a given location. Control may be justified when beet armyworms begin to damage the fruit, especially bolls. Beet army- worms longer than ½ inch may be difficult to control, so early detec- tion of populations helps improve control.
Scouting and Decision Making
Conduct general observational scouting to detect armyworm hits on a 5- to 7-day schedule throughout the growing season. Because infestations within a field may be spotty, scout to determine the need for and the area of the field requiring control. It is not uncommon for only a portion of a field to require treatment.
Once you detect armyworm hits, conduct more extensive scouting. Using the whole plant inspection method described on page 13, thor- oughly inspect 10 to 20 plants from three to four locations throughout the field. Carefully scout weedy areas of the field, especially those infested with pigweed, since beet armyworms are strongly attracted to pigweed and may move from pigweed onto cotton. Migration of the worms from pigweeds to cotton is more of a problem in fields treated with a herbicide to kill the pigweed. Bt cotton varieties usually control beet armyworms effectively.
Table 7. Beet armyworm action threshold
|>10% infested plants with 15 larvae/100 plants|