Beet armyworms (BAW) feeding on escaped pigweeds have been reported from some areas of south Georgia. Hot and dry conditions like we have experienced in recent weeks is favorable for BAW reproduction. We last observed BAWs feeding on pigweed to any degree in 2009. As a whole growers are managing pigweed much better today than in 2009 so there is less opportunity for BAW to get established on pigweed and potentially move to cotton in most fields.
When pigweed is defoliated by BAW or killed by herbicide, plowing or pulling, large BAW larvae may move to cotton and potentially cause injury. Large larvae which move to Bt cottons from pigweed are more tolerant to the Bt toxin compared with just hatched larvae and may not be controlled. Potential damage to cotton from BAW includes foliage, square, and bloom feeding; larvae may also bore into the main stem a few nodes below the terminal causing the terminal to abort.
We have observed egg masses being laid on Bt cottons and some larval survival has been noted. If BAW larvae exceed ¼ inch in length, there chance of survival is greatly increased compared with newly hatched larvae. So scouts should be observant for BAW in all fields. Use good IPM practices, conserve beneficial insects by only applying insecticides for other pests when threshold levels are exceeded.
Our current threshold for BAW is 10 active “hits” or hatchouts per 300 row feet. In situations where BAW are moving from pigweed to cotton, this threshold does not work well; i.e. we cannot count “hits”. Thus we also recommend treatment when 10 percent of squares are damaged or when 10 percent of blooms are infested with larvae. Treatment should also be considered in 10 percent damaged terminals are observed. BAW may potentially feed on bolls, but most feeding we have observed has been in/on squares and blooms. The amount of foliage feeding should also be considered in addition to square and bloom damage.