By Edward Beasley
A field has been found in Berrien Co where armyworm pressure is substantial on Bt cotton. This is not a common situation, but the weather gives this pest optimal performance. Generally this pest is transferred from pigweed which is found prevalent in most fields. Here is some words from Dr. Phillip Roberts on the matter:
We have observed multiple BAW( beet army worm) egg masses on a single pigweed plant; that is a lot of larvae which may move to cotton. When large larvae move to Bt cottons, they are more tolerant to the Bt toxin compared with just hatched larvae and may not be controlled. 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”.
We need a more workable threshold for these situations. In Bt cottons we have observed BAW larvae feeding on squares, blooms, and foliage. As a starting point in fields where BAW are infesting pigweed, treatment should be considered if 10 percent damaged squares or 10 percent BAW infested or damaged open blooms are found. 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.
Source:uga.edu