The Cross-Striped Cabbage Worm is a pest of brassicas, especially cabbage, broccoli, and brussels sprouts. We do see season-to-season and year-to-year variations in the total population size, as well as the population ratio between these and other cabbage worms (such as the Imported Cabbageworm). More often than not, the Cross-Striped Cabbage Worms are the most prolific here.
These caterpillars can quickly defoliate brassica plants, such as the Brussels Sprout plant shown below.
We use several strategies to try to keep damage within acceptable levels. We’ve had some success using shade cloth on hoops over fall cabbage beds to exclude adult moths and prevent them from laying eggs on the plants. Trap crops of collard plants, which are a favored food, can sometimes help, if we keep on top of controlling the populations on the collards plants. When preventative methods fail, we’ll resort to “digital control”, that is, using our fingers (digits) to squish the caterpillars. Repeated periodically with attention to life cycle timing, this can be a relatively effective method (though one that is unpopular among workers).
Though it is allowed for organic production, we have never resorted to spraying Bt (a bacterium that kills cabbage worms). We are even less likely to do so since discovering in 2013 that we also have a parasitoid wasp of the genus Cotesia that eats out the innards of some of the cabbage worm species. As usual, any pesticide, no matter how benign and targeted, has unintended ecosystem consequences, and we prefer methods that keep the beneficials (such as parasitoid wasps) working for us.