Cabbage Worms

Cotesia glomerata and Cotesia rubecula are parasitic wasps that specifically attack imported cabbageworm larvae -- they are present in most gardens and frequently collapse outbreaks without any intervention. If you see a cabbageworm covered in small yellow or white cocoons, leave it -- those are parasitoid pupae and the worm is already dying. Killing it destroys the next generation of your best allies. Trichogramma egg parasitoids attack eggs before they hatch. Plant dill, fennel, and sweet alyssum to sustain these beneficial wasps year-round. Birds -- especially house sparrows -- actively feed cabbageworms to nestlings.

Cabbage worms are the larvae of the white cabbage butterfly -- the small white butterfly you see fluttering around brassica beds is laying single pale yellow eggs on leaf undersides. Check leaf undersides weekly and crush eggs before they hatch. Row covers installed before transplanting completely exclude butterflies from reaching plants -- the most reliable and low-maintenance prevention. One adult butterfly can lay hundreds of eggs across a season -- early exclusion pays dividends throughout the season.

Rotate brassica beds each year to reduce the local butterfly population that has learned your garden location. Red or purple-leaved brassica varieties are slightly less attractive to egg-laying females. Remove crop residues promptly after harvest. Interplant with dill, thyme, and sage at bed edges -- aromatic herbs confuse butterfly navigation and support parasitoid wasp populations. Proper spacing for air circulation reduces the humid conditions that allow larvae to feed undetected.

Floating row covers over transplants exclude adult butterflies completely -- remove at heading or leave on throughout since brassicas do not need pollination. Hand-pick larvae and egg masses on every scouting pass -- larvae are bright green and easy to spot against leaves. Check the dense inner leaves of forming heads where larvae hide. Insect netting at 0.8mm mesh is fine enough to exclude the small cabbage white butterfly while maintaining airflow.

Bt kurstaki while larvae are small (under half-inch) is the gold standard -- it kills caterpillars that eat treated foliage and is completely harmless to everything else including parasitoid wasps. Apply in the evening coating all leaf surfaces. Reapply every 5-7 days and after rain. Spinosad for larger larvae where Bt timing was missed -- apply at dusk. Neem oil deters egg-laying when applied consistently to foliage.