Pest
Cabbage caterpillars
| Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec | |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Watch for | Watch for in May | Watch for in June | Watch for in July | Watch for in August | Watch for in September | Watch for in October |
What it is
Three caterpillars do most of the damage to brassicas in Britain and Ireland. The large white butterfly lays clusters of yellow eggs, and its hairy, yellow-and-black caterpillars feed in groups. The small white lays single eggs, and its velvety green caterpillars are well camouflaged along leaf veins. The cabbage moth produces fat green or brown caterpillars that bore into the heart of the plant.
Symptoms
Ragged holes appear in leaves, often starting on the outer ones, and plants may be reduced to skeletons of veins. Dark green droppings collect where leaves meet the stem. Small white and cabbage moth caterpillars tunnel into hearts and sprouts, fouling them with droppings. Slug damage looks similar but comes with slime trails; flea beetle leaves only tiny round pits.
Cause
Butterflies are on the wing from spring to autumn, with the main damage from July to September when the second generation is at its peak. Females are drawn to the mustard-oil scent of brassicas and nasturtiums. Caterpillars feed for a few weeks before pupating on fences, sheds and walls.
Control
Rub off egg clusters and pick off caterpillars by hand, dropping them in a bucket or onto a bird table. Wasps and parasitic flies take many larvae; caterpillars that turn sluggish and are surrounded by tiny yellow cocoons have been parasitised, so leave those alone. A biological spray based on Bacillus thuringiensis works against young caterpillars. Chemical sprays are rarely needed; if used, follow the label.
Prevention
Cover brassicas with insect-proof mesh or fine butterfly netting as soon as they are planted, keeping the netting off the leaves so butterflies cannot lay through it. Grow a few nasturtiums nearby as a decoy, and check them often. Pull up spent brassicas promptly to break the cycle.
Plants often affected
Could it be something else?
These problems share some of the same symptoms. Compare them, or run the symptom checker with these signs ticked.
Months to keep an eye out: May, June, July, August, September and October.
Sources
Identification and life-cycle details were checked against the references below. Text and advice are our own. See our editorial policy.