Harlequin bug

What you are looking at

Pale, irregular blotches shaped roughly like a four-leaf clover spreading across cabbage, kale or broccoli leaves, where the bug has drained the tissue rather than chewed it, expanding into large dead patches on a heavy attack. Young seedlings take the worst of it and can wilt, blacken or die outright, since they have so little leaf tissue to lose before the damage becomes fatal.

The insect responsible is hard to mistake for anything else: a shield-shaped bug 9 to 12 mm long, boldly patterned in black, red, orange and yellow, with wingless nymphs carrying similarly bold colouring and feeding in groups alongside the adults. Eggs are laid in double rows on the leaf underside, barrel-shaped and banded in black and white, and are conspicuous once you know what to look for.

An American specialist

This bug feeds almost exclusively on plants in the brassica family and turns to anything else only when none is available. Native to Mexico and Central America, it has spread across most of the southern and central United States, where mild winters let adults survive the cold season and build up steadily from spring onward. It does not reliably overwinter north of roughly the latitude running through Philadelphia and northern California, so gardens further north see it as a seasonal visitor riding in on wind currents rather than a permanent resident, and pressure there varies considerably from year to year.

It has not established outside the Americas, so a similarly bold black, red and yellow shield bug found on brassicas elsewhere belongs to a different, unrelated species.

What to do about it

Check plants two or three times a week through the season and hand-pick adults, nymphs and egg clusters into soapy water; the bold colouring makes this more practical than it sounds.

Grow a border of mustard or rapeseed as a trap crop, since harlequin bugs consistently prefer it over the vegetable being harvested, and deal with the trap crop directly once it is carrying a useful share of the population.

Cover young transplants with row cover through the vulnerable seedling stage.

Clear brassica debris and weedy mustard relatives from the bed at the end of the season, in areas where the bug overwinters locally.

Choose fast-maturing varieties or plant early where seasonal pressure builds predictably through summer.

What it gets mistaken for

Capsid bugs are smaller, plain green and leave small ragged holes rather than the pale, cloverleaf-shaped blotches this species produces by draining tissue.

Cabbage root fly damage starts below ground with no insect visible on the leaves, the opposite pattern from harlequin bug, whose conspicuous adults and nymphs are the first thing most gardeners notice.