Cucumber beetle
Acalymma & Diabrotica spp.
also called Striped cucumber beetle, Spotted cucumber beetle, Squash bug
Striped or spotted beetles that chew cucurbit leaves and blossoms and spread bacterial wilt. Cover young plants with row cover until flowering, then remove for pollinators.
How to know it
Slim yellow beetles about 6 mm long, either with three black stripes down the back or with twelve black spots. Both hide inside squash flowers and scatter when a leaf is turned; the larvae are thin white grubs feeding on the roots below.
What you'll see
- Ragged holes in cucurbit leaves and shredded flowers, with beetles inside the blooms.
- Seedlings grazed and checked as soon as they emerge.
- A plant that wilts suddenly and completely despite moist soil — bacterial wilt, carried by the beetle.
- Scarred, pitted skins on maturing squash and cucumbers.
- Slim white grubs on the roots when a failing plant is lifted.
Where it comes from
Adults overwinter in garden debris and hedgerows and fly to the first cucurbits of the season. Eggs are laid at the stem base, the grubs feed on the roots for weeks, and one to three generations follow. The beetle carries Erwinia bacteria in its gut over winter — which is why it matters far beyond the leaf damage it does.
Warm summers, particularly in North America; worst on young plants and on beds where cucurbits grew last year. Any leaf damage on a seedling risks wilt infection, so the numbers that matter are small.
Dealing with it
- Cover young plants with row cover from planting, and remove it at flowering so bees can work the blooms.
- Hand-pick morning and evening, checking inside every open flower.
- Pull and destroy any plant that wilts suddenly — it is a bacterial reservoir for the rest of the bed.
- Use yellow sticky traps at plant height to monitor and thin the adults.
- Mulch with straw, which slows the beetles finding stems and helps the roots through the damage.
Keeping it out
- Rotate cucurbits and clear all debris at the end of the season.
- Sow or plant late, after the first flush of overwintered adults has passed.
- Grow resistant or tolerant varieties, and grow a few extra plants to allow for losses.
- Plant a trap crop of blue hubbard squash at the edge of the bed.
- Encourage soldier beetles, tachinid flies and ground beetles with flowers and permanent edges.
What it goes for
Cucumbers, courgettes, squash, pumpkins and melons; also gourds such as cucuzza, beans, sweetcorn and, for the spotted kind, many other crops.
Trouble it brings with it
Who hunts it for you
- Soldier beetleCantharidae Soft-winged beetles seen mating on flower heads in high summer; adults and larvae both take aphids and soft grubs. Open composite flowers are their landing pads and hunting grounds. Grow to recruit it: Echinacea,Rudbeckia,Sunflower,Yarrow,Zinnia
- SpidersAraneae Webs, ambushes, and night patrols make spiders the garden’s steadiest generalist predators, taking whatever flies or crawls past. Mulch, ground cover, and undisturbed corners keep the population dense. Grow to recruit it: Lavender,Thyme
See also
The Cottage Garden Companion — the gardener's bestiary at cottagegardencompanion.com