Colorado potato beetle
Leptinotarsa decemlineata
Yellow-and-black striped beetles and reddish grubs that can defoliate potatoes and relatives. Hand-pick adults, crush egg clusters under leaves, and rotate crops.
How to know it
A rounded beetle about 10 mm long, straw-yellow with ten black stripes down the wing cases. The grubs are humped, brick-red to orange with two rows of black spots, and feed in gangs; orange egg clusters stand on the leaf undersides. A notifiable pest in the UK — report a sighting rather than dealing with it privately.
What you'll see
- Leaves eaten away to the midribs, working from the top of the plant down.
- Clusters of orange eggs, 20–40 at a time, on the leaf undersides.
- Fat red-orange humped grubs feeding in groups, with heavy dark droppings around them.
- Whole rows of potatoes defoliated within days at peak numbers.
- Striped adults on the stems and on the ground beneath the plants.
Where it comes from
Adults overwinter buried in the soil and emerge in spring as the crop comes up. Each female lays hundreds of eggs; the grubs feed for two to three weeks, pupate in the soil, and two to three generations follow in a warm summer. The beetle is notorious for developing resistance to insecticides, which is why hand control and rotation matter more.
Warm summers across mainland Europe and North America. Absent from the UK as a breeding population, where it is a notifiable quarantine pest — sightings must be reported to the plant health authority.
Dealing with it
- In the UK, report a suspected sighting to the Animal and Plant Health Agency and do not simply squash it.
- Elsewhere, hand-pick adults and grubs into soapy water daily — the most reliable garden control there is.
- Crush the orange egg clusters under the leaves before they hatch.
- Mulch deeply with straw, which genuinely impedes the emerging adults finding the crop.
- Rotate hard and grow early varieties that are lifted before the second generation builds.
Keeping it out
- Never plant potatoes in the same ground two years running.
- Plant early and lift early to dodge the worst generation.
- Cover young plants with insect mesh where the beetle is established locally.
- Encourage ground beetles and tachinid flies with permanent edges and mulch.
- Clear volunteer potatoes and nightshade weeds that carry the beetle between crops.
What it goes for
Potatoes above all, plus aubergines, tomatoes, peppers and other nightshades.
Who hunts it for you
- Assassin bugReduviidae A patient ambusher with a curved beak it drives into whatever it catches — caterpillars, beetles, bugs, and the occasional bee, so count it a generalist rather than a specialist. It will jab a careless finger too, so admire it and leave it be. Grow to recruit it: Dill,Fennel,Goldenrod,Sweet alyssum,Tansy,Yarrow
- Spined soldier bugPodisus maculiventris A predatory stink bug — same shield outline as the pest, but with a sharp spine at each shoulder and a beak it uses to spear caterpillars and beetle grubs. Learn the spines before you squash a shield bug, because this one is on your side. Grow to recruit it: Dill,Fennel,Sweet alyssum,Yarrow
- Tachinid flyTachinidae Bristly, house-fly-lookalike parasitoids that lay eggs on caterpillars, beetle grubs, and earwigs — the maggot consumes the host from within. Adults sip nectar from shallow umbels and daisies. Grow to recruit it: Cilantro,Dill,Feverfew,Parsley,Sweet alyssum
See also
The Cottage Garden Companion — the gardener's bestiary at cottagegardencompanion.com