Colorado potato beetle

What you are looking at

A potato bed that looked fine a week ago and is now bare stems and skeletal leaves. The culprits are hard to miss once you know to look: a beetle about 10 mm long with cream and black stripes running the length of each wing case and an orange-red head dotted in black, alongside soft, humpbacked, orange-red grubs in two sizes clustered on the leaf undersides.

Look closer at an unstripped leaf nearby and there are likely clusters of 20 to 30 bright yellow-orange eggs standing on end on the underside, the next generation queued up.

Potato takes the worst of it, and aubergine and tomato foliage are both eaten too. Defoliation before the plant has flowered costs tuber yield directly, and a heavy attack can strip a bed twice in one summer.

Where this happens, and where it does not

This is a North American beetle first, native to the foothills east of the Rocky Mountains, and it only became a potato pest once European settlers brought the crop within reach in the nineteenth century. It reached France in 1922 and has since spread to predominate across potato-growing regions in 21 European Union member states and through much of temperate Asia.

Britain sits inside that European range geographically and outside it in practice. DEFRA treats it as a notifiable quarantine pest, it has been found and eradicated more than 160 times since it first turned up, and the last established outbreak was in 1976. A beetle matching this description on a UK allotment is a call to the plant health authority, not a pest to hand-pick, because a confirmed finding triggers containment measures well beyond one garden.

Australia and New Zealand have never had it and both run active biosecurity programmes to keep it that way. Anyone in the Southern Hemisphere seeing a similarly striped beetle on potatoes should assume a lookalike and report it for confirmation rather than treat it as this species by default.

The year it has where it is established

Adults overwinter in the soil of the previous year’s potato bed and emerge in spring once the ground warms, typically as the first shoots appear. They mate, and each female lays several hundred eggs over the following weeks in the bright clusters described above.

Larvae pass through four stages over two to three weeks, growing from tiny dark grubs to the fat orange-red fourth instar that does most of the damage; a single adult can strip about 10 square centimetres of foliage a day, and a fourth-instar larva eats faster than that. They pupate in the soil for five to ten days, and a first generation of adults is on the wing by midsummer. A second generation follows by late summer in most years, feeding hard before entering the soil to overwinter without breeding again that year.

What to do about it

Check leaf undersides from the moment shoots break the surface, and pick off every adult, grub and egg cluster you find into a jar of soapy water. This alone controls the pest on a garden scale almost everywhere it occurs.

Rotate potatoes, aubergines and tomatoes as far from last year’s bed as the plot allows, since the overwintering adults walk into a new planting rather than searching far for one.

On a bed too large to check by hand daily, a plastic-lined trench around the perimeter intercepts migrating adults, and insect mesh laid over young plants excludes them until flowering.

Spray Bacillus thuringiensis subsp. tenebrionis on young larvae where it is registered, timed to the first instars rather than the fat fourth-instar grubs that are already past the point where it works well.

Cut down the haulm once the tubers have sized up to deny the second generation its food before it goes into the ground for winter.

What it gets mistaken for

Capsid bugs leave small ragged holes that expand as the leaf grows, never a plant stripped to stems within a fortnight.

Leaf miners tunnel within the leaf tissue and leave the surfaces intact, a pale winding trail rather than an eaten edge working inward.

Flea beetles put many small round holes across a leaf rather than removing it entirely, and their damage stays cosmetic on an established plant rather than skeletonising it.