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Both the 1cm-long black adults and 1cm-long plump, white, legless, brown-headed larvae are a problem. The latter hide in the soil, and feed on roots (severing them) and bore into tubers from autumn to spring. Container plants are most at risk. The adults have six legs and a pair of antennae, moving in the open at night, chewing the edges of leaves. This isn't itself a problem, but the fact that each adult is a female, laying almost 1,000 eggs in summer, is alarming.
Solution
Organic
Go out at night with a torch and pick off the adults. To kill the larvae, in August/early September treat the compost with the predatory nematode Heterorhahditis megidis, which is sold by mail order.
Chemical
Few insecticides are very effective, but compost with the chemical imidacloprid controls the larvae for up to a year. Other formulations provide just short-term protection.
