Expert recommends burying black-headed slug eggs 30 centimetres deep

Black-headed slug eggs can survive winter cold, so gardeners should bury them in a hole about 30 centimetres deep, plant protection specialist Eha Kruus told ERR’s Vikerraadio programme Ökoskoop.

The slugs are still feeding because temperatures have not yet fallen enough to force them to overwinter, Kruus said. They usually spend the winter as eggs, which resist cold because they contain little water.

Black-headed slugs generally damage plants only when their numbers are high and natural food is scarce. They are also decomposers, but as an invasive species they compete with native species, Kruus said. The slugs are less serious pests than Spanish slugs, though their ability to reproduce in large numbers can make them a nuisance.

Wet weather suits the slugs. Kruus said they were seen in particularly large numbers in the second half of July, as well as in August and September.

Finding their eggs and young slugs in a garden can be difficult, but hand-picking larger slugs is effective, she said. Each slug removed is prevented from reproducing. They can lay up to 400 eggs, usually in clusters of about 20. In autumn, gardeners should look for small, translucent white eggs among plant remains and in the top layer of soil.

Kruus also recommended turning over compost once or twice while it is not yet frozen, especially after the first frosts. Fallen leaves provide food for the slugs, she said. Burying plant remains and any eggs in a 30-centimetre-deep hole may prevent the eggs from emerging.

The first black-headed slug was found in Estonia in 2014. Kruus said the species is native to the Caucasus and may have spread with plants, although this is not certain. She suggested that hot, dry summers may help explain why Spanish slugs have not become as big a problem elsewhere in Europe, while Estonia’s damp conditions suit them.