Why Are My Blackberries Full of Grubs?

Discovering a small white grub inside a ripe blackberry — usually when biting into it or washing the fruit — is the unmistakable sign of raspberry beetle (Byturus tomentosus) infestation. This small brown beetle is one of the most widespread fruit pests in the UK, and once its larvae are inside the fruits there is no treatment available. Control must occur before the eggs are laid — either by timing a protective insecticide application correctly or by disrupting the lifecycle in other ways.

The raspberry beetle lifecycle

Adult raspberry beetles emerge from the soil from late May onward and begin feeding on flowers and soft tissue of bramble family plants. The female lays eggs on flowers and developing fruitlets in June–July. The hatching larva bores into the centre of the fruit, feeding on the flesh and the developing seeds. By the time the fruit is ready to harvest, the larva may still be inside, or may have already exited into the soil to pupate. The grub is cream-white, 5–8 mm long, found near the centre or seed area of the fruit.

Control: correct timing

For blackberries, apply an approved insecticide (pyrethrum is available organically; thiacloprid is conventional) when the first flowers are at the pink bud stage but before they open. This timing kills adult beetles feeding on the plant before eggs are laid. Applying after flowers have opened risks killing bees and other pollinators. Check which pesticide formulations are currently approved for domestic use on blackberries as approvals can change.

Disrupting the soil cycle

Larvae that exit the fruit fall to the soil and overwinter as pupae. Cultivating the soil lightly under and around the blackberry plant in winter exposes pupae to birds and frost. Laying a mulch mat on the soil surface under the plant in late May prevents adults that are emerging from reaching the plant. Nematode treatments (Steinernema carpocapsae) watered into the soil in May target soil-stage larvae.