Raspberry beetle
Byturus tomentosus
Its grubs feed at the plug end of summer raspberries and blackberries, drying the fruit. Cultivate lightly around canes in winter to expose pupae; autumn varieties largely escape.
How to know it
A dull, hairy, grayish-brown beetle 4 mm long, found in the open flowers in early summer. What most people meet is the small creamy-white grub with a brown head, tucked into the plug end of a ripe raspberry.
What you'll see
- Fruit dried, browned and shrivelled at the stalk end while the rest looks ripe.
- A small white grub in the hollow plug when the berry is picked.
- Berries that crumble apart rather than pulling cleanly off the plug.
- Dull brown beetles sitting in the open flowers in May and June.
- Summer raspberries badly affected while the autumn crop is largely clean.
Where it comes from
Adults emerge in late spring and feed on flowers, laying eggs in the blossom. Grubs feed on the developing fruit for weeks, then drop to the soil in midsummer to pupate, overwintering as adults or pupae a few centimeters down beneath the canes. One generation a year, and its timing is exactly why autumn raspberries escape.
Warm dry weather during flowering, in established beds where the soil beneath the canes is never disturbed. Damage is far worse in summer-fruiting varieties than autumn ones.
Dealing with it
- Lightly fork or hoe the soil beneath the canes in winter to expose the pupae to birds and frost.
- Hang white sticky traps among the canes at flowering to monitor and reduce the beetles.
- Pick and destroy infested fruit rather than dropping it under the canes.
- Shake canes over a sheet at flowering time to collect the adults.
- Grow autumn-fruiting varieties, which flower after the beetle’s egg-laying is over — the single most effective answer.
Keeping it out
- Switch the balance of the bed toward autumn raspberries where damage is chronic.
- Keep the ground beneath the canes cultivated and free of thick permanent mulch.
- Do not compost damaged fruit near the fruit cage.
- Encourage ground beetles and birds with open ground and cover nearby.
- Prune out and burn old canes promptly after fruiting.
What it goes for
Summer raspberries above all; also blackberries, loganberries and tayberries.
See also
The Cottage Garden Companion — the gardener's bestiary at cottagegardencompanion.com