Why are there small maggots in my raspberries?

The adult beetle emerges in late spring, feeds on the flowers and lays eggs in them. The grub develops inside the fruit, then drops to the soil to pupate directly under the row. That tight local cycle is why cultural control works, and it is also why autumn-fruiting raspberries mostly avoid the problem — they flower after the beetle's egg-laying period has finished.

Step by step

1. Confirm the pest.A dried, browned area at the stalk end of the fruit, and inside, a small creamy-white to pale brown grub up to about 8mm. Often you find it in the bowl after picking rather than on the cane.
2. Distinguish it from other problems.Crumbly fruit that falls apart with no grub is poor pollination or virus. Grey furry mould is botrytis. Sap-sucking damage from capsid bugs distorts the fruit without a maggot.
3. Cultivate the soil under the row in winter.Lightly fork or hoe the top few centimetres beneath the canes to expose overwintering pupae to birds and frost. It is the main cultural control.
4. Or mulch heavily instead.A thick mulch, or a membrane laid under the row in early spring, hinders emerging adults. Choose one approach — cultivation or mulching — rather than doing neither.
5. Hang raspberry beetle traps from flowering.White sticky traps with a floral lure, one per few metres of row, put up as flowering starts. They reduce numbers and tell you the beetles are active.
6. Pick every few days and remove infested fruit.Picking promptly gets the fruit before the grub has done much damage, and removing affected berries takes grubs out of the cycle before they reach the soil.
7. Switch some of the row to autumn-fruiting varieties.They flower after the beetle's main egg-laying period and are usually clean. In a garden with a persistent problem this is the most effective single change.
8. Do not spray open flowers.The flowers are full of bees, and the pest is inside the fruit shortly afterwards. If any product is used at all, check its current approval for edible crops and follow the timing on the label exactly — after flowering, never during.
9. Deal with wild brambles nearby if you can.Raspberry beetle also uses blackberries and wild brambles, so a neglected thicket over the fence is a reservoir.

Tips

• Rinsing picked raspberries in lightly salted water for a few minutes floats the grubs out, which is the standard trick for a crop you are about to eat or freeze.
• The grubs are harmless if eaten and the fruit is perfectly usable — this is an unappetising problem rather than a food safety one.
• Loganberries, tayberries and blackberries get it too, so a mixed cane bed keeps the population going.
• Traps are most useful as monitoring. Combined with winter cultivation and picking discipline they make a real difference; alone they do not solve it.

Common mistakes

• Spraying at flowering — It kills pollinators, and the crop depends on them. The window for any treatment is after petal fall, and cultural control is more effective in a garden anyway.
• Leaving infested fruit on the ground — The grub simply completes its cycle in the soil under the row and comes back next year.
• Neither cultivating nor mulching under the row — That soil is where the entire overwintering population lives. Doing something to it in winter is the highest-value action available.

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