The maggots are eating more raspberries than me
Many thanks for your enquiry regarding the problems you are experiencing with your raspberries. From the thorough description you have provided, it would appear that the Raspberry Beetle is the culprit.
The brown adult beetles lay eggs on the flowers from May through to the middle of July; these eggs hatch and the yellow-brown, brown-headed larvae begin feeding at the stalk end of the developing fruit before making their way inside – these are the maggots you discover in the fruit, and the distortion and discoloration of the fruit is a direct result of this pest. The larvae feed on the developing fruit for around a month before returning to the soil in late summer to pupate; adults emerge from the pupae in the autumn but remain in the soil until the following spring. As you, like ourselves, garden organically, the following control measures are acceptable.
Regular hoeing of the soil around the canes during the summer will unearth any pupae, which are much sought after by the local bird population; applying derris, either in spray or dust form, is the other control method and this must be applied as soon as flowering has finished and the first pink fruits appear. A further application 2 weeks later is also necessary as the key to controlling this pest is to kill the larvae before they tunnel into the fruit. Derris, although an organic insecticide with the active ingredient of rotenone, is harmful to bees so apply in the evening after the bees have finished work for the day. It may also be worth noting that the adult beetles often fly in from wild blackberry and raspberry canes. Hopefully this information is of help and please do not hesitate to contact us should you have any further questions or queries.
Many thanks once again for your enquiry and good luck with your future raspberry crops!