August 2022
- Something is going on with my raspberries. For 2 seasons they produce berries but then seem to die back. I dug around the roots and found what looks like grubs under the soil. What could be going on?
The culprit is most likely the raspberry crown borer (Pennisetia marginata). This is a borer that affects raspberries, blackberries and their hybrids, and wild bramble fruits such as salmon berries.
The noticeable damage is to canes that appear wilted or brittle and pull off easily from the crown, especially on older plantings.
During the life cycle the moths lay eggs from late July to early October on leaf margins. The hatched larvae crawl down to the soil to overwinter beside the crown or crevice in bark. In spring they feed on roots at the base of the canes, burrowing into the crown over the summer. The larvae spend a second winter in the roots. In spring they continue to feed on crowns and sometimes canes. They mature about midsummer then pupate in cocoons at the base of hollowed out canes for 2-3 weeks, with adults emerging in late summer.
To control the borers, keep bramble fruit growing vigorously so there are many replacement canes to choose from at pruning time. Look for weak and wilted canes and brittle canes that break off easily at the crown. At pruning time watch for signs of infestation. Prune canes as close to the ground as possible. If a borer hole is visible at the base of a cane and there is sawdust spilling from the hole, the larvae may still be inside. Sometimes the larvae can be killed by poking a wire into the hole.
If older beds are badly infested dig up the entire planting after harvest or in October. Destroy the crowns and roots and dispose of the material (do not compost). Replant a new bed in the spring in a different location with new stock.
Insecticides are not effective as larvae are well protected inside the crowns.
Use Pyrethrum sprays, natural pest control products, in early April or late October to intercept first year larvae before they tunnel to the crown. This may have variable success.
Chickens who scratch under the canes after fruit has been harvested from fall through late spring may be able to control borers. Studies have found applications of parasitic nematodes (Steinernema feltiae and others) to the crowns can control from 1/3 to 1/2 of the borers in a planting.
Sources:
https://www2.gov.bc.ca/gov/content/industry/agriservice-bc/production-guides/berries/raspberries?keyword=raspberry&keyword=crown&keyword=borer
West Coast Gardening Natural Insect, Weed and Disease Control by Linda A.
Gilkeson, Ph.D Second edition 2013
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