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What's eating your raspberries besides you?
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If you've noticed dieback or wilting tips of raspberry canes, insects may be infesting your raspberry bed. There are three main insects that attack raspberry canes in Utah. Currently, the most prevalent insect infesting raspberries in northern Utah is the raspberry horntail. The raspberry horntail is a wood-boring wasp. It will attack raspberry, blackberry, other related brambles, and roses. Injury is usually confined to first-year, vegetative canes. Horntail larvae (immature stage) are white, cylindrical with dark brown heads and a pointy tail with a spine. In northern Utah, wilting cane tips become noticeable in June and July as larvae bore through the center pith, which becomes soft. It's easy to verify the insect's presence by cutting open wilted canes to check for larvae inside.
Prune and destroy infested canes when wilting becomes apparent; this will remove the larvae and reduce the population. A parasitic wasp attacks horntail larvae, and helps reduce populations, but not before some injury has occurred. The smaller parasitic larvae can be seen crawling on horntail larvae. To control horntail eggs and young larvae in the spring, treat canes with a full cover spray of insecticide when new growth begins. Carbaryl (Sevin), diazinon, malathion and rotenone are effective insecticides. A repeat application can be made 7-14 days later if populations are high. Do not treat with insecticides just before or during bloom to avoid harming pollinators.
Two other insects that attack raspberry canes are the rose stem girdler, a flat-headed beetle, and the raspberry crown borer, a clear-winged moth. Stem girdler larvae form two to five spiral grooves in the cambium (just under the bark), girdling the canes and causing wilt and death. First year canes are attacked more than fruiting canes. Girdling in first year canes produces a gall-like swelling. Larvae are white, slightly flattened and have two short, brown, toothed projections on the tail end.
Raspberry crown borers have a two-year life cycle. The first indication of injury is wilting and dying of foliage on first-year canes in April through June. Infested cane tips may curl into a shepherd's crook. Damaged canes become spindly, may break at ground level, and may be predisposed to winter injury. Larvae over winter in the crown and tunnel upward the second year. Adult moths emerge in summer to fall, leaving pupal skins attached to emergence holes in canes.
Pruning is helpful to reduce infestations of all three raspberry insects. Prune canes below the insect and destroy (burn, bury at least 2 inches deep, or dispose in landfill). Remove entire cane if infested with crown borer. If infestation is substantial, pruning should be supplemented with chemical control. The insecticides listed above are effective for all species. Timing for cane girdler is the same as for the horntail. For the crown borer, first-year larvae can be killed in the fall (mid-October) as they crawl down canes to overwinter in crowns or the following spring when they become active (April to May). Apply a full cane spray and drench to the base of plants allowing the insecticide to soak into the root zone. Treatments must be applied for at least two or more consecutive years for successful control of crown borer.
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