What's Eating Your Raspberries, Besides You?
There are a number of insects that commonly infest raspberry canes and fruits and can dramatically reduce the berry harvest. The most injurious insects are those that bore within the canes resulting in cane dieback and even death. The most common of the borers attacking canes in northern Utah is the raspberry horntail.
The raspberry horntail is a wood-boring wasp that 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), malathion, and permethrin 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.
Other insect pests include the stink bug, earwig, lygus bug, and grasshoppers that may enjoy the ripening berry fruits before you can get them harvested. These insects typically suck or chew into the individual drupelets. Frequent inspections of ripening fruits and physically shaking the canes can remove these insects, but the effect is often short-lived. Spraying with an insecticide to deter fruit-feeding insects can be effective, but you must carefully observe the preharvest or required time interval between application and picking fruits. Recommended insecticides that have low toxicity to humans include neem oil (Azatin) and spinosad (Success, Entrust). Conventional insecticides that will deter fruit-feeders include carbaryl (Sevin), malathion, permethrin, and esfenvalerate.
-Diane Alston, Extension Entomologist