One of the most damaging insects to raspberries is the raspberry cane borer (Oberea affinis). You rarely see the insect itself, but the damage it causes is very visible: the cane wilts and bends downward around 6 inches (15 cm) from the top and eventually the leaves affected dry up and turn brown. You’ll probably start to notice damage at the end of June or in early to mid-July, but they can continue into August.
The raspberry cane borer attacks raspberries (Rubus idaeus), plus blackberries and brambles (various other Rubus species). Sometimes it is also found on roses. The insect is native to North America and found all across that continent.
If you look carefully, just below the wilted part, you’ll notice two “rings” around the stem about ½ inch (1 cm) apart: two series of small holes pierced by the adult female. They are said to look like zippers. The female lays one egg per stem, between the two rings. The white maggotlike larva digs into the stem as it feeds, hollowing it out, and slowly moves downward, overwintering about 2 inches (5 cm) below the lower ring. It then spends a second summer in the stem, this time tunneling more deeply, overwintering near the base of the stem the second year. The following summer, the adult emerges and starts a new infestation.
Because of its 2 year life cycle, you’ll usually see a great deal of cane borer damage one year, but very little the following one, then the problem returns the third year.
The Rarely Seen Adult
The adult raspberry cane borer is a very slender beetle about 12 mm long, dark brown or black with an orange mark behind its head and very long antennae… but few gardeners will ever see one (certainly I’ve never managed to photograph one): they work very discretely.
What to Do
It’s almost impossible to control raspberry cane borers with insecticide treatments, because the adult is present only for a few minutes per plant, the time it takes to pierce its characteristic rings and lay an egg, and then it flies to other plants. However, you could try spraying your raspberries with an insecticide offering some persistence, like neem or pyrethrum, when plant is in bud and also after the flowers have faded. (Never apply an insecticide while the plant is in flower to avoid harming pollinating insects).
But the only really effective treatment is cut the off the wilted stem 6 inches (15 cm) below the rings in June and July. (And yes, you can put them in your compost.) This way you eliminate not only the current season’s larvae, but also seriously reduce the infestation that would have occurred two years later.
It may also be helpful to remove wild raspberries, blackberries, and other brambles from the vicinity of your raspberry or blackberry patch, as they are a major host of future cane borer infestations.
So raspberries and blackberry fans: get out your pruning shears. You have a little chore to take care of!