Raspberry Horntails

When I first saw the name raspberry horntail, my mind conjured up a ripe, red raspberry with a ram’s horn tail. Of course, raspberry horntails are nothing like that, but it was a fun image. The truth about these parasitic wasps is less fun.

Adult male raspberry Horntail (Whitney Cranshaw, CSU) CC BY-SA 3.0

Raspberry horntails are a type of sawfly. Sawflies get their name from the saw-like ovipositor used by females to insert eggs under a plant’s skin. These pests prefer raspberries, but they will also feed on blackberries, other bramble fruits, and roses.

​You may run into some confusion when talking with others about raspberry horntails (Phylloecus (formerly Hartigia) cressonii) because there are raspberry cane borers (Oberea bimaculate), which are frequently called raspberry horntails or raspberry cane girdlers. Or your friend may be talking about raspberry sawflies (Monophadnoides rubi). Yes, it can be confusing sorting all this out sometimes, but photos can help. [Unfortunately, I could not find an available image of a female raspberry horntail.]

Raspberry sawfly (James Lindsey, Ecology of Commanster) CC BY-SA 3.0

Raspberry cane borer (Judy Gallagher) CC BY 2.0

Let’s start with symptoms.
 
As new canes form, typically in spring, raspberry horntails lay their eggs under the surface. If you look closely, you may see small, round, discolored areas where the eggs were laid. If you can get to one, raspberry horntail eggs do not look like what you might expect. They are white, oblong, and they have a flattened, curved point at one end.
 
When those eggs hatch, young larvae spiral upward inside the canes where they feed just below the cambium layer. This causes cane tip wilting. It also weakens the plant and can cause dieback. Infested canes may also twist or become crooked. Infested canes are more susceptible to breaking, frost damage, and other pests and diseases.
 
As larvae grow, they turn around and head lower in the cane, feeding on both the inner cambium layer and the pith. This significantly reduces flower and fruit production. Eventually, the larvae create silk-lined apartments in which they overwinter. As temperatures rise in spring, larvae pupate and then emerge as adults. But it is the larvae who cause the damage.
 
Raspberry horntail larvae are white and can be up to one inch long. They have hard brown heads and a tiny spike on the other end. Pupae are slightly smaller and tan. Adults are only half the size of their fully grown larvae, with narrow bodies and long antennae. Males tend to be black with orange and yellow markings, while adult females are gold to orange with black spots.

Rose sawfly larvae are very similar to raspberry horntail larvae (James Solomon, USDAFS) CC BY 2.0 us

If you suspect a raspberry horntail problem, prune about six inches below the cane tip and slice the removed bit in half lengthwise. If it looks like the infestation extends lower than what you have in your hand, remove more of the cane.
 
These pests are not generally a serious problem, but it pays to regularly inspect your raspberry plants. Bag and dispose of any infested material in the trash. Do not add it to your compost pile.
 
Unless you can time applications to coincide perfectly with egg-laying in your area, insecticides are not recommended, since larvae and pupae are protected inside the cane, and adults simply fly away.

Keeping your raspberry plants healthy is always your first line of defense.