Raspberry horntail—Hartigia cressoni
This sawfly (Cephidae) mines inside the terminals of caneberries (e.g., blackberries and raspberries) and roses, causing shoot tips to wilt and die. This insect differs from the wood-boring horntails (Siricidae), although both groups are wasps (Hymenoptera).
Identification
Cane terminals that wilt, then discolor and die are the obvious symptom of this pest's presence. Shoot dieback can also be caused by inappropriate irrigation, poor soil drainage and aeration deficit, root-decay pathogens, and certain other disorders and pests. It is important to correctly distinguish the cause because some maladies causing look-alike damage can kill affected plants.
To determine whether raspberry horntail is present, examine stem terminals especially where the second and third leaves from the tip attach to the cane. This is where female wasps insert their eggs and cause a small, circular discoloration of the cane. Canes may also become slightly swollen when a larva has fed inside. Cut open the affected area to expose any tunnel with brown, granular frass (excrement) and sometimes an immature raspberry horntail or its cast skin.
Larvae are cream-colored to white and cylindrical with distinct segments. The head is pale brown and at the rear end is a short spine. Larvae have three pairs of true legs immediately behind the head and grow up to 9/10 inch long. Raspberry horntail larvae have no prolegs (leglike appendages on the abdomen), unlike the six or more pairs of prolegs of sawfly larvae that feed openly on plants.
Adults are broad-waist wasps, 1/2 to 3/4 inch long with elongate, cylindrical bodies. The females are mostly yellow and orangish with black. Males are mostly black with some yellow and orange.
Eggs are 1/16 inch long, curved, oval, pearly white, and somewhat flattened with a point at one end. Several eggs can be laid in one stem, but only one larva per terminal survives.
Life cycle
Adults emerge and mate beginning in March and the females lay eggs that hatch within several days. The young larva feeds spirally just beneath the surface, girdling the terminal and causing it to wilt. The larva then chews and lengthens a tunnel in the cane as it feeds and develops through four or five, increasingly larger instars (immature stages) during spring and summer. The mature larva chews downward into cane pith and forms a silk-lined cell where it overwinters as a prepupa. In late winter to early spring, it pupates into an adult and chews a round, emergence hole in the cane.
There is one generation per year in most locations. In some locations there may be two generations per year.
Damage
Infested terminals wilt and discolor during the spring, then die back by late summer and sometimes break if not pruned off. Infestation may reduce the abundance of blossoms and fruit. Raspberry horntail does not seriously threaten the long-term health or survival of infested plants.
Solutions
Inspect canes regularly during spring and summer and prune off and dispose of wilted terminals. Make the cuts below wilted leaves and any stem swelling and discolored, egg-laying wound in canes. Pruning off damaged shoots greatly reduces the subsequent abundance of raspberry horntails and their damage, and within 1 or 2 years may eliminate an infestation.
Insecticide sprays are not recommended. See The Cephid Stem Borers of California for more information.
Adapted from Pests of Landscape Trees and Shrubs: An Integrated Pest Management Guide, University of California Statewide Integrated Pest Management Program (UC IPM).
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