Red-necked Cane Borer in Blackberries
Bruce Bordelon, Professor, Department of Horticulture and Landscape Architecture, Purdue University
There are only a few insect pests that attack blackberries and raspberries. The red-necked cane borer is one of the more common. The Red-necked cane borer (Agrilus ruficollis) and its relative the Bronze cane borer (Agrilus rubicola) may attack all brambles, but are most common on blackberries.
Damage: Infested canes develop galls at the site where larvae are tunneling. The galls are symmetrical swellings, usually 1 to 3 inches in length, usually within a foot of the soil surface (Picture 1). The galls are usually visible on primocanes in late July or August, but are seldom noticed until leaves drop during dormancy. Canes may die overwinter above the galls, or show delayed bud break and shoot growth the following spring (Pictures 2 and 3). In healthy vigorous plants, the galls may only slightly reduce yield and berry size, but in many cases, the canes above the galls fail to produce a crop. Severity of galling (percentage of canes with galls) will dictate potential yield loss.
Appearance: The adult insect is a slender, metallic black beetle, about ¼ inch long, with short antennae and a reddish or coppery thorax or neck (thus the name “red-necked”). Adult beetles are seldom seen. The larvae are white, legless flat-headed grubs about ¾ inch when fully grown.
Life Cycle and Habits: Adult beetles feed along leaf margins during summer months. In May and June, females deposit eggs on the bark of new primocanes, usually within a foot of the soil surface. Eggs hatch into larvae that bore into the primocanes and tunnel upward or downward in a spiral pattern. They feed on the exterior portion of the canes first before entering the pith. Larvae reach full size by fall and overwinter inside the canes. They pupate the following spring and emerge as adults through small (1 mm holes) they bore as they exit the cane (Picture 4). The emergence holes are a characteristic of the pest.
Management and Control: Scout for galls during dormant pruning. Prune out the galled canes and burn, bury, or otherwise destroy them to kill overwintering pupae. This should be done before the adults have emerged from the galls to reduce the population and infestations during the new season. Elimination of nearby wild brambles may help reduce the population further. If more than 5% of the canes have galls, chemical control may be warranted. Begin scouting planting during bloom by looking for adults during daylight hours. Begin insecticide applications after bloom has ended and bees are no longer present. Admire Pro (imidacloprid) may be soil applied for systemic control. Consult the Midwest Small Fruit and Grape Spray Guide (ID-169) for details.
Click image to enlarge
Picture 1: Typical red-necked cane borer gall on thornless blackberry
Picture 2: Normal healthy shoot growth below gall
Picture 3: Delayed and stunted shoot growth above gall
Picture 4: Cane with exit hole near gall from adult emergence