Branch and twig borer—Melalgus (=Polycaon) confertus
The branch and twig borer (family Bostrichidae), also called the grape cane borer, chews and tunnels as larvae in limbs of apricot, avocado, California bay laurel, grape, olive, plum, prune, and occasionally other woody species.
Identification
The first clue that branch and twig borer larvae are present may be broken limbs that are currently or previously infested. Inspecting the rest of the plant or hosts nearby can reveal brown, granular frass (excrement) on bark near entrances to larval tunnels. On avocado, tunnel entrances exude sap which dries to a flaky, whitish material on bark.
Adult branch and twig borers are sometimes observed walking on bark or feeding on bud axils or shoot crotches. The adult is an elongate, cylindrical, dark brown beetle. Adult males are about 1/3 inch long and adult females are up to 3/4 inch long. The head and thorax are narrower than the abdomen.
The larvae are white with a small brown head and distinct segments. They grow up to 3/4 inch long and are distinctly swollen behind the head. When exposed, larvae commonly assume a C shape.
Pupae are oblong and initially white but darken as they age. They are about 1/3 to 3/4 inch long.
Life cycle
Branch and twig borer develops through 4 life stages: egg, larva, pupa, and adult. Adults can be present from early March through May. Mated females lay eggs in bark crevices or wounds. Larvae are present by May or June and feed and tunnel under bark and in wood for about 10 months. Pupation and adult emergence occur in late winter through spring. Branch and twig borer has 1 generation per year.
Damage
This borer prefers or infests only hosts that are dying, injured, severely stressed, or growing slowly. Both adults and larvae chew and bore into hosts. Adults bore into green stems, most commonly at an axil or crotch of the branch. Larvae bore into dead or dying wood and slow-growing hosts. Larvae are mostly found at the site of old pruning scars or other wounds to bark and wood.
When present, the borer causes recognizable holes in branches. This entrance to an adult or larval feeding tunnel can exude sugary sap that turns white and flaky. Branches with tunnels can be easily broken by wind. Where larvae are boring, tunnel entrances are commonly plugged with chewed wood and reddish brown, sawdust-like frass.
Solutions
Remove and dispose of seriously diseased, poorly growing, or borer-infested branches, trees, and vines. Remove host prunings from the growing site and chip or dispose of them to remove any immature borers before they emerge during March to May.
Provide hosts optimal cultural care and a good growing environment to keep them growing vigorously; this greatly reduces hosts' attractiveness to the egg-laying female borers. Appropriate irrigation is especially important.
Protect trees and vines from sunburn and injuries, such as by whitewashing (painting) bark. Paint the main trunk and base of larger limbs with indoor, white, latex paint diluted with an equal amount of water. Minimize pruning or make cuts in mid-summer through early winter so wounds may close some before the adults are active during March through May.
Insecticide application is generally not warranted if the recommended cultural and sanitation practices are followed. Where larval tunnels are found, some control can be achieved by squirting entomopathogenic nematodes (Steinernema carpocapsae) into tunnel entrances. First, clear any frass-packed tunnel entrances so the nematodes can better enter the tree. These nematodes generally must be purchased from an internet vendor that ships them via a home delivery service. The best application times are January and February when the borer larvae are relatively large and have not yet pupated into adults.
Adapted from Pest Management Guidelines: Grape and Pests of the Garden and Small Farm: A Grower's Guide to Using Less Pesticide, University of California Statewide Integrated Pest Management Program (UC IPM).
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Flaky, whitish, dried sap that oozed from the entrance of a branch and twig borer tunnel.
A grape cordon (horizontal limb) broken due to extensive branch and twig borer feeding.
A grape branch twisted and broken due to feeding of branch and twig borer.
An adult branch and twig borer feeding and chewing at the base of a grape shoot.
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