Grape Cane Borer and Grape Rootworm Beetle
(2003 Unpublished)
by Devin Carroll
Unless someone is monitoring carefully, growers may not know that some pests are in their vineyards. Although these “minor” pests are rarely serious, they can sometimes cause significant damage. Two of these pests are the grape cane borer and the grape rootworm beetle.
The grape cane borer is often called the branch and twig borer because it also attacks many trees. This brown beetle thrives in dead wood, but recently dead, not weathered. Unfortunately, both adults and larvae will also bore into living tissue in year-old canes and spurs, which is where they do their most damage.
The adult beetle likes to bore a deep hole near the base of a new shoot, from March through May, causing the shoot to wilt and die. The tiny white eggs, oval with a curved point at one end, can often be found near the tips of spurs or on canes. The hatching larvae burrow beneath the surface, leaving a tiny pile of visible frass, and eventually hollow out the spur centers, filling them with dry powder. The spurs survive, but with weakened growth and reduced production in next year’s spurs. Infested canes may break in high winds.
The mature white pupae overwinter in old canes or dead wood, where they can be found in the midst of the dry powder they made. The best control is sanitation. Remove dead vine parts and place all prunings in the row middles. Then chip and incorporate before March. Encourage your neighbor to do the same.
If you have a history of problems, or find the larvae during pruning, watch for the beetles and look for eggs in late March. If you consistently find more than one in ten minutes search, consider treatment. The best treatment we have found is Imidan at maximum rate, buffered below pH 5, in 50 gallons water. Try to time treatment when most beetles have emerged but not many eggs have been laid. In the San Joaquin Valley, this is usually within a week before or after April 1. Imidan will kill the adults in their burrows, and even newly hatched larvae just beneath the surface. Although it is an organophosphate, Imidan is relatively easy on most beneficial insects.
Larvae of the grape rootworm beetle feed on grape roots. The small black or brown beetles emerge in April and May. Their feeding damage on leaves is easily recognizable: narrow slits, usually many holes on a few leaves per vine. The leaf damage is rarely a problem, although I have seen young vines so shredded that we feared stunting. When populations are heavy, the beetles will also scrape grooves in the new canes, the rachises, and the young berries.
The type of damage depends on the grape cultivar. In most varieties, including all table grapes that I have seen, the beetles damage mainly the leaves and stems, with fruit damage very rare. But on some wine varieties, such as Chenin Blanc, I have seen extensive fruit scars with relatively little leaf damage. I suspect hairy leaves deter the beetles from feeding. Fortunately, the scars do not lead to rot, and although they are unsightly, they do not greatly harm the capacity of the grape to produce wine.
Most damage occurs in April and May, after which the beetles disappear from the vines. I have noticed a second emergence in August, but the damage then was not important. In the rare cases when treatment is warranted, the best timing is late April or early May. Sevin is effective, and probably also Imidan. If treated effectively after the beetles have all emerged from the ground, but before they lay eggs in crevices in the bark, the population should be much smaller the following year.
Pupation occurs near the soil surface in March and April. Disking near the end of March is recommended as the best control method. Indeed, this beetle was a greater problem before disking became widespread, and may be increasing again in vineyards where disking is not routine.