Current year’s leaf damage by black vine weevil has been found east of Effingham. Now is the right time of year to be seeing this in southern Illinois, particularly with our unusually warm spring.
Black vine weevil larvae are thick-bodied, white, legless grubs up to about 3/8 inch long that feed on the roots of yew (Taxus), strawberry, and other plants. They prune off small roots and also damage larger roots. This rootfeeding can be devastating in a nursery because it results in reduced growth or even dieback of the aboveground part of the plant. Anything that lengthens the time a plant takes to reach marketable size is a concern for the nurseryman. It is common to find damage from black vine weevil larvae on the roots of dying yews in the landscape, but these plants typically have obvious additional stresses, such as having poor soil drainage from being near a downspout or soil compaction from being next to a footpath. Nearby yews that show no dieback and do not have these additional stresses commonly show root damage from black vine weevil as well.
The larvae pupate, and adults emerge later in the spring. Adults are about 3/8 inch long; blackish, with indistinct small, yellowish spots; and very hard-shelled. The head tapers into a narrowed muzzle. These insects are all flightless females that must feed 2 to 3 weeks before their ovaries develop enough to lay eggs. These adults hide in debris below the plant during the day and come up onto it during the night to feed. Their small mouthparts nibble 1/8-inch-long, crescent-shaped notches in the leaf margins. This feeding damage is very characteristic and is common on the leaves of yew, strawberry, euonymus, clematis, wisteria, and many other plants. The adults also can migrate indoors, where they feed heavily on house plants. Eighty other feeding hosts are known for the adults of this insect.
For the nursery, control has been achieved against the larvae with soil drenches of bendiocarb (Dycarb or Turcam), carbofuran (Furadan), and acephate (Orthene or Pinpoint). The insecticidal nematodes Heterorhabditis bacteriophora and Steinernema feltiae are also effective. Larval control is much more certain in potted plants than in field-grown plants. Thus larval control in the landscape is uncertain.
Controlling the adults, particularly in the first 2 to 3 weeks after emergence, not only reduces adult damage but also indirectly reduces the larval population by killing the adults before they lay eggs. Spray attacked foliage with acephate (Orthene), cyfluthrin (Tempo), or bendiocarb (Dycarb or Turcam); and allow a liberal amount to run from the foliage onto the soil to kill hiding adults. Bendiocarb is being phased out of the marketplace and may be difficult to find.
Finally, it is often not necessary to treat for the damage by black vine weevil in landscapes. Larval feeding is likely not to be a problem to established plants. Adult feeding is not obvious on yew, so frequently the client is unconcerned about that damage. Adult feeding is more obvious on euonymus and some other plants. Avoid siting these near yews to reduce adult damage and avoid insecticide application.