Michael E. Reding, Michael G. Klein, Ross D. Brazee, and Charles R. Krause, U.S. Department of Agriculture, Agricultural Research Service, Application Technology Research Unit, Wooster, Ohio.
In 2002, the authors conducted several trials related to the management of black vine weevil (BVW). We tested traps to use for monitoring adults; we are evaluating various spray programs against adults in field-grown Taxus (yews); and we tested a number of insecticides and entomopathogenic nematodes as rescue treatments against larvae in containerized plants.
Black vine weevil is a serious pest of field-grown Taxus and various ornamental species in containers. It is also a pest of many small fruits, including strawberry, raspberry, grape, and cranberry.
In general, damage to ornamental plants is caused by the larval stages, which feed on the roots and girdle stems. BVW has a one-year life cycle. Eggs are laid in the soil during mid- to late summer, and BVW spends the winter as larvae in the soil. Adult weevils emerge from the soil during late May through early July.
After emergence, adult BVW require about three to four weeks of feeding to mature their ovaries so they can lay eggs. All BVW are females, so mating does not occur.
In field-grown Taxus, current guidelines for management of BVW are based on insecticide treatments against adults. Sprays should begin when adults first emerge.
However, detecting the first emergence of adults is not easy. Adult BVW are active at night and seek out concealed sites in the canopy or debris on the ground during the day.
Currently, monitoring consists of either beating plants to dislodge adults onto the ground or by searching for fresh feeding injury (notching on the needles). An efficient reliable trap that would allow growers to detect adults when they begin emerging from the soil would make monitoring much easier and more effective.
In 2002, we tested three types of traps in a commercial Taxus field (ca. 1.5 acres). Two of the traps were effective. One was made from 12 in. x 12 in. x 1 in. boards with 1/4-inch grooves cut into one side. These traps were placed grooved side down beneath the canopy of plants, creating a "concealed" site for the adults during the day.
The other successful trap was a pitfall trap made from two 16-oz. plastic drinking cups. A hole was dug under the canopy of a plant, and the first cup was placed into the hole so that it was level with the soil surface. The second cup had a barrier of grease smeared around the inside rim and was placed inside the first cup. The weevils fell into the second cup and could not get out because of the grease. This cup could be removed to count the BVW. The third type of trap was a "deep-pan" trap made from a small dog-food dish. This trap was not very effective.
Twenty-one of each trap type were deployed in a grid pattern throughout the field. The board traps appeared to be the most effective, because they captured BVW on two dates before the pitfall traps made their first capture. Weekly plant-beat sampling was conducted in this field by Ohio State University graduate student Gina Penny, and board traps made captures before she detected adults by beating (11 and 13 June vs. 17 June, respectively).
This trial was conducted in a heavily infested field that was unsprayed (no insecticides) since 2000; therefore, testing in fields under normal management conditions is needed.
In another project, the authors are evaluating various spray programs against adults in field-grown Taxus. There appears to be confusion regarding the number of treatments necessary and when spraying can stop.
In 2002, the authors began a trial to determine the number of adult sprays necessary to achieve acceptable control of BVW. However, this trial will not be evaluated until April 2003. At that time, we will dig up the plants and count the number of BVW larvae present. Because there is very low tolerance for BVW, the number of larvae in successful treatments will have to be zero or almost zero.
In Spring 2002, we evaluated a number of insecticides and species of nematodes for efficacy as rescue treatments against BVW larvae in containerized plants (Astilbe, 1-gallon pots). Unfortunately, none of these treatments provided 100% control. The best control we achieved was 81% with an unregistered pyrethroid that was injected into the soil and 79% control with entomopathogenic nematodes (Heterorhabditis bacteriophora HP88) drenched onto the soil surface.
The nematodes and some of the insecticides might have worked better if we had given them more time. The trial was evaluated seven days after treatment, and 14 days might have been more appropriate. In research trials, nematodes have often been effective against BVW larvae in containers. However, one constraint to using nematodes is soil temperature. Nematodes are generally most effective when soil temperatures are 60°F or above. This trial was kept in the lab at room temperature (ca. 73°F) after treatment.