Black Vine Weevil: A Rhododendron Pest
That May Be Resistant to Dieldrin in Your Area
D. G. Nielsen Department of Entomology,
Ohio Agricultural Research and Development Center
The black vine weevil, Otiorhynchus sulcatus (Frabricius), is a flightless and parthenogenetic (all are females) insect. It is one of the most common and destructive pests of Rhododendron spp. in the north central and northeastern United States. It is referred to as black vine weevil because it was a pest of grapes for a number of years in Europe before its importation to the U. S. Nurserymen often call 0. sulcatus the taxus weevil because taxus is the most common preferred host in much of its North American range.
Although adults feed on foliage of many woody plants, including rhododendron and taxus, feeding damage is inconsequential. Adults deposit eggs at random while feeding. The eggs fall to the ground or become lodged in the foliage of the adult host. Larvae usually begin hatching in the late-June in Ohio: hatching is probably complete by the end of August. After hatching, larvae (or grubs) feeding on roots cause the damage for which the black vine weevil is well known.
Because this insect is parthenogenetic and each female is capable of laying 1,000 or more eggs, an infestation can be started by the presence of only one weevil (mating does not occur, there are no males). For this reason, infested nursery stock or plants suspected of harboring weevils have been and are restricted from interstate shipment. Consequently, nurserymen have made every effort to prevent black vine weevil infestation in their nurseries.
In 1972, a few nurserymen in Ohio began to complain about poor black vine weevil control with dieldrin, a so called "hard" insecticide used for decades against soil arthropods. I initiated a research program to determine why growers were having increased black vine weevil problems even though they were following proper control recommendations. The study began with a bit of detective work to learn something about the history of taxus, rhododendron, and black vine weevil in the problem nurseries.
The first facts of interest came from a nurseryman with a black vine weevil problem who has a small collection of rhododendron planted in an arboretum near his taxus production area. He indicated that several of his specimen rhododendron came from a propagator who had also been experiencing difficulty with black vine weevil control. Upon discussing black vine weevil with this propagator, I learned the following: (1) Approximately 30 years ago when his nursery became infested with this pest he eliminated the problem with a single application of chlordane; (2) A second black vine weevil infestation developed about 10 years later and it was eliminated with a single application of aldrin; (3) Black vine weevil became a problem for him a third time in the early 1960's following importation of mulch from New Jersey. Repeated attempts to control the pest with dieldrin at recommended and higher rates failed to eliminate the infestation. Although spraying two or three times each year as recommended, he acknowledges the probability that he has shipped black vine weevils to at least some of his clients.
We compared susceptibility to dieldrin of black vine weevil adults from the problem nursery and from nurseries where it had not been a problem. Populations from Ohio, Oregon, and Washington proved susceptible to 0.1% dieldrin (100% mortality) applied by a Potter Spray Tower. But the problem Ohio population was resistant to dieldrin at a concentration of 1.0% (less than 10% mortality). If this is the source of the resistant population, we should expect to find widespread resistance in the U. S. and elsewhere since this propagator has shipped widely. We hope to evaluate as many black vine weevil populations as possible throughout the U. S. for resistance to dieldrin. We require approximately 100 healthy adult weevils for this test.
We are currently studying developmental biology and control of black vine weevil in an effort to find alternative control measures for this pest. While some populations remain susceptible to dieldrin, it is no longer registered and should no longer be used by nurserymen. We and others have worked with a carbamate insecticide which is toxic to black vine weevil adults and larvae. We now need nursery infestations for additional insecticide screening studies to obtain efficacy data needed for registration of this and other alternative insecticides for black vine weevil control.
Research entomologists in Illinois, Indiana, Connecticut, and Ohio are willing to develop alternative control methods. However, we can not do this until you tell us about your infestations and invite us to help you control them. You can obtain this service and contribute to development of black vine weevil control methods by contacting your nursery inspector or county extension agent, asking him to contact the research entomologist in your state who is responsible for woody ornamentals insects.
The black vine weevil is an economic problem for nurserymen. Research entomologists will solve it when we learn about infestations and are invited to work with them.
Experts and Answers
In response to several queries from home gardeners about weevil control, we wrote to R. Lee Campbell, Associate Entomologist, Western Washington Research and Extension Center, Washington State University, Puyallup, Washington for the most up-to-date information.
Unfortunately the chemical control picture for root weevils is rather glum at the present time. It appears that the only materials that can legally be recommended are clordane, malathion and diazinon. These are effective in killing adults as they feed on the foliage if applied frequently enough. As new adults emerge or migrate into the area they will notch unprotected leaves and deposit eggs. Since the adults predominately feed at night it is wise to time sprays for late afternoon or early evening rather than in the morning. Chlordane has the longest residual, but none of the three materials will give long term protection and repeat applications will be necessary if new feeding notches appear. In all cases pesticide label directions should be followed. R. Lee Campbell, Associate Entomologist, Washington State Univ.