Black Vine Weevil Biology and Management
Journal of the American Rhododendron Society
Richard S. Cowles
Conn. Agric. Expt. Station, Valley Lab.
P. O. Box 248
Windsor, CT 06095
The importance of controlling the movement of adult black vine weevils has not been appreciated by nursery owners. To show why adult exclusion barriers should be considered a first line of defense in black vine weevil management, I will first describe two common scenarios.
The first example is heavily infested container grown Belgian azalea nurseries in California. I have observed that the number of larvae required to kill these azaleas is inversely proportional to the extent of root development. During the first year of Belgian hybrid azalea growth (following planting from liners), there are insufficient roots for black vine weevil larvae to complete development. However, these same plants can be easily killed by the larval feeding because they are dependent on those few roots to survive. The result is that both the plants and the larvae have extremely high mortality; the usual scene is a group of pots with girdled and dead plants, from which live black vine weevil larvae cannot be recovered. Since larval development in these pots is largely suicidal, where do the adults come from that cause repeated infestations? The answer lies with the juxtaposition of the small pots next to old plants, sometimes of the same variety of azalea, in 5 - 15 gallon tubs. The girth of the trunk on old plants is large enough that girdling becomes improbable, and there are sufficient roots for the larvae to complete development; therefore, these plants are rarely killed by black vine weevils. When these large tubs are placed next to gallon pots, they act as asymptomatic carriers, supplying the adult black vine weevils that invade the gallon pots each year.
Another example could illustrate a small nursery in the Pacific Northwest, the Midwest, or the Northeast. Rhododendrons in many locations are grown under shade cloth. The sides of the shade house may be open, or have driveways allowing efficient movement of plants onto trucks. Close to the growing area, ideal hosts, such as a thicket of blackberries, or a nursery block planted with yew, provides an ideal breeding ground for black vine weevils. Each year the adults developing from the alternate hosts disperse to the rhododendron plantings, where they then lay eggs.
In both cases, the problem lies with the movement of gravid females from alternate hosts to susceptible plant material. One solution is to provide a barrier that prevents the movement of adults. Since they can only crawl, an effective barrier can be constructed with 6-inch wide sheet aluminum (commonly sold as flashing strips), with 2 - 3 inches of the bottom edge buried in the ground, completely enclosing plantings that need to be protected. The upper 2 - 3 inches of the perimeter barrier is then coated with an unclimbable lubricant such as grease. The canopy of plants on opposite sides of the barrier cannot be allowed to touch, as adult weevils will readily use such a bridge to cross over a barrier.
Because black vine weevil larvae are commonly transported as larvae in nursery stock, an additional use for exclusion barriers is to quarantine plant materials brought in from other nurseries. Plants brought into quarantine can be held until it can be determined whether these plants are infested, and to isolate them where they can be treated separately from other plant material.
Finally, exclusion barriers could be used to define "weevil-free" areas within nurseries, where plants can be grown from the liner stage until final sale with the assurance that adults do not have access. If adults are totally excluded, then other management practices for black vine weevil become unnecessary.
Often, the first sign that black vine weevils are present is when plants start dying. In such a situation, nurseries are required to establish "commercial plant cleanliness," which means that there has to be an effort to kill the larval stages. Otherwise, plant shipments may legitimately be rejected.
Early workers amended lead arsenate into potting media to kill black vine weevil larvae, however, this was phytotoxic to several plant species (Smith 1932). The invention of chlorinated hydrocarbon insecticides, such as aldrin and dieldrin, gave very long residual control of black vine weevil larvae; one treatment of potting medium before planting would be sufficient for protecting the plants until they were sold. These insecticides were banned in the United States in the 1970's, but continued to be used in the United Kingdom until very recently (Blackshaw 1987). The loss of these materials has been lamented by many growers, however the appearance of black vine weevil populations resistant to dieldrin suggests that these materials may not have been useful for much longer anyway (Nielsen et al. 1975).
The current situation is that nearly all insecticides labeled for use on ornamentals have relatively short residual activity in soil. Because adult activity and egg laying can extend from spring through autumn, insecticides drenched into soil to control the larval stages may have to be repeatedly applied, or else used in early autumn to control mixed ages of larvae. Late instar larvae are notoriously difficult to control with conventional insecticides (acephate, chlorpyrifos, or carbofuran [Evanhuis 1982]) so growers choosing to disinfest plants with late instar larvae currently must use root ball dips to meet strict phytosanitary standards.
Two new advances may lead to products that will give long residual control of black vine weevil larvae. The first, formulation of chlorpyrifos into slow release granules (SuSCon Green, Incitec, Ltd.), has readily been accepted by European growers and is now being tested in the United States. The active ingredient leaches slowly from a plastic resin, creating zones in the soil that are toxic to young larvae. An advantage in using this technology is that formulation in resins safens the insecticide by making very little of the active ingredient available at any one time. Consequently, leaching of the active ingredient out of the pot and into surface run-off water is negligible compared with conventional pot drenches. In order to work, this material has to be distributed throughout potting soil, including the volume closest to the stem. Therefore, cuttings must be rooted in treated medium, or soil can be washed from around the roots on liners before being transplanted into the treated medium. In several trials (Buxton et al. 1992), SuSCon Green has been shown to last through two growing seasons, but is also known to no longer be effective after the second year (D. G. Nielsen, personal communication). Therefore, once plants have reached the second year, the root volume is no longer protected and may become infested by black vine weevil larvae.
One area that bears investigation is the use of new classes of insecticides in the soil. Imidacloprid (Merit or Marathon), belongs to a new class of chloronicotinyl insecticides, and has shown very good activity against scarab larvae (white grubs) (Power et al. 1993; Smitley & Davis 1993; Cowles, unpublished data). It has demonstrated residual activity for some pests up to two years after treatment, but is most effective against early instar larvae. Therefore it may only be effective as a preventive drench in spring or early summer rather than as a curative autumn treatment. Another group of insecticides, insect growth regulators, is showing promise against scarab larvae. These can directly kill larvae, but also, by shortening the duration of larval stages, could cause black vine weevil to enter the prepupal stage when soil temperatures are too hot, or to enter the pupal or adult stage at the onset of winter. In either case, nearly complete mortality would be predicted from exposure to adverse temperature conditions.
|Introduction & General Biology||Management Options & Adulticides||Adult exclusion & Conventional larvicides|
|Biological control of larvae & Summary||References|
|YEYS IN CONNECTICUT ARTICLE|