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
Control of black vine weevil is best approached with an understanding of how the various aspects of its biology can be turned in our favor. Specific control practices have been developed using: (1) host plant resistance, (2) treatments to kill adults, (3) exclusion of adults, (4) conventional insecticides directed against larvae, and (5) biological control agents directed against the larvae. Each of these techniques is not perfect, so each grower must decide which level of management and which techniques are appropriate for their situation.
The ability of female black vine weevils to develop ovaries and produce a large number of eggs depends on the quality of the plants she eats (Maier 1981). Therefore, if all the available plants for her to feed on are unpalatable, then a female's ovaries will not mature. Host plant resistance has been evaluated for strawberry and rhododendron varieties; in both cases, phenotypes with large numbers of trichomes (plant hairs or scales) interfered with egg development (Nielsen et al. 1978, Doss et al. 1987, Doss 1983). The trichomes present on lepidote rhododendrons are also glandular in function: besides being scaly, these structures produce essential oils that deter feeding by adult black vine weevils (Doss 1983, 1984). Thus, resistance to black vine weevil adult feeding could be accomplished by selecting lines of plants that have low concentrations of feeding stimulants (Doss & Shanks 1985) and have high concentrations of deterrents (Doss 1984).
The black vine weevil resistance of indumented and "bad tasting" rhododendrons cannot be predicted, however. According to Hanula (1988), adult black vine weevil had to contact hostplant foliage in order to be stimulated to lay eggs. However, certain varieties of blueberries are not fed on at all by adult females, yet are suitable for larval feeding, and consequently suffer severe root damage (Cram 1970). Evidently, gravid adults may feed on susceptible, readily fed upon plants, then wander to plants resistant to adult feeding, where they lay their eggs.
A more reliable form of hostplant resistance would be to develop rhododendrons that can either tolerate root feeding (through rapid callus and root formation) or antibiosis, the ability to kill larvae that attempt to feed on the roots. An example of tolerance can be seen with some hybrid azaleas, such as the 'Gumpo' varieties, which tend to be able to callus quickly enough to survive black vine weevil feeding.
The maturation feeding stage during adult development offers a long window of opportunity to attempt to break the black vine weevil life cycle. Insecticides applied to foliage are a well-accepted method for reducing the adult population. Materials labeled for use vary in different states; however, those commonly used include an organophosphate (acephate), carbamates (bendiocarb and carbofuran), and pyrethroids (bifenthrin, cyfluthrin, fenvalerate, and fluvalinate). Each material has its advantages and disadvantages. Orthene (acephate) will only kill adults for approximately three days following application (Nielsen & Montgomery 1977), however, in some states this may be the only registered and effective insecticide, and it has low mammalian toxicity. Furadan (carbofuran) is highly effective against adults, however it is also highly toxic to mammals. The pyrethroids have to be applied at their highest labeled rates to achieve control; even then the detoxification mechanism in adult weevils is alarming. A common observation with these pyrethroids is that adults are "knocked down," only to recover one or more days later (Nielsen 1983, personal observation). To get the greatest effect from any foliar treatment, sprays should be timed approximately two hours after sunset to maximize direct contact of weevils with spray droplets as they feed. Furthermore, sprays should be timed to eliminate overwintering adult weevils, with follow-up sprays scheduled approximately every four weeks (depending on temperature) to eliminate groups of adults following their emergence and before laying any eggs (Nielsen et al. 1978, Phillips 1989, Hanula 1990).
One insecticide that has been neglected for use against black vine weevil adults is cryolite (sodium aluminofluoride). This mineral has the disadvantage in being abrasive to spraying equipment, and in having very high application rates that result in highly visible residues on foliage. However, the material itself has low toxicity to mammals, is accepted by organic growers, and is especially effective against chewing insects, even those (such as Colorado potato beetle) that have become resistant to most other insecticides.
Control of black vine weevil adults historically began with application of baits laced with lead arsenate (Smith 1932). Recent work by Ocean Spray Cranberry (A. Broaddus, personal communication) suggests that the use of baits with more modern insecticides (such as cryolite) can be an effective way to control adults. The advantages of using broadcast baits would be that disruption of other pests (such as mites) resulting from foliar sprays can be avoided, a smaller quantity of insecticide would be needed on a per-acre basis, and simple mechanical equipment can be used to apply these products. I predict that significant improvements in control of black vine weevils will be accomplished by combining newer stomach-acting insecticides (such as pyrroles) or microbial toxins with baits. The formulation of baits could itself be an active area for research. Apple pomace or bran-based baits are 60-year-old technologies (Smith 1932), which could potentially be improved by an understanding of attractants (Doss 1983) and phagostimulants (Doss & Shanks 1984) influencing black vine weevil behavior.
Monitoring adult activity is necessary to be certain that adults are present and to time sprays properly. Several monitoring methods have been studied, each has merits for specific situations. Four general categories of techniques are used to monitor adults: (1) evidence of feeding activity, (2) direct counts, (3) use of man-made refugia, and (4) pitfall traps.
One of the simplest monitoring methods is to look for the notches in leaf edges characteristic of root weevil feeding. The disadvantages of this method are that (1) there is no way to determine which of the many species of root weevil caused the feeding, (2) it is difficult to tell when the feeding took place (which is important to know when scheduling sprays), and (3) other physical damage, such as wind whip or katydid feeding, can resemble root weevil feeding. Black vine weevils feed most intensely between emergence and the initiation of egg laying. This feeding tapers off during egg laying activity until feeding stops and the adults seek out overwintering sites (Smith 1932). Therefore, a constant cannot be defined that relates the count of new feeding notches in leaves to the number of active adults, and the lack of feeding in the autumn can give a false sense of security that adults are absent.
One of the best ways to use observations of feeding notches is to determine whether weevils are present or absent in specific areas within nurseries. Certain plants, especially weedy Epilobium spp. are a highly preferred adult food compared with Rhododendron spp. Therefore, Epilobium can be used as indicator plants: they will be fed upon (and adults can be found during the day under these plants) long before any notches are found on Rhododendron.
Direct counts of adult weevils is a difficult monitoring method because these beetles are active at night and are also inconspicuous. Sweep net sampling is generally unsuccessful, however, container grown plants can be shaken over a sheet to obtain the feeding adults. The best time to use this technique is from a couple hours after sunset until midnight (D. G. Nielsen, Ohio State University, personal communication).
Adult weevils seek out places to hide during the day; this behavior can be used to trap them in man-made refugia. Burlap sacks can be used as trunk wraps, and are useful for single or multi-stem plants. Make 4-inch lengthwise accordion folds in the burlap bag, then hold one end against the base of the trunk, and loosely spiral the wrap (Mulgrew 1991). Do not tie the burlap to the trunk since this method requires adult weevils to climb into the openings in the vertical folds. To count captured weevils, unwind the burlap and shake it over a white sheet or concrete paving. Repeated and frequent removal of the adults could be sufficient for suppressing the weevil population in small residential plantings. Another method is to place a board on the ground so that it touches the base of the trunk (Maier 1983). Trap boards can be checked more quickly than burlap trunk wraps, but do not catch as many of the beetles because they do not surround the trunk. Also, adults may hide under leaves or other duff rather than under the trap board (personal observation).
In nurseries, especially with container grown plants, pitfall traps are an effective adult monitoring method (Hanula 1990; Cowles, unpublished data). Pitfall traps work by capturing crawling animals in a depression from which they cannot escape (ant-lion pits are a good example from nature). A simple pitfall trap can be made by placing a 4-inch diameter plastic cup in the ground so that the edge is at or just below the soil surface. Coat the upper edge on the inside of the cup with a 2-inch band of an oily lubricant, which prevents the captured beetles from exiting. To prevent the trap from filling up with irrigation water or rain, I invert a 1-gallon pot over the trap, after having first cut three legs into what originally was the rim of the pot. Preliminary results from trapping adults in a commercial azalea nursery suggest that pitfall trap catches are increased when placed near the inside of a right-angle bend of an exclusion barrier (discussed below). Beetles meeting a sharp-angled obstruction may be forced to turn, consequently increasing the likelihood of falling into the pitfall trap.
|Introduction & General Biology||Management Options & Adulticides||Adult exclusion & Conventional larvicides|
|Biological control of larvae & Summary||References|
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