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Volume 49, Number 2
Spring 1995

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Black Vine Weevil Biology and Management
Richard S. Cowles
Connecticut Agricultural Experiment Station, Valley Laboratory
Windsor, Connecticut

Nearly every rhododendron nursery owner or enthusiast has experienced the disappointment of obtaining a choice shipment of plants, only to have some of those plants suddenly wilt and die. The two immediate suspects are the nurseryman's banes, root rot organisms and black vine weevil. Lying hidden within the soil, these organisms destroy root function, causing similar symptoms of drought stress and eventual death. Damage caused by feeding from black vine weevil larvae can easily be distinguished from root rots, because they leave characteristic girdling of the underground main stem and roots.
        This article has the purpose of describing what is known about black vine weevil biology and relating that information with past, present, and possible future management strategies. It is my hope that a thorough understanding of biology and pest management principles will lead to rational, sustainable, and economical solutions for managing this pest.

General Biology
Though not known with complete certainty, black vine weevil, Otiorhynchus sulcatus (F.), is thought to have a northern European origin but was present in North America by 1835 and was a notable pest in Missouri by 1871 (Smith 1932). Throughout all areas of the world where it is found, the larvae will feed on the roots and damage a tremendously varied number of species, with favored food plants in the families Ericaceae, Pinaceae, Primulaceae, Rosaceae, Saxifragaceae, Taxaceae, and Vitaceae. Such a broad host range, with an ability to develop on most gymnosperms and broad-leaved plants (Smith 1932, Masaki et al. 1984), has enabled this beetle to establish itself in nurseries, greenhouses, and landscapes around the world in Mediterranean, temperate, and northern climates (Smith 1932).

Adult Otiorhynchus sulcatus
Figure 1. Adult Otiorhynchus sulcatus

        The adult black vine weevil (Fig. 1) is typical for beetles from its family, Curculionidae, in having a head projected forward into a snout and in playing dead when disturbed. Other characters of the adult are that it is parthenogenetic (only females are known), 8.5 to 11.5 mm long, and jet black, with a beaded appearance and small, asymmetrically arranged tufts of short orange hairs on the elytra (the modified front wings covering the top of the abdomen). The beads on the elytra are arranged lengthwise in rows. Under the microscope the entire upper surface of the thorax (the middle body segment) and the abdomen can be seen to have a general covering of short yellowish hairs. The elytra are weakly fused along the mid-line of the abdomen, which means that the adult cannot fly. Each leg femur (homologous to our thigh) is club-shaped. The coloring of the adult is sometimes disguised by soil clinging to its surface.

Figure 2. Black vine weevil feeding notches 
in rhododendron leaves.
Figure 2. Black vine weevil feeding notches in rhododendron leaves.

        The adult life stage begins upon emergence from the pupa in the soil, usually in the spring or early summer (during late winter in greenhouses and nursery hoop houses). Adults emerging from pupae are initially white, turn brown, then black over a few days. Newly eclosed adults have appendages called mandibular cusps that aid in their exit from the pupal cell and in digging to the soil surface. The adult completes the hardening process in 6-10 days while resting just below the soil surface (Smith 1932). Following emergence from the soil, the adult loses the mandibular cusps and starts feeding on the edges of leaves, creating characteristic feeding notches (Fig. 2). This period is called maturation feeding, because consumption of foliage is required before eggs may develop. Black vine weevil, like all insects, has little control of its own body temperature, so the temperature of its surroundings determines how rapidly development takes place, with ovarian development being completed more rapidly for adults emerging in midsummer than those emerging in the spring (Phillips 1989). The duration of the maturation feeding stage is reduced and the number and viability of eggs increased when black vine weevils feed on leaves containing adequate foliar nitrogen (Cram 1965, Hesjedal 1984) from preferred hosts (Cram & Pearson 1965, Shanks 1980, Maier 1981, Nielsen & Dunlap 1981,Shanks & Doss 1986). For example, the minimum time at constant 24°C conditions before initiating egg laying was 21 days when females fed on Taxus cuspidata, and 50 days when feeding on Cornus florida (Maier 1981).

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        Foliar feeding mostly takes place at night; however, in overcast conditions adults can be found on foliage during daylight hours (Kirk A. Smith, Vancouver, WA, personal communication). On large shrubs and on trees, adults may find daytime hiding places under bark scales or in litter lodged in branch crotches (Smith 1932); however, on smaller plant material adults usually migrate to the base of the plant and hide under leaf litter, under foliage touching the ground, or in the soil (Nielsen et al. 1978, Montgomery & Nielsen 1979). Eggs are laid at night, either dropped to the ground while feeding, inserted into crevices on plants (Smith 1932), or deposited in the same hiding places where adults are found during the day, usually under the soil surface and up to a depth of 20 cm (Nielsen et al. 1978). Eggs are very sensitive to drying, so the indiscriminate egg laying reported from greenhouse situations (Smith 1932) may only occur when adults experience high humidity. Once egg laying commences, the amount of feeding by the adults decreases. As long as moderate temperatures last, egg laying continues into autumn (Nielsen et al. 1978). Some adults overwinter and then emerge the following spring to recommence egg laying. In areas with mild winters, the overwintering adult population can cause management problems because they start laying eggs before the eclosing adults have completed maturation feeding. These overwintered adults may lay an average of 600-700 eggs during the growing season, while adults during their first year may lay 200-400 eggs apiece (Smith 1932). Therefore, control strategies directed at adults have to take into account both overwintered adults and the prolonged emergence of a new cohort of weevils.
        Viable eggs laid by black vine weevil are initially a yellowish white and then over 1-3 days turn an opaque brown (Smith 1932). Even under the best conditions, at least 10% of eggs are non-viable and remain white (Montgomery & Nielsen 1979). Humidity and temperature interact to determine the time required for eggs to hatch (Shanks & Finnigan 1973, Montgomery & Nielsen 1979). Under field conditions, females probably lay eggs where humidity is favorable for egg development, so temperature is probably the most important influence. With at least 85% relative humidity, eggs hatch in 40 days at 10°C, 20 days at 16°C, and 10 days at the optimal 25°C (Montgomery & Nielsen 1979).

Figure 3. Last instar black 
vine weevil larvae.
Figure 3. Last instar black vine weevil larvae.

        Black vine weevil and related root weevil larvae are a yellowish to pinkish white (Fig. 3), with a brown, hardened head capsule, and are legless, which distinguishes them from scarab larvae (e.g., chafers and Japanese beetle), which have six legs. Black vine weevil larvae usually continue through six (sometimes seven) larval instars (they molt, shedding their skins, five or six times to allow growth). Because the rest of the larva is soft and impossible to measure accurately, the brown, hardened head capsule must be used to determine the stage of the larvae. At each molt, the head capsule becomes larger in a roughly geometric progression; the range for head capsule widths for each larval instar is 0.27-0.32, 0.36-0.48, 0.58-0.65, 0.83-0.99, 1.13-1.23, and 1.45-1.59 mm (LaLone & Clarke 1981). When feeding on rhododendrons, the first three to four instars feed principally on small roots, as long as these are available. The last two instars proceed to feed on larger roots, especially on the phloem and cambium tissues near the soil surface (LaLone & Clark 1981). Interestingly, this switch in feeding behavior also is related to a reduction in growth rate. During the first three molts, larvae follow a nearly constant 1.46-fold increase in size, while the last two molts have an average 1.28-fold increase, suggesting that bark and cambium may be a poorer quality food source for these larger weevil larvae. Azaleas and rhododendrons attacked this manner may die, even with only one to three larvae present. When the plant dies, the larvae present in these pots cannot complete development to the adult stage unless they have already completed their larval feeding. In spite of being legless, these larvae can actually girdle plant stems up to 1 cm above the soil surface (personal observation).
        The duration of larval development, like maturation feeding and development of embryos within eggs, is temperature dependent. The duration of larval development is a minimum of 74 days under optimal conditions and a maximum of 225 days when larval development is interrupted by a winter freeze. Most larvae cease development following their last instar and overwinter as prepupae (larvae ready to pupate) in underground chambers before resuming development.
        Smith (1932) noted that although prepupae overwinter well they are sensitive to high temperatures and that larvae entering the prepupal stage when soil temperatures averaged 81°F invariably died. Some black vine weevils pupate before the onset of winter. The resulting pupae or newly emerged adults are not as hardy as larvae and prepupae and are usually killed in northern climates by winter temperatures. Pupae are a transitional, non-feeding stage (Fig. 4). They are initially white, but as the transformation of tissues to the adult takes place, the extremities turn a light brown. Pupation takes approximately 21-24 days and is nearly synchronous for all individuals overwintering as prepupae (Smith 1932).

Figure 4. Black vine weevil pupae.
Figure 4. Black vine weevil pupae.

Management Options
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 imperfect, so each grower must decide which level of management and which techniques are appropriate for his 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 etal.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 host plant 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 and then wander to plants resistant to adult feeding, where they lay their eggs.
        A more reliable form of host plant 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 (bendio-carb 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 adult weevils must 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, 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.

Adult Exclusion
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 isolated 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.

Conventional Larvicides
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 makes the insecticide more safe by releasing very little of the active ingredient 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 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.

Biological Control of Larvae
Biological control (biocontrol) is defined as suppression of a population through the action of predators, parasites, or disease organisms. There has been considerable interest in using insect pathogenic nematodes and fungal diseases against black vine weevil larvae (Zimmerman & Simons 1987, Soares et al. 1983, Simons & Van der Schaaf 1987). However, recent efforts at commercializing a fungal pathogen, Metarhizium spp., have been discontinued by some producers due to high production costs.
        Unlike the more familiar plant parasitic nematodes, insect pathogenic nematodes do not have stylets and thus are incapable of feeding on plant tissues. The commercially available species belong to two families of nematodes, Steinernematidae and Heterorhab-ditidae, and have similar life histories. In both families, the infective juvenile stage larva moves through moist soil, using CO2 emissions and other host associated cues to find insect larvae. Once a larva is found, the nematodes generally enter through any body opening (mouth, anus, or spiracles) and penetrate to the insect's open circulatory system. Once there, the nematode then releases symbiotic bacteria retained in a special region of their gut. In two to three days in warm soil temperatures, these bacteria multiply and produce toxins that kill the insect. The insect's cadaver becomes a bacterial soup, complete with bacterially secreted antibiotics that prevent competition with other bacteria or fungi. Over the next two weeks, the nematodes grow to adults, mate, and produce another generation. Two to three generations later (depending on the size of the grub initially infected), third stage juvenile nematodes exit from the cadaver and seek more insects. The adults of these nematodes generally are not observed unless a cadaver is broken open (Gaugler & Kaya, 1990).
        Commercially available species effective against black vine weevil are Steinernema carpocapsae, and Heterorhabditis bacteriophora. H. bacteriophora has consistently given higher percent kill of black vine weevil larvae (Bedding & Miller 1981, Rutherford, et al. 1987, Shanks & Agudelo-Silva 1990), which may partly be due to its ability to penetrate the grub through body openings and directly through soft cuticle, using a tooth at their anterior end. This species is more difficult to grow and store, so S. carpocapsae has been more readily commercialized. A common observation has been that these nematodes can give excellent kill of black vine weevil larvae or pupae under greenhouse conditions (Evanhuis 1982; Georgis & Poinar 1984), but tend to give poor results when applied to field-grown plant material. The most likely explanations for failure of these nematodes in the field are that the soil temperatures are generally too cold for the nematodes to disperse effectively (Rutherford, et al. 1987) and that nematode antagonists may cause high mortality of these nematodes.

In spite of being flightless as adults, black vine weevils have proven to be a formidable pest with which nurserymen and rhododendron enthusiasts have had to cope. Black vine weevils' success has probably arisen from two important aspects of biology. First, the fact that larvae feed underground and the adults are nocturnal allow them to slip in and colonize, like a Trojan Horse, previously uninfested sites. Secondly, the fact that the adults can feed on such diverse and potentially toxic plants as Rhododendron and Taxus is significant because it allows populations to develop throughout the landscape, and it also predisposes populations of this organism to develop more finely tuned metabolic machinery to detoxify man-made insecticides.
        The challenge in integrated pest management is to turn a pest's traits to our advantage. The fact that adults are flightless means that we have an opportunity to exclude them from feeding on plants we wish to protect. Unclimbable barriers are probably the most underutilized, most common-sense tool to combat root weevils. Since the adults hide during the day, we can respond by using trunk wraps for monitoring populations and for mass trapping. Finally, since adult weevils have strong feeding preferences, we may be able to devise baits for more efficiently applying insecticides.
        For large-scale nurseries, it is important to realize that adults may be emerging and be ready to lay eggs at various times of the year. Overwintering prepupae will develop to adults in late winter if held within containers in warm hoop houses. The next flush of adults may be the overwintering 2-year-old beetles and the last group to emerge would develop from prepupae overwintering in cold field soils. This complexity, and the inability to obtain adequate control of adults with currently registered insecticides, has led to chronic, low level infestations in many susceptible plant materials. Only by combining a fresh approach of maintaining growing areas free of adult weevils (using exclusion), by monitoring adult populations (to detect failure in exclusion), and more effective use of insecticides can black vine weevil be eliminated from the nursery trade.

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Volume 49, Number 2
Spring 1995

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