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
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 damage. 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.
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).
The adult black vine weevil 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, 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.
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. 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 mid-summer 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).
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 dig into 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, 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).
Black vine weevil and related root weevil larvae are a yellowish to pinkish white, 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 in 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 those larvae entering the prepupal stage when soil temperatures averaged 81ºF 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. 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).
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