|Pest Management Office 491 College Avenue Orono, ME
(207) 581-3880 1-800-287-0279 (in Maine) Fax (207) 581-3881
|STRAWBERRY ROOT WEEVIL
|BLACK VINE WEEVIL
DESCRIPTION & LIFE HISTORY
There are more than 20 species of root weevils that attack strawberry in the United States. In the Northeast, the three major species are the black vine weevil, Otiorhynchus sulcatus (Fabricius), the strawberry root weevil, O. ovatus L., and the rough strawberry weevil, O. rugostriatus Goeze. Root weevils are also pests of raspberries and rhododendrons.
The root weevil adult is a brown to black beetle, with rows of pits or punctures along its back. Like other weevils, its mouthparts are extended into a snout. The three species discussed here look similar but differ in size. The strawberry root weevil is the smallest in size, about 5 mm (0.2") and black to light brown; the rough strawberry weevil is generally an even chocolate brown and 6.4 mm (0.25") long; and the black vine weevil sometimes has small flecks of yellow on its black body and can reach 1 cm (0.4") in length.
Adults of Otiorhynchus generally emerge in late May through June from puparia in the soil. They feed at night on foliage and hide during the day. After a period of approximately 30 to 60 days (for the black vine weevil) or 10 to 14 days, they begin to lay eggs. Some larvae of these species do not pupate in the spring and will remain in the soil throughout the summer. They then pupate in the fall and overwinter as adults, to emerge the following spring.
Depending on the species, peak egg laying occurs from late July through August. Eggs are laid in the soil around the plants; they are pearly white when laid, but soon change to an amber color. Eggs of the strawberry root weevil are 0.4 mm by 0.5 mm (0.02" long); those of the black vine weevil are 0.6 mm (0.03") spheres.
Larvae, or "grubs," are creamy-white or dirty-white to brown, have no legs, and lie in a characteristic "C" position in the soil. Grubs of the strawberry root weevil are about 6 mm (0.25") long when fully grown; those of the black vine weevil are 12 mm (0.5") long. By October, most of the eggs have hatched into larvae; hatching occurs about ten days after the eggs are laid. Young larvae feed on fine roots and crowns in mid-summer, overwinter in the soil, and cause their heaviest damage in the spring. Black vine weevil pupae are soft and white. Adults emerge after a short pupation period in April and May. There is only one generation per year.
Adult root weevils eat notches in the leaves, but this damage is seldom important. The larvae, however, cause serious damage by tunneling in the roots and crowns as they feed on strawberry plants. Most damage to the roots is caused by the later instars of larvae in March and April. Plants become stunted and darkened, and this damage can weaken or kill the plant. Injured plants have a stunted appearance; the leaves are closely bunched and are dark and blue-green. The fine roots have been destroyed, and sometimes even the hard fibrous roots have been eaten.
Heavily damaged areas in the field can be large--sometimes up to 0.2 ha (.5 acre)-- and circular, because of the beetles' behavior of gathering in groups. Without control, damage can be so severe by the second fruiting year that early termination of the planting is necessary. Newly transplanted strawberry plants can be particularly susceptible to black vine weevils.
Because there are three or more species of root weevils in the Northeast, precise identification is essential for adequate control. Adults should be collected for proper identification
To prevent the spread of insects to other beds, plow under old beds as soon as possible. Rotate fields to an unsuitable host (e.g. corn, pumpkins) for at least two years. Fall plow infested beds. These pests do not fly. Keep new beds far from infested sites and clean farm equipment before moving from infested fields to new fields to prevent them from spreading.
No economic thresholds established. Two to eight black vine weevils per plant are known to cause economic damage. Soil fumigation provides effective pre-plant control of larvae. Chemical applications to the soil following bed renovation can reduce populations. Material: carbofuran (Furadan 4F). This material is registered under a special local needs permit (24c). Applicators must be certified and have a copy of the special label. Postharvest foliar sprays to control the adult beetles during the summer the most common treatment. Present research is directed at optimum timing of this treatment; it should be delayed until as many adults as possible have emerged, yet applied before egg laying begins. This approach will not control those individuals overwintering as adults.
Parasitic nematodes have been used effectively on black vine weevil larvae in cranberries but have not yet been successfully used in strawberries. Larvae are also attacked by some general predatory insects including carabid and staphylinid beetles. Some plant resistance has been noted in various strawberry clones, but this has not yet been incorporated into commercially acceptable varieties.
DESCRIPTION & LIFE HISTORY
The adult form of this insect are beetles that are small (1/8") round, and copper-colored with dark markings on their backs. The immature root-feeding grubs are also small (1/8"), creamy white in color with 3 pairs of legs, and are actively feeding on roots in the late spring to early summer. The new generation of adults appears after renovation (late July or early August).
This insect can be most easily observed in the field as adult beetles feeding on leaves. Feeding occurs at two times in the growing season in Massachusetts (May and July-August), and results in shot-holes in the leaves. In Maine, the second feeding period may extend into October. The second feeding period usually is more evident because a greater number of beetles are feeding then. The earlier feeding is done by the overwintering population.
As with all root-feeding insects, control of the root-feeding stage is very difficult. Therefore, control measures for strawberry rootworm should be directed toward the adult stage of the insects. Presence of adults can be detected by the feeding injury or direct sightings of the adult beetles in the field. Sticky traps used for monitoring tarnished plant bug may aid in sighting strawberry rootworm adults since they feed primarily at night. Some of these beetles find their way onto the traps.
If feeding injury is observed in May or June, an insecticide spray at this time will reduce the number of egg laying females and therefore, the number of grubs feeding during the summer. When the next generation of adults emerges in July or August, control measures may be needed again.
No threshold is established for this insect. Feeding injury, as with all the root-feeding insects, is most damaging if root diseases (i.e. black root rot) infect the plants as a result of wounding. Therefore, it is advisable to keep the root-feeding population low. See pest management schedule for recommended materials and timing.
James F. Dill, Pest Management Specialist
Clay A. Kirby, Insect Diagnostician
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