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Ministry of Agriculture, Food and Rural Affairs

Root Weevil

Root weevil pupa in soil Root weevil larvae feeding on strawberry crown Reddish granular “frass” indicative of root weevil feeding Leaf notching caused by root weevil adults Adult black vine weevil Root weevil larvae Root weevil damage Adult strawberry root weevil Click to enlarge.

Beginner

Scientific Names
Otiorhynchus ovatus, Otiorhynchus suclatus

There are several species of root weevils which feed on strawberries. The most common in Ontario are the black vine weevil (O. sulcatus) and the strawberry root weevil (O. ovatus).

Identification

  • Larvae are found in the soil around the plant or imbedded in the crown
  • They are cream-coloured, or pinkish-white, legless, with c-shaped bodies and brown heads
  • Mature larvae range in size depending on species
  • Adults are black or brown beetles with a characteristic long, probing mouthpart called a snout
  • They feed on strawberry leaves causing characteristic c-shaped notches on the leaf edge
  • The injury alone is not serious, but it indicates a potential problem with the larval feeding next year

Often Confused With
White grubs
Winter injury
Root or crown disease

Period of Activity
Root weevils overwinter as larvae in soil. Larvae feed extensively on plant roots in spring. Adults begin to emerge from the soil during harvest. Adults are in the field throughout and early fall July and early August. Peak emergence and egg laying by adults occurs in late July through mid August. Root damage is not usually evident until the next spring. Although there is only one generation a year, populations can build rapidly within two years of planting.

Scouting Notes
Scout fields in spring through bloom for areas of stunted growth. Carefully dig up the roots of a plant about 15 cm (6 in.) into the soil and look for grubs. If grubs are found, control measures should be taken after harvest when adults emerge. In mid to late summer look every 1- 2 weeks for leaf notching caused by adult feeding. Black vine weevil adults can cause extensive and obvious damage to leaves. However, not all species of root weevils cause noticeable notching.

Thresholds
None established.

Advanced

Scientific Names
Otiorhynchus ovatus, Otiorhynchus sulcatus

There are several species of root weevils that feed on strawberries. The most common in Ontario are the black vine weevil (O. sulcatus) and the strawberry root weevil (O. ovatus).

Identification
Weevil larvae, or grubs, feed on strawberry roots. They are found in the soil around the plant or imbedded in the crown. Young larvae are very small and difficult to detect. Mature larvae range in size depending on species. They are cream-coloured, or pinkish-white, legless, with c-shaped bodies and brown heads.

Adult weevils have a characteristic long, probing mouthpart called a snout. The adults are wingless and migrate by crawling across land. They feed at night and chew c-shaped notches on edges of strawberry leaves. The leaf injury alone is not serious, but it indicates a potential problem with the larvae next year.

 

Distinguishing Root Weevils
  Adults Larvae
BLACK VINE WEEVIL Black, 8- 11 mm (1/3- 1/2 in.) long with beige short hairs on wing covers Up to 8 mm (1/4 in.) long
STRAWBERRY ROOT WEEVIL Brown with reddish brown legs, 5- 8 mm (1/5- 1/3 in.) long 3- 5 mm (1/8- 1/5 in.) long

Often Confused With
White grubs (Both feed on plant roots as larvae. White grub larvae have legs and claws and they are found in new plantings after sod or where grassy weeds are high risk. Root weevil larvae are legless and more likely found in in older plantings.)
Winter injury (In spring, plants are stunted, or slow to develop.)
Root or crown disease (Symptoms of stunted growth and eventual collapse of plants occurs with root and crown diseases as well as with root feeding from weevil larvae.)

Period of Activity
Root weevils overwinter as larvae in soil. Larvae feed extensively on plant roots in spring. Adults begin to emerge from the soil during harvest. Adults are in the field throughout July and early August. Peak emergence and egg laying by adults occurs in late July through mid August. Eggs hatch in about 10 days under ideal conditions. Dry soil conditions can delay egg hatch. Very small larvae are present in late summer and fall. Root damage is not usually evident until the next spring. Although there is only one generation a year, populations can build rapidly within two years of planting.

Biology
Most root weevils overwinter as larvae in the soil. Root feeding resumes in the spring as soil temperatures become warmer. Feeding occurs usually within 2 to 40 cm (4/5- 16 in.) of the soil surface. Towards the end of May, larvae pupate in earthen cells below the plant. Adult weevils emerge early to mid-June. Since they cannot fly, they crawl up the host plants to begin feeding. Strictly a night feeder, adult weevils hide in leaf litter on the soil surface, or on stems during the day. When a host plant is disturbed, weevils fall to the ground and lie motionless for a while. After a period of feeding of 10- 14 days adults begin to lay eggs. One female can lay up to 150- 200 eggs in the soil. Eggs hatch within about 10 days and larvae feed on roots until the fall. There is generally one generation per season. However, late emerging adults may overwinter.

Scouting Notes
Usually root weevils move into a field from the edge. Sometimes they move in along irrigation pipes or other sheltered conditions.

Examine strawberry plants in the early spring if you notice patches of stunted growth. Carefully dig up the roots of a plant about 15 cm (6 in.) into the soil and look for larvae. If grubs are found, control measures should be taken after harvest when adults emerge. As the season progresses, dig to determine the presence of pupae and newly emerging adults.

In mid-to-late summer look every 1- 2 weeks for leaf notching from adults feeding on foliage.

Thresholds
None established. Healthy plants can tolerate feeding from 1- 2 larvae per plant. Black vine weevil larvae cause more damage than strawberry root weevil larvae, therefore thresholds should be higher for strawberry root weevil larvae.

Assessment of leaf notching in late summer could also be used to predict problems with black vine weevil the following spring. Some researchers have suggested that leaf notching on 25- 50% of the leaves is a problem. Not all root weevil species cause leaf notching however.

Management Notes

  • Root weevils can be a serious pest on strawberries. Damage can be extensive; a pocket of damage in a corner of the field can expand and cause devastating losses the following year. Recognizing root weevils and their damage is the first step in preventing this problem.
  • Till the soil between the strawberry rows at renovation to expose root weevil populations and reduce their numbers.
  • After harvest, apply an insecticide labeled for root weevil adults in strawberries. There are no labeled insecticides for control of larvae.
  • Spray after renovation, at night, when adults are most active.
  • Where weevils are a problem do not narrow the rows with gramoxone, because the plant debris and undisturbed soil will favour weevil activity.
  • Disc infested fields as soon as possible after harvest, but leave a trap row or two of the old planting at the edge of the planting to prevent mass exodus from the field.
  • Do not establish new plantings near older, infested plantings.
  • Beneficial nematodes can be applied to the soil for root weevil control. Field trials have proven this method to provide fair to good control, depending on application timing and conditions.