Root Weevils

Table of Contents

1. Introduction
2. Host Plants
3. Description
4. Damage
5. Life Cycle
6. Control
7. References

Introduction

Strawberry root weevil, O. ovatus and black vine weevil, Otiorhynchus sulcatus, can be problems both in the nursery and the landscape. While these pests are generally not considered a significant problem, high populations can cause conspicuous damage. Because of the similarities in their life cycles, they will both be described here.

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Host Plants

Both adults and larvae feed on a wide range of host plants.

• Taxus, Japanese yew;
• Tsuga, hemlock;
• Thuja, white cedar;
• Pinus, pine;
• Picea, spruce;
• Euonymus;
• Rhododendron;
• and many small fruits as well.

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Description

The adult strawberry root weevil (SRW) and black vine weevil (BVW) have an extended mouth part forming a "snout". The adult SRW is about 5 to 6 mm in length and brown in colour. BVW adults are black and 9-13 mm in length. Neither fly, and their hard outer wing cases, or elytra, have rows of small round pits in them. In both species, no males exist. Weevils develop from unfertilized eggs through parthenogensis. The larvae of both weevils are similar. They are small, legless grubs, nearly 1 cm in length with a brown head.

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Damage

Adults feed on the margins of foliage, giving the plant a rather ragged appearance. SRW feeds on foliage but can girdle small branches of some species such as Thuja or Picea. BVW leaves a characteristic "C" shaped notch in leaf margins. Generally injury begins close to the ground and towards the inside of the plant. Weevil larvae are root feeders, feeding both on fine roots and also at the base of the stem. Infested plants grow slowly or fail to grow. Infected, newly planted stock dies without becoming established.

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Life Cycle

SRW and BVW overwinter in the ground as partially developed larvae. Root feeding resumes in the spring as soil temperatures become warmer. Feeding occurs usually within 2 to 40 cm of the soil surface. Towards the end of May, larvae pupate and the 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, 10-14 days for SRW and 30 days for BVW, 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.

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Control

Control strategies should be aimed at the adults. Larvae are difficult to control with chemicals presently available. Sprays applied to foliage in the evening are more effective in controlling adults. In a recent study at the University of Massachusetts, burlap traps were used to establish when adults emerged and control programs should begin. A 3' x 4' sheet of burlap, gathered lengthwise to create folds, is wrapped around the base of plants suspected of being infested. Adult beetles will hide in the burlap during the day.

To determine the time of adult emergence, place several traps in different locations in the field. Traps should be inspected once or twice a week. In order to determine the severity of the infestation increase the number of traps per area. The more traps used, the more accurate the estimation. If infestations appear to be isolated, spot treating those plants can be an option. Treatments for adult control can be started when Spiraea x vanhoutei finishes blooming (3).

For recommended chemical controls, consult the Ontario Ministry of Agriculture, Food and Rural Affairs Publication 383, Production Recommendations for Nursery and Landscape Plants.

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References

1. Johnson W. T. and H. H. Lyon 1976. Insects that Feed on Trees and Shrubs. Cornell University Press, Ithica N.Y.

2. Mulgrew S. M. 1991. Improved Black Vine Weevil Trapping. Nursery Notes. Ohio Cooperative Extension Service 26 (7).

3. Orton D. A. 1989. Coincide: the Orton System of Plant Management. Plantsmen's Publication. IL.

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