Strawberry Clipper Weevil: A Major Pest of Strawberry

Agdex#:	232/620
Publication Date:	09/99
Order#:	99-031
Last Reviewed:	09/99
History:	Replaces Factsheet 92-104, Strawberry Clipper Weevil: A Major Pest of Strawberry
Written by:	A.M. Watson - Pest Management Advisor, Plant Industry Branch; G. Walker - Pest Management Advisor, Plant Industry Branch; P. Fisher - Berry Crop Specialist/OMAFRA

Table of Contents:

1. Biology
2. Damage
3. Monitoring
4. Control

The strawberry clipper weevil (SCW) Anthonomus signatus (Say) is one of two major insect pests that can cause economic losses in strawberry plantings. It is believed to be a pest, native to North America. Alternate hosts are blueberries, wild strawberry, and wild and cultivated brambles.

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Biology

In Ontario, there is one generation of the SCW per year. The adult is a small reddish brown beetle, 3 mm or 1/8th of an inch, in size with a pronounced snout (Figure 1). The SCW overwinters as an adult in protected areas such as fence lines, hedgerows and under mulch.

Figure 1. Clipper weevil on strawberry blossom. Note snout.

The SCW migrates into the strawberry field from its overwintering site when maximum daytime temperatures are above 16°C. This occurs generally about the time when flower buds are visible and beginning to extend from the strawberry crown (well before blossoms are visible). After mating, the female cuts a hole in the flower bud and lays one egg inside. She then partially cuts off the blossom stalk a few centimetres below the bud (Figure 2). This is where the common name the "CIipper Weevil" comes from. The damaged bud will not open; it wilts, turns brown and dries up, providing a place for the egg and larva to develop (Figure 3). Once the egg hatches the larvae develop inside the bud for approximately four weeks. The larvae pupate inside the bud with adults emerging in mid-summer. After emerging, adults feed on pollen from flowers and weeds until the fall when they seek out overwintering sites. A resident population (one that becomes established inside the strawberry field) may develop in plantings older than 3 years.

Figure 2. Close up picture of clipped bud. Note feeding on sepals of bud.

Figure 3. Dried up clipped bud in patch.

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Damage

The major damage caused by the SCW takes place during egg laying. Damaged buds do not open, therefore berries do not form, reducing strawberry yield. In a Quebec study, the percentage of buds attacked ranged from 9%-46%

Small holes in the petals of the strawberry blossom are another sign of SCW activity (Figure 4). This damage occurs before bloom, when the SCW cuts into the unopened buds to feed on pollen. Pollen feeding by SCW adults often occurs a few days prior to egg laying and clipping of buds. The occurrence of pollen feeding in unopened buds can be determined by observing brown to purplish holes in the sepals (green leaf-like structures surrounding the flower blossoms) (Figure 2). The affected flower buds are not necessarily "clipped" and some develop normally. Losses due to pollen feeding are not usually significant and no controls are recommended.

Figure 4. Holes in petals from pollen feeding.

The population pressure and subsequent potential damage by SCW usually increases once the strawberry field is 3 years old.

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Monitoring

Monitoring for SCW is based on looking for and counting the number of clipped buds in a specific area. It should begin once buds start to extend from the crown. A delay in the start of monitoring, especially in fields with a history of damage, can result in increased levels of damage. Check the field at least twice per week. At the beginning of the season, consider monitoring more than twice per week to accurately time the first insecticide spray. As most SCW will be entering the patch from overwintering sites, concentrate monitoring on the outer 2-3 rows of the field closest to suspected overwintering sites.

Figure 5. Monitoring for clipped buds.

The standard way to monitor for SCW is with a 1 foot by 2 foot (30 cm by 60 cm) wooden frame. The frame is placed on a row and all the clipped buds within the frame are counted (Figure 5). A total of 5 locations in the perimeter of the field are checked for clipped buds. Historically an old threshold of 1.3 clipped buds per 2 ft² of row was used. Recent research from New York confirms that the threshold can be raised to at least 13 clipped buds per 2 ft² of strawberry row without causing economic losses. Apply insecticides when a threshold of 13 buds per 2 ft² of strawberry row is reached.

Apparently most varieties grown today compensate for this number of lost buds when injury occurs before the flower truss begins to open by producing larger fruit on the remaining intact flowers. Make occasional checks of interior rows, as a resident population of SCW may be present. Re-examine fields for newly clipped buds 5-7 days after the date of the first insecticide application. When reassessing the need for further insecticides, it is important to only count newly clipped buds. Newly clipped buds will still be green, with only a little browning. Clipped buds, over 5 days old, may still be attached to the stem and will be greenish brown, shriveled and dry or they will have dropped to the ground. Monitor fields continuously for clipped buds every 3 days until the end of bloom, to assess the need for additional controls.

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Control

Cultural

Renovation should take place promptly after harvest. For information on renovation practices, consult OMAFRA Publication. 513, The Strawberry in Ontario. Good weed control, especially the elimination of broadleaved weeds, can reduce the survival of newly emerging SCW adults by eliminating a favoured habitat. A weed-free strawberry field discourages summer pollen feeding and the establishment of overwintering SCW adults inside the strawberry field.

In the final fruiting year, plough down the field immediately after the last picking. Follow this by summer fallowing and crop rotation, especially if you are replanting strawberries into the same field. This helps reduce populations of SCW. Waiting 2-3 years before planting strawberries back into the same field is recommended for insect and disease control.

Chemical

Once the action threshold has been reached, apply an insecticide immediately. A delay in application results in increased damage within a short period of time. Insecticide recommendations are outlined in OMAFRA Publication 360, Fruit Production Recommendations. Do not apply insecticides when bees are working; spray in the evening. Be aware of the number of days from the last spray to harvest for the insecticides.

Note: The repeated use of pyrethroids is discouraged, as they may induce two spotted spider mite problems. Pyrethroids are broad spectrum insecticides and can kill beneficial insects and mites, which in an unsprayed environment would control pests.

In most situations, only one corrective spray is required. Fields with severe SCW pressure may require a second spray, if the action threshold is reached 7 days after the application of the first insecticide. Late SCW damage is often prevented when tarnished plant bug sprays are applied.

The authors wish to thank, the Co-operative Extention Service, Cornell University, New York State for permitting the use of certain photographs. In addition, the authors also wish to thank Agriculture Canada, Vineland Station for the use of photographic facilities.

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