Forages: Alfalfa Weevil

Excerpt from Agronomy Guide for Field Crops (Chapter 5)

Order OMAFRA Publication 811: Agronomy Guide for Field Crops

Table of Contents

1. Description
2. Life History
3. Damage
4. Scouting Technique
5. Action Thresholds
6. Management Strategies
7. Updates on Forages: Alfalfa Weevil
8. Related links...

Description

The adult weevil is a brown snout beetle, about 5 mm long, with a dark brown stripe extending from the head down the centre of the back. Larvae are bright green with a black head, six legs and a distinctive white stripe down the centre of the back. At full size, they are about 8 mm long.

Plate 92. Alfalfa weevil adults are about 8 mm long. Note the dark stripe running along the back.

Plate 93. Alfalfa weevil larvae feed on leaves, causing skeletonization. Note: Larvae have black heads.

Do not confuse the alfalfa weevil larvae with the clover leaf weevil larvae, which are larger, have a light brown head instead of a black head and a white stripe edged with pink. Also, do not confuse alfalfa weevil larvae with predacious fly larvae, which have no head capsule and are legless.

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Life History

Adults overwinter in plant debris and emerge in spring to feed on new alfalfa growth and lay their eggs in alfalfa stems in May. Larvae hatch from eggs and crawl to the tops of alfalfa where they feed on the developing leaf and flower buds. After feeding, larvae form loosely woven white cocoons in leaf masses and enter the pupa stage, usually in late June or early July. Pupa hatch in 1 or 2 weeks into the adult stage.

Damage

The larvae cause most of the damage as they feed within the leaf buds and then move to the tips of the plant. Damage starts out as pinholes and progresses to feeding between the leaf veins, resulting in a skeletonized appearance. In heavy infestations, larvae shred the leaves so badly that fields take on a greyish-white or frosted appearance. Adult feeding throughout the summer does not cause significant damage.

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Scouting Technique

Examine each field twice a week from mid-May to June. Check several areas throughout the field. Look for damage to show up first on shallow soils or on southerly slopes, particularly during warm, dry springs. Experience in Ontario has shown that the peak of larval attack usually coincides with the bud stage of the first crop. To count larvae, collect 30 stems in an M-shaped pattern. Place them inside a white pail and beat them against the side to knock off the third-to-fourth-stage instar larvae. First and second instars are smaller (3 mm or less), pale yellow-to-light green, with the white stripe not yet distinguishable. They may be in the upper leaves but do not include these younger larvae in the count. Check to see whether the weevil larvae look active and healthy. Larvae infected by the fungus pathogen are slow-moving, yellow or tan larvae.

Action Thresholds

Leaf tip damage and weevil counts are used in assessing threshold levels and appropriate action of either cutting or insecticide application. If there is 40% leaf tip feeding, with two or three active weevils per stem, and there is more than 7-10 days to the preferred harvest date, consider applying an insecticide. Leaf tip feeding refers to the percentage of plant tips showing obvious signs of damage, which is not to be confused with the percent defoliation.

• Less than one active larva per stem does not require action, however, continue to monitor the situation.
• If the alfalfa is less than 30 cm (12 in.) high with more than one larva per stem, action is required.
• Two larvae per stem requires action if the alfalfa is less than 40 cm (16 in.) high.
• If there are more than three active larvae, immediate action is required.

Occasionally, if weevil populations are high on a late first cut, surviving larvae will feed on the regrowth. Such feeding can eliminate alfalfa regrowth which may lead to a loss of the stand. With a severe infestation, be sure to monitor stubble regrowth. The characteristic symptom is the alfalfa plant not "greening up" due to weevils feeding on the developing crown buds. The presence of two or more active larvae per crown, or 4-8 larvae/ft2 indicates a need to spray the stubble with insecticide.

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Management Strategies

Fungal disease and several species of parasites have been established in Ontario and in most years, forages stands do not warrant treatment with insecticides. The key to weevil control is proper timing of harvest or insecticide application based on field inspection. When threatening infestations occur, fields should be cut immediately to eliminate feeding damage. The bud stage is also the best time to cut the crop for maximum quality. Occasionally, warm May weather will result in an early hatch of weevil and feeding damage will show before it is practical to harvest the alfalfa. In those situations, an insecticide may be warranted. Refer to OMAFRA Publication 812, Field Crop Protection Guide, for insecticide information. Insecticides for alfalfa weevil may also control other pests such as potato leafhopper and lygus bug. However, it should be noted that spraying insecticides on alfalfa will also kill beneficial insects, the natural enemies of these pests. This increases the potential for future outbreaks of these pests. (Order OMAFRA Publication 812, Field Crop Protection Guide)

Updates on Forages: Alfalfa Weevil

• Field Crop Protection Guide - Publication 812
  • Forages: Alfalfa Weevil

Related links...

• Insects
• Forages

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