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Managing weevil pests in alfalfa hay

Rachael Long and Ian Grettenberger for Progressive Forage Published on 29 April 2019
Weevil damage

Alfalfa weevils are key pests of alfalfa across the U.S. Larvae feed on the foliage, causing yield and forage-quality losses to the first and sometimes second hay cutting. These weevils can be challenging to control.

There are no resistant plant varieties, biocontrol is limited in much of its range, and cultural practices have significant trade-offs with yield and forage quality. For example, overseeding fields with other forages not preferred by weevils (such as berseem clover or oats) makes up for a loss in alfalfa damage (by weevils) but changes the forage quality and marketability of the hay.

Alfalfa weevils have one to two generations per year: a primary one in late winter or early spring (depending on field location) and sometimes a smaller second one in late spring or early summer, which can be heavily attacked by parasitoid wasps (Figure 1).

Alfalfa weevils have up to two generations per year

Adults become active at the start of the growing season, fly back into fields and lay eggs in old alfalfa stems. When eggs hatch, larvae feed and create holes in buds and leaves, producing tattered foliage. Stubble fields that are beginning to break dormancy are most at risk of injury because weevils can feed on developing buds; once the alfalfa is growing, it is more resilient. Adult weevils become dormant in the summer. Some remain in the field, whereas others leave and aggregate in protected areas.

The primary management tool for managing alfalfa weevils are insecticides. However, insecticides are becoming more difficult to use with increased restrictions for chlorpyrifos (e.g., Lorsban) and the development of insecticide resistance. Weevils are clearly resistant to pyrethroids in some areas of California, and there are reports of resistance in other Western states and in Canada (Alberta).

Because pyrethroids have been effective against weevils and often relied upon, this makes management problematic. In the intermountain and low desert areas of California, some growers are only getting 20 percent weevil control or less with pyrethroids (e.g., Warrior). Some populations are resistant to multiple pyrethroid materials.

Through a grant with the California Department of Pesticide Regulation, we are examining other materials for weevils but have not found ones as effective as Lorsban, Warrior or Steward (Table 1).

Insecticides evaluated in alfalfa hay in California

This increases the necessity of implementing management practices that extend the useful life of current insecticides, including rotation of different available modes of action. For example, one might use indoxacarb (Steward) one year and a pyrethroid the following. Steward does not control aphids, so a separate material may be necessary for aphid control.

To time insecticide treatments and manage weevils, alfalfa fields should be monitored. Typically, sampling entails sweep net or stem-and-bucket sampling, with thresholds and sampling methods available from local university extension. Sprays applied to fields too early may need a second treatment, which is costly, and could contribute to insecticide resistance problems.

Watch for plant damage in stubble fields that cannot be sampled with a sweep net. If there is weevil damage to an alfalfa stand, controlling the weevils and then allowing the plants to outgrow weevil damage before harvesting should help avoid yield losses from the damage.  end mark

PHOTO: Weevil damage results in yield and quality loss. Photo provided by Rachael Long.

Ian Grettenberger is an entomology specialist with University of California – Davis Extension.

Rachael Long ia a farm adviser at Yolo County UCCE. Email Rachael Long.