Weevils (Alfalfa and Egyptian Alfalfa)

Two identical-looking weevils infest alfalfa in California. In general, they are distinguished by their biology and distribution in the state. The alfalfa weevil has tended to be an annual pest in alfalfa districts east of the Sierra Nevada mountains and in the northernmost counties of California. In most other areas of California (e.g., the intermountain areas) it may have been displaced by the Egyptian alfalfa weevil. The weevils are serious pests due to the lack of natural enemies that can effectively control them, especially the Egyptian alfalfa weevil.

Adult weevils of both species are dark gray and about 0.2 inch. The legless larva of the alfalfa weevil is about 0.25 inch when fully grown. It is pale green with a thin white line down the center of the back and has a brown head. Larvae complete their growth in about 3 to 4 weeks. They will then spin a cocoon and pupate either in the leaves of the plant or on the ground.

Both weevils spend the summer as adults under the loose bark of trees, especially eucalyptus, or in any place they can wedge their bodies, such as in rough-barked trees (walnut) or under shake shingles on homes.

Alfalfa Weevil

Alfalfa weevil overwinters as an adult in field trash or other secluded hiding places and emerges in late winter or early spring. Soon after emergence and mating, the adult females begin inserting their eggs into the alfalfa stems. Hatching larvae make their way up the stem to feed on alfalfa terminals and drop to spin a cocoon and pupate by early summer. This species generally has only one generation a year.

Egyptian Alfalfa Weevil

In late fall or early winter, Egyptian alfalfa weevils migrate to alfalfa fields. Soon after entering the fields, adult females begin inserting their eggs into the stems of alfalfa, and hatching larvae make their way into the alfalfa terminals. Egyptian alfalfa weevil may be found in the field throughout the year, although damage is most serious in spring. This species usually has one generation, but a smaller second generation has been documented in the southern San Joaquin Valley.

Young larvae damage alfalfa by feeding on terminal buds; larger larvae feed on the leaflets. Feeding by older larvae is the most damaging and is characterized as skeletonization and bronzing of the leaves in spring. Under severe pressure, complete defoliation can occur. Damage from both weevils is most commonly seen before the first cutting. However if weevil damage continues after the first cutting, Egyptian alfalfa weevil is more likely to cause significant damage to the second cutting (and occasionally the third cutting) than alfalfa weevil.

Adult weevils feed on alfalfa but generally do not cause significant damage.

Weevil management in alfalfa is focused on the period before the first cutting. Control options are insecticides and early harvest. Biological control is not effective at preventing economic damage in most areas because populations of natural enemies are not sufficient to provide control in the spring.

Alfalfa weevils have natural enemies, but they are not effective enough to provide good control early in the season when most needed.  Two parasitic wasps, Bathyplectes curculionis and Bathyplectes anurus, have been introduced into California for control of the larval stage of the alfalfa weevil and the Egyptian alfalfa weevil. Bathyplectes curculionis is present throughout the range of both alfalfa weevil species in California. It can provide up to 30% parasitism of larvae late season, but is frequently encapsulated and killed by the Egyptian strain. Bathyplectes anurus has become established in Central Valley alfalfa as well as other locations; however, at the present time it is only found at very low levels. Microctonus aethiopoides, a parasite of the adult weevil, was established and had been recovered from some counties in California in the past, but recent studies indicate that the parasite is absent or present at very low levels throughout the state and does not provide adequate weevil control. The parasitoid Oomyzus incertus is very effective, with up to 50% parasitism of the weevil larvae late season, which, together with B. curculionis, likely explains the smaller second generation, as appears to be under good biocontrol.

An alfalfa weevil-specific fungus occurs in many alfalfa-growing regions in California that aids in biological control. This soil-dwelling fungus (Zoophthora phytonomi) sporulates and infects the larval stage, causing death of weevil larvae within days of infection, providing up to 30% weevil suppression. In some regions in California, the fungus maintains numbers of weevils below the economic threshold of 20 per sweep and may help minimize the need to apply a pesticide for the weevil. However, the fungus infection level depends on sustained soil moisture—in dry years there is little infection.

After alfalfa weevil larvae begin to appear, check fields at 2- to 4-day intervals. Cutting the crop as soon as most of the plants are in the bud stage can sometimes prevent serious damage by the weevil. Also, the process of harvesting and curing the alfalfa kills most weevils. However, early cutting to control weevils concentrates the survivors in the windrows. Closely monitor alfalfa regrowth for the second cutting to detect feeding damage because both larvae and adults can cause injury.

The primary organically acceptable management method is cutting the crop early if damage seems imminent. Sprays of the Entrust formulation of spinosad are also acceptable to use on an organically certified crop.

• Begin monitoring for weevils in early January in southern and central areas of the state and in April in the far northern intermountain area. If the alfalfa is too short to sweep, look for signs of feeding damage on the leaves.
• Sweep fields with adequate plant height weekly after weevil larvae begin to appear in late winter or early spring. Take five sweeps in four sections of the field.
• As thresholds are approached, monitor every 2 to 4 days to determine if numbers decline or a pesticide application is required. (For details on sweep net sampling, see SAMPLING WITH A SWEEP NET.)
• Record your observations on a monitoring form (PDF). Research is underway to reevaluate threshold levels, but currently the recommendation is that a pesticide application is warranted when the weevil larvae count reaches an average of 20 or more larvae per sweep.
• Continue to monitor weekly:
  • Central Valley: typically late winter through early spring or after a pesticide application (occasionally a second generation can carry into late spring).
  • Southern deserts: January through March.
  • Northern intermountain areas: mid- April through early-June.

In some situations early harvest can be used to manage larval numbers when they reach damaging levels. This tactic minimizes the killing of predators and parasites of aphid pests by pesticides. However, before making a decision to harvest early, consider stand vigor and economic practicality.