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How to Manage PestsUC Pest Management Guidelines
DESCRIPTION OF THE PESTSTwo identical-looking weevils infest alfalfa in California. They are distinguished by their biology and distribution in the state. The alfalfa weevil is 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, it has been displaced by the Egyptian alfalfa weevil, which is a far more serious pest. Adult weevils of both species are dark gray and about 0.20 inch long. The legless larva of the alfalfa weevil is about 0.25 inch long 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. 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, and 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 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. In late fall or early winter, they emerge and 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 has three to four generations a year and may be found in the field throughout the year, although damage is most serious in spring. DAMAGEYoung 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, while alfalfa weevil may occasionally damage the second or third cutting, Egyptian alfalfa weevil is more likely to cause significant damage to the second cutting and occasionally the third cutting if a second generation occurs. Adult weevils feed on alfalfa but generally do not cause significant damage. MANAGEMENTWeevil 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.
Biological Control 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. An alfalfa weevil-specific fungus occurs in many alfalfa growing regions in California that aids in biological control. In years experiencing heavy rainfall, this soil-dwelling fungus (Zoophthora phytonomi) sporulates and infects the larval stage, causing death of weevil larval within days of infection. In some regions in California, the fungus maintains weevil populations below the economic threshold of 20 per sweep and may help minimize the need to chemically treat for the weevil.
Cultural Control
Organically
Acceptable Methods
Monitoring
and Treatment Decisions Sweep fields with adequate plant height weekly after weevil larvae begin to appear in late winter or early spring. As thresholds are approached, monitor every 2 to 4 days to determine if populations decline or a treatment is required. (For details on sweep net sampling, see SAMPLING WITH A SWEEP NET.) Record your observations on a monitoring form . Research is underway to reevaluate threshold levels, but currently the recommendation is that a treatment is warranted when weevil larvae count reaches an average of 20 or more larvae per sweep. Continue to monitor weekly during the spring through June or after a treatment through in the Central Valley, March in the southern deserts and mid-June in the northern intermountain areas. In some situations early harvest can be used to manage larval populations 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.
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