Description of the Pest
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 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.
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. Before the Egyptian alfalfa weevil invaded California and spread into most of the areas occupied by the alfalfa weevil, B. curculionis effectively reduced the numbers of alfalfa weevil in the mid-coastal and likely intermountain areas.
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 fields with sustained moisture, this soil-dwelling fungus (Zoophthora phytonomi) sporulates and infects the larval stage, causing death of weevil larvae within days of infection. 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.
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.
Organically Acceptable Methods
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.
Monitoring and Treatment Decisions
- 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 . 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.
|Common name||Amount per acre**||REI‡||PHI‡|
|(Example trade name)||(hours)||(days)|
|Not all registered pesticides are listed. The following are ranked with the pesticides having the greatest IPM value listed first—the most effective and least harmful to natural enemies, honey bees, and the environment are at the top of the table. When choosing a pesticide, consider information relating to air and water quality, resistance management, and the pesticide's properties and application timing. Always read the label of the product being used.|
|(Steward EC)||6.7–11.3 fl oz||12||7|
|MODE-OF-ACTION GROUP NUMBER1: 22A|
|COMMENTS: Make no more than one application per cutting. Do not apply when bees are in the area. Not for use in alfalfa grown for seed or for sprouts for human consumption.|
|(Warrior II with Zeon)||1.28–1.92 fl oz||24||See comments|
|MODE-OF-ACTION GROUP NUMBER1: 3A|
|COMMENTS: Preharvest interval (PHI) is 1 day for forage and 7 days for hay. Do not apply when bees are actively foraging.|
|(Baythroid XL)||1.6–2.8 fl oz||12||7|
|MODE-OF-ACTION GROUP NUMBER1: 3A|
|COMMENTS: Not for use in alfalfa grown for seed. Do not apply more than 2.8 fl oz/acre per cutting or more than 11.2 fl oz/acre per season.|
|(Malathion 8-E)||1–1.25 pt||12||0|
|MODE-OF-ACTION GROUP NUMBER1: 1B|
|COMMENTS: Use only when other products cannot be used. Do not apply when bees are present. In fields where 10 or 15 larvae per sweep are present at the time of cutting, malathion at 1.5 pint/acre can be applied as an under-the-windrow spray. This reduces the larval numbers as it filters down to the stubble from windrowed alfalfa. Thus, feeding damage to new regrowth is reduced and the alfalfa's vigor is preserved. Under-the-windrow spray requires mounting a spray unit on the swathing machine.|
|(Entrust SC)#||2–4 fl oz||4||See comments|
|MODE-OF-ACTION GROUP NUMBER1: 5|
|COMMENTS: Preharvest interval (PHI) is 0 days for forage and 3 days for hay and fodder.|
|**||See label for dilution rates.|
|‡||Restricted entry interval (REI) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (PHI) is the number of days from treatment to harvest. In some cases the REI exceeds the PHI. The longer of two intervals is the minimum time that must elapse before harvest.|
|1||Rotate chemicals with a different mode-of-action group number, and do not use products with the same mode-of-action group number more than twice per season to help prevent the development of resistance. For example, the organophosphates have a group number of 1B; chemicals with a 1B group number should be alternated with chemicals that have a group number other than 1B. Mode-of-action group numbers are assigned by IRAC (Insecticide Resistance Action Committee).|
|#||Acceptable for use on an organically grown crop.|
|*||Permit required from county agricultural commissioner for purchase or use.|