How to Manage Pests
UC Pest Management Guidelines
Weevils (Alfalfa and Egyptian Alfalfa)
Alfalfa Weevil: Hypera postica
Two 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 one generation per year in most of its California range; a small area in the Central San Joaquin Valley is documented to have a second annual generation.
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, 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.
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 EAW. 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 suppressed alfalfa weevil populations in the mid-coastal area.
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.
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, most weevils are killed by the harvest and curing process. 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.
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. 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.
|Common name||Amount per acre**||REI‡||PHI‡|
|(Example trade name)||(hours)||(days)|
|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. Not all registered pesticides are listed. 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.|
|(Lorsban Advanced)||1–2 pt||24||See comments|
|MODE-OF-ACTION GROUP NUMBER1: 1B|
|COMMENTS: Will also kill aphids. Do not apply when bees are present. Avoid drift and tailwater runoff into surface waters. Preharvest interval is 14 days for 1 pt/acre and 21 days for more than 1 pt/acre. Certain formulations emit high amounts of volatile organic compounds (VOCs); use low-VOC formulations. Regulations affect use for the San Joaquin Valley from May 1 to October 31, 2015 and 2016. Review the Department of Pesticide Regulation's updated fact sheet .|
|(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. Where populations of 10 or 15 larvae per sweep are present at the time of cutting, malathion at 1.5 pt/acre can be applied as an under-the-windrow treatment. This treatment reduces the larval population 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 treatment 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). For additional information, see their Web site at http://www.irac-online.org/.|
|#||Acceptable for use on an organically grown crop.|
|*||Permit required from county agricultural commissioner for purchase or use.|
UC IPM Pest Management Guidelines: Alfalfa
UC ANR Publication 3430
L. D. Godfrey, Entomology, UC Davis
E. T. Natwick, UC Cooperative Extension, Imperial County
P. B. Goodell, Entomology, Kearney Agricultural Center, Parlier
R. F. Long, UC Cooperative Extension, Yolo County
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