How to Manage Pests

UC Pest Management Guidelines

Alfalfa

Weevils (Alfalfa and Egyptian Alfalfa)

Alfalfa Weevil: Hypera postica
Egyptian Alfalfa Weevil: Hypera brunneipennis

(Reviewed 11/06, updated 4/08, corrected 3/13)

In this Guideline:


DESCRIPTION OF THE PESTS

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.

DAMAGE

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.

MANAGEMENT

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.

Biological Control

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.

Cultural Control

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.

Organically Acceptable Methods

The primary organically acceptable management method is cutting the crop early if damage seems imminent.

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. 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 (PDF). 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/Acre** R.E.I.+ P.H.I.+
(trade name)   (hours) (days)

  Calculate impact of pesticide on air quality
The following materials are listed in order of usefulness, taking into account efficacy and impact on natural enemies and honey bees. When choosing a pesticide, also consider environmental impact. Not all registered pesticides are listed. Always read label of product being used.
 
A. INDOXACARB
  (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. Do not apply more than 45 fl oz/acre/crop season.
 
B. PHOSMET
  (Imidan) 70W 1 lb 120 14
  MODE OF ACTION: An organophosphate (Group 1B)1 insecticide.
  COMMENTS: Do not apply more than once/cutting or during bloom period.
 
C. LAMBDA-CYHALOTHRIN*
  (Warrior) 2.56–3.84 fl oz 24 1 - forage; 7 - hay
  MODE OF ACTION: A pyrethroid (Group 3)1 insecticide.
  COMMENTS: Do not apply when bees are actively foraging. Do not apply more than 0.24 pt/acre/cutting or 0.96 pt/acre/season
 
D. CYFLUTHRIN*
  (Baythroid) 2E 1.6–2.8 fl oz 12 7
  MODE OF ACTION: A pyrethroid (Group 3)1 insecticide.
  COMMENTS: Not for use in alfalfa grown for seed. Do not apply more than 2.8 fl oz/acre/cutting or more than 11.2 fl oz/acre/season.
  ...OR...
  (Renounce) 20WP 2–4 fl oz 12 7
  COMMENTS: Do not apply to alfalfa grown for seed. Do not apply more than 4 oz/acre/cutting or more than 16 oz/acre/season.
 
E. CHLORPYRIFOS*
  (Lorsban) 4EC 1–2 pt 24 see comments
  MODE OF ACTION: An organophosphate (Group 1B)1 insecticide.
  COMMENTS: Will also kill aphids. Do not make more than 4 applications/year or apply more than once/cutting. 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.
 
F. MALATHION 8E 1–1.5 pt 12 0
  MODE OF ACTION: An organophosphate (Group 1B)1 insecticide.
  COMMENTS: Use only when other products cannot be used. Do not apply when bees are present. Where populations of 10 or 15 larvae/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.
**   See label for dilution rates.
+ Restricted entry interval (R.E.I.) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (P.H.I.) is the number of days from treatment until the field can be grazed or cut. In some cases the R.E.I. exceeds the P.H.I.; the longer of these two intervals is the minimum time that must elapse before harvest may take place.
* Permit required from county agricultural commissioner for purchase or use.
1 Modes of action are important in preventing the development of resistance to pesticides. 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. 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 is assigned by IRAC (Insecticide Resistance Action Committee). For additional information, see their Web site at http://www.irac-online.org/.

[Precautions]

PUBLICATION

[UC Peer Reviewed]

UC IPM Pest Management Guidelines: Alfalfa
UC ANR Publication 3430

Insects and Mites

  • C. G. Summers, Entomology, Kearney Agricultural Center, Parlier
  • L. D. Godfrey, Entomology, UC Davis
  • M. Rethwisch, UC Cooperative Extension, Riverside County (Blythe)
  • D. R. Haviland, UC Cooperative Extension, Kern County
  • P. B. Goodell, Entomology, Kearney Agricultural Center, Parlier
  • R. F. Long, UC Cooperative Extension, Yolo County

Top of page

PDF: To display a PDF document, you may need to use a PDF reader.


Statewide IPM Program, Agriculture and Natural Resources, University of California
All contents copyright © 2014 The Regents of the University of California. All rights reserved.

For noncommercial purposes only, any Web site may link directly to this page. FOR ALL OTHER USES or more information, read Legal Notices. Unfortunately, we cannot provide individual solutions to specific pest problems. See our Home page, or in the U.S., contact your local Cooperative Extension office for assistance.

Agriculture and Natural Resources, University of California

Accessibility   /PMG/r1300511.html revised: January 8, 2014. Contact webmaster.