Alfalfa Weevils, Insecticides & Resistance: Update from the Western US
Kevin Wanner, Montana State University, email@example.com
Ian Grettenberger, University of California Davis, firstname.lastname@example.org
Michael Rethwisch, University of California Cooperative Extension, email@example.com
Alfalfa weevil (Hypera postica) resistance to pyrethroid insecticides is now well established in the western US
Summary: In areas of high pyrethroid resistance, the most commonly
used insecticides no longer provide adequate control, including products
such as Warrior II, Mustang Maxx, Baythroid, Fastac, Proaxis & many
generic formulations. These products are based on pyrethroid active
ingredients that belong to Mode of Action Group 3A (MoA3A).
Chlorpyrifos (Cobalt, Lorsban & generic formulations, MoA1B) has
been discontinued and it is no longer available for alfalfa weevil control.
Rotating the insecticide mode of action (MoA), applying insecticides
only when economic thresholds have been met based on monitoring, and
alternative non-chemical options such as early harvest, are the current
primary recommendations for slowing further insecticide resistance.
Insects tend to develop resistance to a specific MoA Group when those
active ingredients are used repetitively – rotating the MoA slows
resistance. With the loss of chlorpyrifos, the only insecticide with a
different MoA that is currently available and known to be effective is
Steward, whose active ingredient is indoxacarb (MoA22A).
Resistance Management: The good news - many areas remain susceptible to the pyrethroid insecticides. In these
areas, we are initially recommending that MoA3A pyrethroid insecticides be used at most only once every three
years to preserve their effectiveness. Methods to reduce insecticide applications based on Integrated Pest
Management (IPM) should be used as well when possible.
1) Rotate insecticide MoA. Currently, only two effective MoA Groups are available for rotation, MoA3A
(pyrethroids) and MoA22A (Steward) – use Group 3A once every three years, and Group 22A once
every three years, at most. If multiple applications are required in the same season, use the same MoA
within that year. Areas with high pyrethroid resistance are an exception, where Steward is the only
effective option until resistance levels decline.
2) Maximize the effectiveness of insecticide applications. Calibrate spray equipment, apply the high end of
the label rate, optimize spray coverage, and monitor insects and weather for best timing and control.
These practices reduce the exposure of insects to “sublethal doses” that can favor and speed up
development of resistance. Also, higher rates can compensate for variable application conditions.
3) Monitor for economic thresholds - No insecticide application unless thresholds are exceeded. Apply
insecticide only when necessary, based on monitoring larval populations and their numbers exceeding
economic thresholds (check with local Extension for thresholds and monitoring protocols in your area).
Do not mix insecticides with early herbicide applications as routine practice.
4) Maintain healthy alfalfa stands. Healthy stands can better tolerate feeding damage. Follow agronomic
best practices and ensure that your stand goes into the spring weevil season as vigorous as possible.
5) Harvest early. The most common non-insecticidal option is to harvest early when weevil populations
exceed economic thresholds and the crop is within 7-10 days of normal harvest time (and the weather is
favorable for rapid curing). Harvesting early kills many larvae but in some conditions enough survive to
damage regrowth; monitor regrowth to determine if insecticide applications are required.
6) Determine resistance levels. Apply test strips using backpack, ATV or commercial ground spray
equipment. Compare your pyrethroid spray to Steward and an untreated strip. Sample the strips 7-10
days after spray application using a standardized method (sweep net, shake-bucket) and count the
number of larvae. Lack of control by the pyrethroid (MoA3A) and good control from Steward
(MoA22A) may indicate a pyrethroid-resistant alfalfa weevil population (Fig. 1).
7) Know your active ingredient, MoA Group and product name. Permethrin, alpha-cypermethrin, betacyfluthrin,
gamma-cyhalothrin, lambda-cyhalothrin and zeta-cypermethrin are all MoA3A pyrethroid
insecticides available commercially as a variety of product names.
Field Trial Results: In recent trials near Blythe CA, the high rate of lambda-cyhalothrin (Warrior II, MoA3A)
reduced larval populations by only 50-60% compared to Steward (MoA22A), which reduced populations more
than 94%. Before the development of resistance (and in areas that remain susceptible) lambda-cyhalothrin also
reduced alfalfa weevil larvae by more than 94% (Arthropod Management Tests, https://doi.org/10.1093/amt/32.1.F4
and https://doi.org/10.1093/amt/21.1.197). Fig. 1 summarizes recent field trial data from four western states, with two
rates of Steward compared to the high label rate of Warrior II. Images of a larva (white stripe along the back
and black head), larval feeding damage, and the adult weevil are illustrated in Fig. 2.
Fig. 1. Percentage control (% reduction
compared to untreated plots) of alfalfa weevil
larvae 6 – 14 days after applying Steward at
6.7 and 11.3 oz / acre and Warrior II at 1.92
oz / acre. Utah (UT) data cited from
Arthropod Management Tests (Price, Gale &
Ramirez 2020) https://doi.org/10.1093/amt/tsaa094.
*Known pyrethroid-resistant population.
** Probable pyrethroid-susceptible
Steward: The low and high rates of Steward generally perform well when applied within two weeks of harvest
or within two weeks of the weevil population declining naturally. However, in some cases effectiveness has
been reported to be below 90% control; further field trials will evaluate factors such as timing and spray
coverage. Timing and rates of Steward may be more important in areas where weevil pressure is prolonged over
1-2 months. Factors to consider:
Ingestion is an important route of insect exposure for Steward. Spray coverage is important. Spreading and
sticking adjuvants are recommended along with higher spray volumes (15-20 gal/acre).
Steward does not have plant systemic activity. New plant growth will not have insecticide protection.
Preventing the development of alfalfa weevil resistance to Steward is critical to its future management in
forage alfalfa. Avoid routine yearly applications of Steward.
Inclusion of a common chemical or trade name does not imply endorsement of that product or brand of pesticide. Read and follow all
product labels carefully and contact the manufacturer with any product specific questions. Funding provided in part by the USDA
NIFA Alfalfa Seed and Alfalfa Forage Systems Program and the National Alfalfa Forage Alliance checkoff program.