Biocontrol Agent Overview

Common Names


Relative effectiveness

Alfalfa weevil biological control is fairly successful across eastern North America due to a suite of natural enemies which includes Bathyplectes anurus and B. curculionis. This is not the case in the western United States where biological control is often not sufficiently effective at controlling alfalfa weevil. Generally, B. anurus appears to out compete B. curculionis and better control alfalfa weevil except for areas where B. anurus is not well established.

Where to use


Bathyplectes anurus and B. curculionis may be in the field wherever alfalfa and alfalfa weevil are present.

About Bathyplectes anurus and B. curculionis

Adult Bathyplectes anurus and B. curculionis parasitize alfalfa weevil larvae by depositing a single egg into their larval host in spring. The parasitoid egg develops into a larva which survives in the host often until the alfalfa weevil larva has spun a cocoon. The parasitoid larva will then kill and emerge from its weevil host and spin a cocoon of its own for pupation. The number of generations per year and overwintering habits differ between Bathyplectes anurus and B. curculionis.

  • Native/Non-native: Non-native
  • Preferred climate: Temperate
  • Region: Bathyplectes parasitoids can be found anywhere in North America where alfalfa and alfalfa weevil are found.
  • Established: Yes
  • Where established: Bathyplectes curculionis has become well established across North America where alfalfa and alfalfa weevil are found. Although B. anurus is often considered a more effective parasitoid of alfalfa weevil it is not as well established as B. curculionis in western North America.

Appearance of Bathyplectes anurus and B. curculionis

Adult Bathyplectes anurus and B. curculionis are similar looking wasps that are small and mostly black in color with lighter toned legs. They have long antennae, constricted waists, and are around 1/8 inch in length. Bathyplectes anurus and B. curculionis cocoons are small and brown with a white medial band. This band is slightly raised on B. anurus cocoons and flattened on B. curculionis cocoons.

Dark colored adult wasp with long antennae

Adult Bathyplectes curculionis. Photo by Judith Herreid

Two brownish, oval cocoons both with a wide light colored medial stripe. One cocoon is darker brown and one is lighter brown.

The light brown cocoon is an example of a non-diapause cocoon and the darker one is a diapause cocoon. Photo by Judith Herreid

One brownish, oval cocoon with a thin light colored medial stripe.

A single Bathyplectes anurus cocoon. Photo by Dalton Ludwick

How to Use Bathyplectes anurus and B. curculionis

Biocontrol category: Classical—released once and persists

When to use: Bathyplectes anurus and B. curculionis are active in the spring and summer. During the winter they diapause (hibernation like state) as larvae or adults.

Pest stage: Larvae, especially young larvae

Mode of action: Parasitoid

Conservation: Although explicit strategies to conserve Bathyplectes parasitoids have not been developed there are ways to conserve natural enemies and parasitoids in general. Decreasing use of broad spectrum insecticides can help conserve natural enemies in a variety of biological control systems. Also, floral resource plantings can potentially provide parasitoids with important nectar resources and have been shown to benefit natural enemies in agricultural systems. Though sugar resources provided by floral plantings could contribute towards Bathyplectes conservation there is evidence showing they may not need additional sugar resources because Bathyplectes often can take advantage of aphid honeydew in alfalfa fields.

Compatibility: Bathyplectes anurus and B. curculionis can provide support for alfalfa weevil control along with a suite of other parasitoids and predators. Also, both parasitoids are alfalfa weevil specialists therefore their impact on other beneficial insects within the system is likely minimal. Bathyplectes wasps are susceptible to broad spectrum insecticides like many other insects.

Any time you use a pesticide, you must read and follow the label directions and comply with all applicable laws and regulations related to pesticide use. Also be sure that any pesticide used is approved for use in your country and state/province. 

Risk: No specific risks are associated with Bathyplectes wasps.

Commercially available: No

Pests Targeted by Bathyplectes anurus and B. curculionis

Alfalfa Weevil, Hypera postica

Alfalfa weevil are considered one of the most destructive pests of alfalfa within the US. As larvae, they exhibit light green coloration with a white line running down the center of their body and a black head capsule. Adult alfalfa weevils have an elongate snout and are brown with a darker stripe running down their back. Adults are ¼ inch and mature larvae are up to 3/8 inches.

Alfalfa Weevil Damage

Damage is caused primarily by alfalfa weevil larvae feeding but adults can also contribute to alfalfa damage. Weevils feed on terminals, foliage, and new crown shoots. Excessive feeding pressure can result in skeletonized leaves which can make alfalfa fields have a silver or white appearance. Significant alfalfa weevil pressure can also have future consequences including slowed alfalfa regrowth, reduced yield, and decreases in stand density.

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Bathyplectes anurus and B. curculionis Differences

In the spring both B. curculionis and B. anurus adults emerge from overwintering cocoons. Their lifecycles are similar with a few key developmental differences. Bathyplectes anurus has a single yearly generation while B. curculionis has 1 complete and part of another generation each year. In the spring after B. anurus parasitizes an alfalfa weevil larva the new parasitoid larva emerges, spins a cocoon and diapauses over the summer. In fall, B. anurus larvae become active again, pupate and develop into adults. These adults remain within their cocoon and enter diapause to overwinter. In contrast, the first cocoons produced by B. curculionis in the growing season are often non-diapausing cocoons, which tend to be lighter in color than late season cocoons. Individuals in these cocoons develop and emerge as adults that parasitize alfalfa weevil still present. This second generation is generally composed of darker colored diapause cocoons. This generation of B. curculionis remains in the larval form to overwinter. Their lifecycle is completed the following spring. 

Bathyplectes anurus cocoons possess the unique ability to jump through larval movement within its cocoon. This ability is not shared by B. curculionis. It has been proposed that this jumping habit may contribute to survival in B. anurus by helping prevent predation and parasitism or by allowing it to avoid harmful environmental conditions.

Bathyplectes curculionis was first introduced to Utah in 1911 where it quickly became established. Subsequent introductions that led to successful establishment include California starting in 1933, the Northeastern US from the 1950s to 1960, and Ontario, CA in 1970. Releases of B. curculionis in Arizona in 1941 led to less success and low levels of establishment in the southwestern US. Bathyplectes curculionis has been extremely successful at establishment and can be found virtually everywhere alfalfa weevil is located in North America. It has been recorded in Canada from British Columbia to Quebec and across the United States.  

Bathyplectes anurus was not established in North America until much later than B. curculionis. The first introduction that led to establishment was in the Northeastern US in 1960. Although B.anurus establishment was successful it spread slowly so redistribution efforts and introductions to Ontario in 1970 helped grow its range across eastern North America. Bathyplectes anurus is documented as established in much of the United States including in the northeastern states, Florida, and California. It is not established in many states in the Western US and in Canada it is only found in Ontario. 

Although Bathyplectes curculionis is more widely distributed, B. anurus, where established, tends to control alfalfa weevil better and displace B. curculionis. This could be due to lifecycle and fitness differences, and varying defensive abilities. In the southern part of their range, B. anurus may be more compatible with alfalfa weevil due to their single generation lifecycle while the second, later season, generation of B. curculionis that emerges must locate a host when they are not abundant. Also, B. anurus preferentially parasitizes slightly older alfalfa weevil larvae which may increase the chance of completing their development. Other factors which have been documented and may contribute to the competitive abilities of B. anurus are higher fecundity, superior host searching and handling, and its ability to quickly eliminate conspecifics as a larva in its host. Finally, survival may be higher in B. anurus due to the jumping ability their cocoons have and their encapsulation differences. 

Encapsulation is the ability of alfalfa weevil larvae have to encapsulate certain parasitoid eggs allowing them to prevent parasitoid development. The Egyptian and eastern alfalfa weevil strains commonly encapsulate B. curculionis while the western strain does not often encapsulate B. curculionis. Bathyplectes anurus is not susceptible to encapsulation by any of the alfalfa weevil strains found in North America. This may potentially contribute to B. anurus ability to out compete B. curculionis outside of the western US.


Judith Herreid and Randa Jabbour
University of Wyoming

Date: December 2021

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Portrait of Amara Dunn
Amara Dunn-Silver

Senior Extension Associate

NYS Integrated Pest Management

Amara Dunn-Silver