Alfalfa weevil, Hypera postica
Order: Coleoptera
Family: Curculionidae
Description
The alfalfa weevil was introduced to the United States from Eurasia and North Africa. In the United States, this pest was first detected in Utah in 1904 and is the most destructive pest of alfalfa grown in low deserts. This pest is especially injurious prior to first alfalfa cut but may also be present in the second and third cuttings. Adult alfalfa weevils are oval-shaped brown to gray beetles that measure about 6.4 mm (1/4 inch) long and have a dark brown band on the back, although the stripe may be less apparent on older weevils. They also have an elongated snout at the front of their head. The larvae of alfalfa weevil are more easily identified with a 10x hand lens. They have a dark brown head capsule with creamy yellow or light green bodies and a stripe running down the middle of the back that becomes more pronounced as the larva matures. After the first instar, larvae are about 8 mm (~1/3 inch) long and have wrinkled bodies. Pupation occurs within a brown cocoon. At 0.5 mm long, the oval eggs are very small. Initially the eggs are bright yellow but turn dark brown prior to hatching. The black head capsules of developing larvae can be seen through the egg casing.
In certain states west of the Rocky Mountains such as California and Arizona, a second species called the Egyptian alfalfa weevil (Hypera brunnipennis) is morphologically indistinguishable from the alfalfa weevil. In addition, a third species of weevil, the clover root curculio (Sitona hispidulus), may be misidentified as the alfalfa weevil. The clover root curculio is slightly smaller and lacks the brown band observed on the alfalfa weevil. Larvae of the clover root curculio feed on roots and can be problematic in older alfalfa fields.
Quick Facts
- The alfalfa weevil is not native to the United States. This species is the most important insect pest of alfalfa grown in Colorado. Both larvae and adults can injure plants.
- In the United States, alfalfa is the third most valuable field crop after corn and soybean. It is an important commodity for dairy and livestock enterprises.
- This species is considered an early season pest of alfalfa and is most problematic prior to the first cutting.
- Alfalfa weevil can be controlled via crop management practices, biological control, and chemical control, and monitoring is crucial for making effective management decisions.
Alfalfa weevil adult. Note the elongated snout and dark band running along the back. Image credit: Adam Sisson, Iowa State University, Bugwood.org
Alfalfa weevil larvae. Note the white strip running along the back. Image credit: Frank Peairs, Colorado State University, Bugwood.org
Adult of the clover root curculio. Note the absent dark brown band running down the back, which can be used to distinguish this species from adults of the alfalfa weevil. Image credit: Anyi Mazo-Vargas, University of Puerto Rico, Bugwood.org
Alfalfa weevil damage. Image credit: Phil Sloderbeck, Kansas State University, Bugwood.org.
Cocoon of B. anurus, which is an important parasitoid of alfalfa weevil. Image credit: United States Department of Agriculture – Agricultural Research Service.
Life history and habits
This species overwinters as adults under plant litter, or the bark of trees located near field edges. In Colorado, adults emerge in spring once average temperatures reach 15.6°C (60°F) and begin feeding on young alfalfa leaves. About several weeks later in early April, females chew holes in alfalfa stems and deposit anywhere from five to 20 eggs in each cavity. Egg hatch occurs two weeks later, and the newly emerged larvae feed on the tightly folded leaves of stem buds until nearing maturity, at which point they migrate to open terminal leaves where resume feeding until reaching the fourth and final instar. Larvae are typically found in the field for one to two months, mostly during May and June. Mature larvae move into plant crowns or in soil debris where they spin a loosely woven, net-like cocoon, which serves as a pupation chamber. The adult emerges after 10 to 12 days. In Colorado, adults can be found in the field until early fall before they move to their overwintering sites. The exact development time depends on environmental conditions, with optimal development typically occurring in dry warm weather. Alfalfa is the preferred host, but this species can also feed on other legumes such as white clover (Trifolium repens).
Injury
This species is considered an early season pest. Both larva and adult stages of alfalfa weevil cause injury to plants by chewing holes in leaves and stems of alfalfa. Larvae begin feeding in folded leaves, which can cause significant injury to stem terminals. The initial feeding injuries appear as pinholes and may not be noticeable. Leaves can be skeletonized and dry quickly during heavy infestations, which can lead to stands with a gray or frostlike appearance due to the large numbers of dried leaves. These feeding injuries can result in substantial reductions in biomass, with yield losses ranging from 30-40% under high pest densities.
After the first cutting, larvae tend to concentrate their feeding in strips that align with the location of windrows, especially under heavy weevil infestations where larvae survive early first cuttings. Feeding injuries on regrowth can reduce plant growth, reduce yields, and increase the prevalence of weeds.
Scouting
To detect feeding injuries, the closed overlapping foliage on stem terminals of randomly selected plants can be carefully unfolded and inspected. When feeding injuries are present on 30-50% of terminals, additional scouting is highly recommended. Sweep-net sampling and the shake-bucket method are two scouting approaches that can be used to assess larval densities in alfalfa fields. The ideal window for scouting is when most larvae are in the first and second instars since mature larvae can inflict more severe feeding injuries. It is worth noting that sweep-net sampling may be less reliable than the shake-bucket method depending on the experience and training of operators in using sweep nets effectively.
Sweep-net sampling can be conducted on calm sunny days when alfalfa is at least 20-25 cm (~10 inches) tall by using a 38 cm (15 inch) sweep net after 148-degree days when larvae are expected to be in the second instar. Each sample should consist of ten sweeps taken along a 180° arc while walking through the field, and the number of larvae should be counted after each sweep. A minimum of three samples should be taken in fields up to 20 acres, four samples in fields up to 30 acres, and five samples in fields greater than 30 acres. It is important to sample alfalfa weevil in every distinct section of the field since infestations may be patchy depending on terrain, weather, and soil conditions. A simplified economic threshold for sweep sampling is an average of 20 larvae per sweep. This number, in combination with the size of the plants, the value of the hay, and the cost of treatment, can be used as an economic threshold obtained from the table found at: https://extension.psu.edu/alfalfa-weevil.
The shake-bucket method involves sampling stems with a light-colored three- or five-gallon bucket and white cloth. These are used to collect 30 stems from the field and carefully record the number of larvae per stem. Individual stems can be sampled by taking 20 steps from the field edge while looking at the horizon, rather than looking at plants. After 20 steps, the base of an alfalfa stem should be grasped and cut at the tip to gently break it from the crown. It is important to avoid dislodging larvae as stems are placed in the bucket. This process should be repeated until six stems are collected in the bucket, at which point the stems can be firmly grasped and vigorously shaken inside the bucket to dislodge larvae, which can then be counted using a hand lens if needed. This number should be recorded and divided by the total number of stems collected across the entire field to obtain an average. A simple economic threshold for stem sampling is 1.5-2 larvae per stem.
Cultural control
Harvesting seven to ten days earlier than normal harvest can reduce weevil populations if the economic threshold is reached late in the growth of the first cutting. After cutting, rapidly removing hay is recommended to expose larvae to direct sunlight and help increase mortality due to desiccation. If large numbers of larvae survive under windthrows and baling of hay is delayed due to rainfall or other factors, injury to regrowth can be expected. Raking postharvest stubble is recommended to reduce survival of larvae.
In addition to early harvest, grazing and intercropping can also provide some control. Grazing by livestock may be beneficial in dormant alfalfa fields. The grazing recommendations depend on environmental conditions. For example, alfalfa weevil eggs do not survive in colder climates, but may survive winter in warmer regions. Livestock activity can help destroy overwintering eggs. For effective weevil reductions, grazing should occur in early spring in northern regions when adults begin laying eggs.
Several alfalfa cultivars such as Arc, Perry, Liberty, Team, and Weevelchek can tolerate some feeding of alfalfa weevils. These varieties have a greater degree of auxiliary branching and associated buds that continue growing after weevil larvae defoliate stem terminals. However, it should be noted that the ability of these cultivars to outgrow feeding injuries may not necessarily eliminate the need for treatment. Furthermore, these varieties are not resistant to several important plant pathogens of alfalfa grown in the High Plains, such as Phytophthora root rot, Verticillium wilt, and stem nematode. Additional considerations include the dormancy requirements, winter hardiness, seed price, and availability.
Biological control
Several natural enemies of the alfalfa weevil can be found in Colorado, such as lady beetles (Coleoptera: Coccinellidae), damsel bugs (Hemiptera: Nabidae), lacewings (Neuroptera: Chrysopidae) and certain species of parasitic wasp (Hymenoptera: Ichneumonidae). To help conserve resident populations of natural enemies, it is recommended that refuge strips of alfalfa be left during harvesting, insecticide applications be reduced or eliminated if possible, or fields be planted near undisturbed habitat. In addition, the presence of flowering plants near alfalfa fields will provide nectar for parasitoid wasps. Purchasing predators or parasitoids for release in alfalfa fields has not been shown to reduce densities of alfalfa weevil. In addition to these natural enemies, fields can be treated with the entomopathogenic fungus Beauveria bassiana to control economically damaging infestations of alfalfa weevil.
Of the natural enemies, parasitoid wasps are some of the most effective control agents of alfalfa weevil. Several species of importance inhabit the High Plains, some of which are native, while others were intentionally introduced to help suppress the pest population. Three prominent species in the region include Bathyplectes curculionis, Bathyplectes stenostigma, and Bathyplectes anurus. While these parasitoids often do not reduce economically significant densities of alfalfa weevil, they may play an important role in preventing the pest from causing severe annual yield losses in many areas.
Chemical control
Insecticides can be effective but should only be applied when the economic threshold is met due to the high cost of treatment, negative impact on beneficial insects, and their association with increased outbreaks of aphids in alfalfa, such as the pea aphid. The protocols for calculating the economic threshold, along with examples, are available here. Traditionally, pyrethroids have been used for managing this pest. However, decades of overreliance have resulted in widespread resistance to certain pyrethroids such as lambda-cyhalothrin throughout much of the western United States.
Generally, control measures are justified when weevil populations build to a point at which the economic losses due to yield reductions are expected to exceed the cost of management. Certain insecticides are available for controlling alfalfa weevil such as phosmet and chlorantraniliprole. A more complete list with detailed information for these insecticides is available here.
CSU Extension Fact Sheet
Download or view the CSU Extension’s PDF fact sheet for your reference.
References
Beckman, B. (n.d.). Alfalfa Weevil. University of Nebraska-Lincoln – Extension. Available https://extension.unl.edu/statewide/cedar/alfalfa-weevil-0/
Peairs, F., G. Hein, and M. Brewer. 2018. Alfalfa Weevil. High Plains Integrated Pest Management. Available https://wiki.bugwood.org/HPIPM:Alfalfa_Weevil
Pellissier, M., Z. Nelson, and R. Jabbour. 2017. Ecology and Management of the Alfalfa Weevil (Coleoptera: Curculionidae) in Western United States Alfalfa. Journal of Integrated Pest Management. 8(1): 1-7.
Available https://colostate.primo.exlibrisgroup.com/discovery/fulldisplay?docid=cdi_oup_primary_10_1093_jipm_pmw018&context=PC&vid=01COLSU_INST:01COLSU&lang=en&search_scope=MyCampus_FC_CI_PU_P&adaptor=Primo%20Central&tab=Everything&query=any,contains,alfalfa%20weevil&facet=searchcreationdate,include,2000%7C,%7C2023&offset=0
Harrington, K., Y. Carriere, A. Mostafa, and S. Rondon. 2021. Re-evaluating the Economic Injury Level for Alfalfa Weevil (Coleoptera: Curculionidae) Control in Low Desert Irrigated Alfalfa. Journal of Economic Entomology. 114(3): 1173-1179. Available https://colostate.primo.exlibrisgroup.com/discovery/fulldisplay?docid=cdi_unpaywall_primary_10_1093_jee_toab070&context=PC&vid=01COLSU_INST:01COLSU&lang=en&search_scope=MyCampus_FC_CI_PU_P&adaptor=Primo%20Central&tab=Everything&query=any,contains,alfalfa%20weevil&facet=searchcreationdate,include,2000%7C,%7C2023&offset=0
Michaud, J.P. 2013. Alfalfa Insects. Kansas State University – Department of Entomology. Available https://entomology.k-state.edu/extension/insect-information/crop-pests/alfalfa/alfalfa-weevil.html
Rim, K., J. Clarine, S. Price, and R. Ramirez. 2020. Alfalfa Weevil (Hypera postica G.). Utah State University – Extension. Avalable https://extension.usu.edu/crops/research/alfalfa-weevil
Rodbell, E., M. Hendrick, I. Grettenberger, and K. Wanner. 2022. Alfalfa Weevil (Coleoptera: Curculionidae) Resistance to Lambda-cyhalothrin in the Western United States. Journal of Economic Entomology. 115(6): 2029-2040. Available https://colostate.primo.exlibrisgroup.com/discovery/fulldisplay?docid=cdi_openaire_primary_doi_dedup_89a68f433bbc2e78620483479ae094dc&context=PC&vid=01COLSU_INST:01COLSU&lang=en&search_scope=MyCampus_FC_CI_PU_P&adaptor=Primo%20Central&tab=Everything&query=any,contains,alfalfa%20weevil&facet=searchcreationdate,include,2000%7C,%7C2023&offset=0