Aphids in alfalfa
Order: Hemiptera
Family: Aphididae
Description
There are four aphid species considered to be economically important pests of alfalfa (Medicago sativa) grown in Colorado: the pea aphid (Acyrthosiphon pisum), cowpea aphid (Aphis craccivora), blue alfalfa aphid (Acyrthosiphon kondoi), and spotted alfalfa aphid (Therioaphis maculata). Aphids are small, soft-bodied insects with pear-shaped bodies and piercing-sucking mouthparts for extracting fluid (phloem) from plant cells. Plant phloem contains a high concentration of sugars, which are excreted as sugar-rich honeydew by aphids. These insects often have long antennae and have a pair of short tubes, called cornicles, at the rear of their abdomen. Most actively feeding aphids are wingless but winged forms are present under certain conditions, which allows them to disperse to new host plants or overwintering sites. The exact coloration and body morphology depends on the species, and each species common in alfalfa in Colorado has a different potential for damaging alfalfa. Accurate identification is essential to determine which aphids are present in a field and help inform management decisions.
Pea aphid
Relatively large aphids that measure 3 mm (1/8 inch) long. Pea aphids are pale green or occasionally pink with very long legs and black tipped cornicles. This species resembles the blue alfalfa aphid but has antennae with narrow dark bands on each segment.
Spotted alfalfa aphid
Adults are about 2 mm (<1/8 inch) long. They have very short cornicles and pale-yellow bodies with four to six rows of dark spots running along the back that can only be seen with a hand lens.
Cowpea aphid
Adults of this species have a distinct polished, black body that measures about 2.5 mm (<1/8 inch). The cornicles are also black.
Blue alfalfa aphid
Adults are large blue-green aphids with long legs, antennae, and cornicles. This species resembles the pea aphid but has antennae with dark tips instead of bands.
Quick Facts
- In Colorado, the pea aphid, blue alfalfa aphid, cowpea aphid, and spotted alfalfa aphid are the most common aphids in alfalfa. Overlapping populations of these aphids can be found in an alfalfa field.
- Pea aphids and blue alfalfa aphids are generally early season pests, while cowpea aphid and spotted alfalfa aphid tend to occur later in the season.
- Accurate identification is necessary for effective management since treatment thresholds differ among aphid species.
- Many insecticides can be applied to alfalfa for managing aphids. However, it is important that they be used effectively to minimize impacts on beneficial insects such as pollinators.
Pea aphid adult and nymphs. Image credit: Joseph Berger, Bugwood.org
Cowpea aphid. Image credit: Clemson University, USDA Cooperative Extension Slide Series, Bugwood.org
Spotted alfalfa aphid. Image credit: Kansas Department of Agriculture, Bugwood.org
Adult blue alfalfa aphid. Note the long antennae that darken at the tip. Image credit: Kansas Department of Agriculture, Bugwood.org
A colony of aphids parasitized by Aphidius spp. Note the bronze appearance and presence of exit holes, which are key features of aphid mummies. Image credit: David Cappaert, Bugwood.org
Life history and habits
Aphids give birth to live young and reproduce through parthenogenesis. While much of the aphid life cycle involves asexual reproduction, individuals of some species reproduce sexually during fall conditions in temperate regions. The pea, cowpea, and blue alfalfa aphid feed on young leaves and new growth at the tips, while the spotted alfalfa aphid feeds on older foliage. Feeding is typically more concentrated on lower leaves and gradually spreads upward as the lower leaves die. While the exact life history depends on species, aphids produce multiple generations each growing season.
Pea aphid and blue alfalfa aphid
The pea aphid overwinters as eggs laid on fallen alfalfa stems and leaves in the fall. The eggs hatch early the following spring, and the emergent nymphs begin feeding on new growth near the growing tips of a host plant. Dry, cool conditions of 13-15.5°C (55-60°F) in the spring or late summer tend to support high densities of pea aphids, although infestations in the High Plains can occur at any time. The blue alfalfa aphid has a similar life history to the pea aphid, and populations tend to decrease quickly at temperatures of 29.5°C (85°F) or greater.
Spotted alfalfa aphid
During feeding this species introduces a toxic substance to plant tissues. Populations of the spotted alfalfa aphid tend to peak in the late summer during periods of warm weather and low humidity. However, these aphids have also been observed in dryland alfalfa during spring.
Cowpea aphid
Cowpea aphids are usually present in low densities in cotton (Gossypium spp.), alfalfa, and various weeds. This species can attack a wide variety of cultivated plants and weeds including but not restricted to apple (Malus spp.), carrot (Daucus carota), dandelion (Taraxacum officinale), goldenrod (Solidago canadensis), lambsquarters (Chenopodium album), lettuce (Lactuca sativa), and pepperweed (Lepidium latifolium). This species has been extremely abundant recently in the arid parts of the Southwest, especially California and Arizona, and its distribution appears to be expanding in the High Plains region. Like the spotted alfalfa aphid, the cowpea aphid injects a toxin into the plant during feeding.
Injury
Newly planted fields are more susceptible to aphid infestations. The saliva of some aphid species has toxic properties. In the case of the spotted alfalfa aphid, the toxin can result in yellowing of the leaf veins, while the toxin introduced by the cowpea aphid can cause wilted and stunted plant growth. Low to moderate feeding may not produce noticeable injuries on host plants. Prolonged feeding can lead to yellowing at the veins and curling and wilting of leaves, which can kill leaves or entire seedlings. High densities of any of these species can lead to decreased growth rates, weakened plants, and increased susceptibility to stress. Weeds may also become more prevalent in affected stands.
In addition to feeding injuries, aphids also secrete droplets of honeydew during feeding, which can lead to sooty mold growth that contaminates alfalfa grown for hay. Pea aphid and blue alfalfa aphids prefer cool, dry conditions and are more problematic during the first cutting, while cowpea aphid and spotted alfalfa aphids prefer hot, dry conditions and are generally a problem in later cuttings.
Monitoring
Generally, monitoring for pea aphid and blue alfalfa aphid can be conducted in the fall, winter, and spring. Monitoring for the spotted alfalfa aphid can be conducted in summer, while monitoring for the cowpea aphid can be conducted in spring and summer. It is recommended that 20 stems be sampled during periods of high aphid activity, even in fields containing resistant alfalfa varieties. To determine if aphids have reached economically damaging levels, alfalfa stems can be clipped at the base of plants. The length of the stem and number of aphids present per stem should be recorded. Both sides of clipped stems should be examined for aphids. Extra care should be given when inspecting the growing tips and older leaves where aphid feeding is likely to be concentrated. When monitoring aphids, it is important to consider the presence of natural enemies, which often suppress aphids below economically damaging levels.
After counting aphids, the average number of aphids per stem can be obtained by dividing the total number of aphids by the number of stems collected. This average can help inform decisions related to chemical management. It is worth noting that the use of broad-spectrum insecticides for controlling alfalfa weevil can reduce the abundance of resident natural enemies, which can lead to an increase in the aphid population. It is recommended that fields be reexamined following periods of hard rain or high winds, which can dislodge aphids from plants. Action thresholds vary based on aphid species, although multiple aphid species can simultaneously inhabit an alfalfa field:
| Treatment guidelines for aphids per stem on alfalfa. | ||||
| Growth Stage | Pea aphid | Blue alpha aphid | Spotted alfalfa aphid | Cowpea aphid |
| Seedling | 5 | 1 | 1 | 5 |
| <10 inch | 40 | 10 | 10 | 40 |
| >10 inches | 75 | 30 | 30 | 75 |
| 20 inches | 100 | 50 | 100 | 100 |
Cultural control
Maintaining healthy plants can be an effective means of minimizing feeding injuries. Certain alfalfa varieties have varying degrees of tolerance against pea aphid, blue alfalfa aphid, and the spotted alfalfa aphid. However, aphid biotypes can adapt and infest previously resistant varieties. There are currently no alfalfa varieties resistant to the cowpea aphid. For more information on resistant alfalfa varieties, visit the 2024 leaflet produced by the National Alfalfa & Forage Alliance.
Biological control
There are numerous predators of aphids including ladybird beetles (Coleoptera: Coccinellidae), lacewing larvae (Neuroptera: Chrysopidae), syrphid fly larvae (Diptera: Syrphidae), damsel bugs (Hemiptera: Nabidae), big-eyed bugs (Hemiptera: Geocoridae), and minute pirate bugs (Hemiptera: Anthocoridae). Certain species of parasitoid wasp belonging to the genus Aphidius (Hymenoptera: Aphidiidae) can parasitize aphids. The aphid becomes mummified as the wasp develops internally, and after emerging from the aphid host, the parasitoid wasp leaves behind an aphid mummy which appears as a bronze or black aphid husk with an exit hole. Moreover, certain entomopathogenic fungi are naturally occurring pathogens of aphids. Aphids infected with these fungi initially appear pink to brown and eventually shrivel and die. If natural enemies, aphid mummies, or aphids infected with fungi are abundant in fields, insecticide control should be delayed, and aphid densities re-examined. For example, insecticide applications should be avoided if more than one adult lady beetle per 5-10 aphids is observed per stem, or when three or more lady beetle larvae are observed per 50 aphids on each stem.
Chemical control
Aphids become secondary pests of alfalfa following the use of broad-spectrum insecticides for alfalfa weevil control during the first hay crop. This is the most common way that pea aphids become a problem and occurs because the insecticides that produce effective alfalfa weevil control are also highly toxic to many beneficial insects, including natural enemies. During periods of warm temperatures, reduced populations of predators and parasitoids allow aphid populations to increase rapidly in a short period of time.
A complete list of insecticides for managing aphids in alfalfa grown in the High Plains is available here. In addition to reducing resident populations of natural enemies, some insecticides can have negative effects on pollinators. To minimize the negative impact of insecticides on pollinators applications should only be made when aphids reach action thresholds. If aphids increase during periods of bloom, it is recommended that alfalfa crops be harvested rather than treated with insecticides. Local beekeepers should be notified prior to applying insecticides, and treatments should be made early in the morning or later in the evening when pollinators are less likely to be foraging. Treating fields with flowering weeds should be avoided, and insecticides that are relatively safe for honeybees should be selected when possible.
CSU Extension Fact Sheet
Download or view the CSU Extension’s PDF fact sheet for your reference.
References
Blodgett, S., M. Brewer, F. Peairs, and J. Donahue. 2018. High Plains Integrated Pest Management. Available https://wiki.bugwood.org/HPIPM:Pea_Aphid_Blue_Alfalfa_Aphid_Spotted_Alfalfa_Aphid
Federici, B. 1999. A Perspective on Pathogens as Biological Control Agents for Insect Pests. University of California – Department of Entomology. Biology, Environmental Science. Available https://www.sciencedirect.com/science/article/pii/B9780122573057500655?via%3Dihub
NAFA. 2024. Leaflet: Alfalfa Variety Ratings 2024. National Alfalfa & Forage Alliance. Available https://alfalfa.org/pdf/2024_Alfalfa_Variety_Leaflet.pdf
OSU. (n.d.). Spotted Alfalfa Aphid. Oklahoma State University – Extension. Available https://extension.okstate.edu/programs/digital-diagnostics/insects-and-arthropods/spotted-alfalfa-aphid-therioaphis-maculata/#:~:text=The%20spotted%20alfalfa%20aphid%20is,secretes%20large%20amounts%20of%20honeydew
Stern, D. 2008. Quick guides: Aphids. Current Biology. 18(12): 504-505. Available https://www.cell.com/current-biology/pdf/S0960-9822(08)00376-X.pdf
UCIPM. 2021. Aphidius spp. University of California Agriculture and Natural Resources – Integrated Pest Management. Available https://ipm.ucanr.edu/natural-enemies/aphidius_spp.html
UCIPM. 2017. Spotted Alfalfa Aphid. University of California Agriculture and Natural Resources – Integrated Pest Management. Available https://ipm.ucanr.edu/agriculture/alfalfa/spotted-alfalfa-aphid/
UCIPM. 2006. Natural Enemies of Aphids. University of California Agriculture and Natural Resources – Integrated Pest Management. Available https://ipm.ucanr.edu/agriculture/alfalfa/natural-enemies-of-aphids-identification/
Wickwar, D., and S. Eigenbrode. 2023. Pea Aphids. University of Idaho – Extension. Available https://www.uidaho.edu/extension/ipm/ag-pests/arthropods/pea-aphids