Pea aphids

Aphids are small, soft-bodied insects with piercing/sucking mouthparts. Adult aphids can be either wingless or winged (Figures 1-2), whereas nymphs (immatures) are always wingless. Aphids have cornicles (small rear-facing “tailpipe” like appendages near the end of the abdomen), a key trait in distinguishing them from other small arthropods.

Pea aphids are usually a bright shade of green, similar in color to host leaves, though a pink form can arise under certain conditions. Fully grown adult pea aphids are larger than adults of many aphid species, around 1/8 inch (3 mm) long, with notably long legs. Though aphids can be difficult to identify, pea aphids are the only aphid found in significant numbers in legume crops in most production areas in the PNW. Some useful information on identification of aphids you may find in legumes can be found on the University of California IPM website.

Biology

Unlike many aphids that are pests in cropping systems, pea aphids are monoecious, with all life stages occurring on legume hosts. On these legume hosts, pea aphids go through a complex life cycle that includes both asexual and sexual reproduction.

Pea aphids overwinter as eggs. In the spring these eggs hatch into both winged and wingless females that give birth without mating. Individuals in this generation can disperse great distances, moving on prevailing winds from central Washington and northern Oregon to eastern Washington and northern Idaho. Early in the season they can colonize feral vetch or pulse crops that have emerged. Subsequent generations of pea aphids disperse more locally, meaning they can colonize fields at any point during the growing season.

As days shorten in the fall, sexual forms of pea aphids are produced and move from legume crop hosts to perennial hosts where they mate and lay eggs that will overwinter. In some parts of the PNW, in the dry falls, most pea aphids die and eggs are unable to overwinter, so new infestations each year must come from warmer regions.

Damage

Pea aphids can inflict direct damage to their host plant, feeding on plant sap and reducing the host plant’s health, growth and hindering nitrogen fixation. Severe infestations of pea aphid can kill host plants (Figure 3).

Pea aphids can also inflict indirect damage by vectoring a number of viruses. Notable viruses transmitted by pea aphid in legumes include Pea seed-borne mosaic virus (PSbMV), Pea enation mosaic virus (PEMV), Bean leafroll virus (BLRV) and Pea streak virus (PeSV). Yield reductions of 50% or more have been documented in years of heavy pea aphid infestation.

PSbMV can cause significant losses, particularly when infection occurs before or during bloom. Plant symptoms include stunting and delayed maturation, as well as shortening of internodal lengths. Leaves may show swollen or cleared veins and mottling or mosaic discoloration. Seeds are often malformed, discolored and shriveled. PSbMV can be transmitted by infected seed or aphids. PSbMV can infect legumes, as well as alfalfa, sugar beet and many weeds (e.g., black medic and shepherd’s purse). Pea aphid and green peach aphid both transmit PSbMV relatively effectively (Figure 4).

PEMV causes plant symptoms such as general stunting and development of unusually dense basal branches. Leaf symptoms include mosaic coloration and characteristic blister-like outgrowths called enations. Pods may have enations as well, be split open or be generally malformed. PEMV is not seed-borne and only can be transmitted by aphids. PEMV is transmitted by many aphid species, but the pea aphid is by far the most important vector in PNW legumes (Figures 5-6).

In legumes, BLRV causes stunting in plants and chlorosis of young leaves, and downward leaf-rolling. BLRV can also cause 50-90% reduction in pod production. BLRV also infects alfalfa, white clover and vetch, which can serve as reservoirs of virus. Pea aphid is the primary vector of BLRV but many other species of aphid, including black bean aphid, potato aphid and green peach aphid, can also vector the virus (Figure 7).

Pea streak can be caused by many different viruses, but symptoms generally include streaking of the stem in colors ranging from gray to dark brown to purple. Leaves often wither, with dieback starting from the tip. Pods are sometimes deformed. Alfalfa and red clover are primary sources of inoculum in the PNW (Figure 8).

Monitoring

Pea aphids can be monitored visually by inspecting individual plants, where aphids may be present on any plant part (leaves, stems, stipules and pods). Sweep net sampling is recommended to estimate aphid density and guide management decisions. A general threshold above which management of pea aphid may be recommended is an average of nine to 12 aphids per 180 degree sweep during bloom to pod-filling plant growth stages.

Visit the Idaho Pest Monitoring and Legume Virus Project sites for current information on aphid abundance and viral risk.

Management

Primary management tactics

Effective management of pea aphids is largely based on the local and seasonal risk of virus transmission, and careful cost-benefit analysis. Where infection with pea aphid transmitted viruses is deemed likely, aggressive management is recommended until approximately three weeks after plant emergence. Where infection is deemed less likely, direct injury thresholds or abundance rules-of-thumb should be used. Treatment after symptoms of PEMV and BLRV are observed in the field is ineffective, as field-wide inoculation will likely have already occurred.

Cultural

• Early sowing may allow plants to mature beyond vulnerable stages before aphid numbers reach economic levels
• Crop rotation between pulses and cereals
• Remove infected plants

Biological

Avoiding the use of broad-spectrum insecticides can help conserve resident predators (e.g., lady beetles, larval syrphid flies and lacewings) and parasitoids (e.g., Aphidius ervi) that help keep pea aphid populations in check (Figure 9).

Chemical

• Pea aphid management using insecticides may occur at multiple points in the growing season: seed treatment, early spray or late spray
• There are calculators available to help determine if spraying is likely to be economically advantageous at the Legume Virus Project site
• Treatment after symptoms of PEMV and BLRV are observed in the field is ineffective, as field-wide inoculation will likely have already occurred
• Viruses transmitted in a non-persistent manner by aphids can be more difficult to control with insecticides. These include PSbMV and PeSV.
• Viruses transmitted in a persistent manner by aphids can often be effectively controlled with insecticides. These include PEMV and BLRV.
• Recommendations for pesticides to use in the management of pea aphid can be found on the PNW Pest Management Handbooks website

Further reading

Pesticide warning

Always read and follow the instructions printed on the pesticide label. The pesticide recommendations in this University of Idaho webpage do not substitute for instructions on the label. Pesticide laws and labels change frequently and may have changed since this publication was written. Some pesticides may have been withdrawn or had certain uses prohibited. Use pesticides with care. Do not use a pesticide unless the specific plant, animal or other application site is specifically listed on the label. Store pesticides in their original containers and keep them out of the reach of children, pets and livestock.

Trade names — To simplify information, trade names have been used. No endorsement of named products is intended nor is criticism implied of similar products not mentioned.

Groundwater — To protect groundwater, when there is a choice of pesticides, the applicator should use the product least likely to leach.