**Pea Aphids** (*Acyrthosiphon pisum*) are large, soft-bodied insects and one of the most serious pests of legumes globally, primarily attacking peas, alfalfa, clover, and vetch. Like all aphids, they feed using piercing-sucking mouthparts to extract sap from the plant’s phloem. They cause damage through direct sap removal (leading to stunting and wilting), excretion of sticky honeydew, and, most critically, acting as highly efficient **vectors of plant viruses** (e.g., Pea Enation Mosaic Virus, Bean Yellow Mosaic Virus) that cause severe yield loss.
Taxonomy and Classification
Pea Aphids belong to the family Aphididae (aphids) in the order Hemiptera. They undergo incomplete metamorphosis. They are renowned for their complex life cycle, which includes the ability for females to reproduce asexually (**parthenogenesis**), giving birth to live female clones, allowing for explosive population growth, especially during the spring and early summer.
Physical Description
Pea Aphids are relatively large for an aphid, about $1/8$ inch long.
- **Appearance:** Usually bright, striking green, but sometimes pink or red. They have a pear-shaped body.
- **Cornicles:** Possess two distinct, tube-like projections (**cornicles**) extending from the rear of the abdomen, used to excrete defensive, waxy fluid.
- **Forms:** They exist as both **wingless** forms (which reproduce rapidly) and **winged** forms (which develop when the host plant quality declines or populations become too dense, allowing for migration).
- **Damage Sign:** Colonies massed on terminal growth, wilting, yellowing, sticky leaves, and the presence of black **sooty mold** growing on the honeydew.
Distribution and Habitat
Pea Aphids are cosmopolitan. Their habitat is restricted to the tender terminal growing points, stems, and undersides of leaves of leguminous plants (Fabaceae family). They are common in alfalfa fields, pea and bean crops, and clover pastures.
Behavior and Damage
Their rapid asexual reproduction allows for exponential growth, leading to massive populations quickly stressing the host plant.
- **Sap Removal:** High populations extract large volumes of plant sap (phloem), leading to stunted growth, reduced pod set, and lower overall yields.
- **Virus Transmission:** Aphids are highly effective vectors, spreading viruses from infected to healthy plants during brief, test-feeding probes. Virus diseases cause irreversible stunting, mottling, and plant death.
- **Honeydew and Sooty Mold:** The sticky honeydew attracts ants and supports the growth of black sooty mold, which reduces photosynthesis and is a cosmetic issue in marketed products.
Management and Prevention
Control focuses on early season monitoring and conservation of natural enemies.
- **Biological Control (Key):** Natural enemies, including **Lady Beetles**, **Lacewing larvae**, **Hoverfly larvae**, and specialized **Parasitic Wasps** (*Aphidius ervi*), often provide excellent control. Conservation of these predators is paramount.
- **Monitoring:** Regular sweep-net sampling or stem inspections are used to determine if aphid counts exceed the economic action threshold.
- **Chemical Control:** Insecticide application should be timed carefully to minimize impact on beneficial insects. Selective insecticides or insecticidal soaps/oils are preferred for spot treatments.
- **Host Plant Resistance:** Planting resistant or tolerant varieties of peas and alfalfa is a fundamental, long-term management strategy.
Conservation and Research
Pea Aphids are managed as high-priority crop pests. Research focuses on understanding the molecular mechanisms of their symbiosis with host plants, their genetic basis for reproducing asexually, and maximizing the efficacy of biological control agents in large-scale legume agriculture.