Manitoba Agriculture, Food and Rural Initiatives
Aphids on Peas
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Host Crops
Field peas, alfalfa and clovers are the main hosts of the pea aphid. Other legumes, including vetches, sweet peas, trefoil and dry beans, are also attacked.
Biology
Adult pea aphids are soft-bodied, slow-moving, and range in colour from light to dark green. They are pear shaped, about 3 mm (1/8 in.) long and 1.5 mm (1/16 in.) wide, with long slim legs. The antennae of the pea aphid show narrow dark bands at the tip of each segment. The nymphs are smaller but closely resemble the adults.
Pea aphids overwinter as eggs on leaves and stems of perennial legumes (ex, on the crowns of clover or alfalfa). In the spring, when plants resume growth, a small, light green, wingless female hatches from each egg. These aphids, which are all females and are called "stem mothers", can reproduce without mating. They feed on the growing plants and give birth to female young. Some aphids of the second and third generations become winged and migrate to field peas and other acceptable host plants. There they feed and produce wingless females that give rise to winged and wingless females.
Aphids develop from birth to maturity in 5 to 50 days depending on the weather. All pea aphids are female throughout spring and summer. A summer female can produce 50-150 young during her life. If the host crop is cut during spring or summer, the winged aphids leave to search for new plants on which to live.
In late September or October, winged males and wingless females are produced. These mate and the females lay eggs on leaves and stems. There may be seven to 15 generations per year.
May June July August September Overwintering eggs hatch Feeding by nymphs begins, new adults emerge New adults disperse and lay eggs, nymphs feed New adults disperse and lay eggs, nymphs feed Adults disperse, lay eggs that overwinter
Scouting Techniques
The key to aphid management is early detection through careful, periodic scouting. A 10X or 20X hand lens is needed to identify aphids when scouting in the field. The fields should be sampled weekly for aphids, from the time of plant emergence to harvest. Populations may increase two to three times on a biweekly basis.
If strong southerly winds occur over a period of several days, in the early summer, this may indicate a subsequent outbreak of aphids. Remember that you have to look over the entire field. If the aphids fly in, they may land anywhere in the field. If they migrated in from an adjacent field, chances are that they will be found on the edge of the new field.
Counts should be at least 50 m (150 ft.) apart and observations should be made well into the centre of the field. Carefully examine 20 stems in each of five areas of the field, examining at least 100 stems per field.
Look for symptoms of damage, including leaf discolouration in host plants. Severe infestations may appear as large, bronzy or brown patches in the field, often easily visible from the road. Later in the season, these patches may appear as thin stands.
In younger plants, aphids may cluster near the base of the plant, on the upper sides of the leaves. In older plants, check inside leaf whorls for the aphids.
Some agronomists routinely take stem samples for aphid counts. Cut the stem close to the ground and then hit it sharply against a stiff piece of white paper or into a white pan. This dislodges the aphids so they can be counted.
Economic Thresholds
Sampling to determine aphid density should be done when 50 to 75% of the pea plants are in flower. The threshold in Century peas is two to three aphids per 20 cm (8 in.) of plant tip at flowering. Trapper peas can tolerate higher levels. Plants infested before the flowers open recover without loss of yield.
The threshold in alfalfa is ten aphids per stem at two weeks before hay cuttings.
Control Tips
Any practice that reduces crop stresses should be emphasized (ex, good seed bed preparation, proper seeding rate, fertilization, irrigation where available). On irrigated alfalfa, control may be unnecessary if adequate water is provided to the plants. Early seeded crops grow and become vigorous and pass the susceptible stage before aphid populations reach damaging levels.
Avoid any unnecessary insecticide applications to let ladybird beetles, lacewing larvae, syrphid fly larvae, damsel bugs, big-eyed bugs, minute pirate bugs, and parasites keep populations in check.
For further information, contact your GO representative.
