Pea crops have relatively few insect pests of economic importance, but the few that can affect pea plants must be monitored to prevent yield loss.
Pea Aphid: the pea aphid has recently been recognized as a yield-threatening pest in pea. The population must reach threshold levels prior to the plant nearing maturity to cause damage. The pea aphid is blown in from the United States. If it arrives early enough and the environment is conducive for rapid reproduction, multiple generations of the insect eventually result numbers high enough to cause economic losses. Economic threshold is considered to be 30 to 40 aphids per 180° sweep of a 38 cm net when few natural predators are present and aphid numbers do not decline over a two-day period. If the majority of the pea seeds are close to full size, the aphid is less likely to cause significant damage. Economic loss can occur if there are more than 10 aphids per plant during the period between formation of the tenth node, and the appearance of the first flower. Population estimates should be calculated by averaging the counts taken from at least five separate areas of the field. To avoid a reoccurrence of the problem after spraying, delay application of insecticide until late-flowering. One application per season should give satisfactory control. Pea aphid populations usually begin to decline in mid-to-late August due to drying of the crop, parasitic wasps, diseases and other factors. Numerous insecticides are registered to control aphids on pea. Insecticides with both contact and systemic action can be advantageous.
Pea Leaf Weevil: the pea leaf weevil has been expanding its infestation eastward across southern Saskatchewan. The grey adult weevil feeds on pea seedlings leaving scalloped or notched leaf margins in seedlings. The adult is difficult to observe; therefore economic thresholds are determined by severity of notches in plants at various points in the field. Usually pea plants will survive this defoliation; however, adult females will lay large numbers of eggs at the base of pea plants. The larvae hatch and burrow into the soil causing more serious damage when they feed on nitrogen fixing nodules on the roots of the plant. Research at Agriculture and Agri-Food Canada (AAFC), Lethbridge, Alberta, suggests the best yield response from insecticides results with a seed treatment. If a foliar application is required, the current threshold for insecticide application is one notch on the clam leaves per three plants prior to the sixth node stage of crop growth.
The pea leaf weevil produces one generation per year. The adult (light grayish-brown colour with three stripes on the back and on the wings) overwinters in alfalfa, other perennial legumes, or tree shelters. In May through June, eggs are laid on or near developing pea plants. The adult moves (up to a few kilometers) mainly by flying when temperatures are above 17°C, but can also walk short distances. The larvae hatch in one to three weeks and feed on the nodules on the roots. Once the larval stage is complete, the insect pupates and emerges as an adult once again in late July through September. It will search for any legume plants, including pea and other pulse crops, and continue to feed until it finds an overwintering spot.
Cutworms: can cause damage to newly emerged pea seedlings, and at times may require insecticide application. Below ground feeding cutworms (pale western and red-backed cut worms) cut plants off at or near the soil surface. Foliar feeding cutworms (dingy and army cutworms) feed above ground, consuming the plant foliage. Pea crops can often recover from cutworm damage if cool, moist growing conditions occur. However, plants are set back and may not be competitive.
Fields should be monitored from late-May to late-June. The economic threshold for cutworms is two to three per m2 (0.2 to 0.3/ft2) in the top 7.5 cm of soil. Insecticides that are currently registered for the control of cutworms are listed in the Ministry of Agriculture publication, Guide to Crop Protection. Best control is achieved by applying in the evening due to their nocturnal feeding habits.
Grasshoppers: pea is not a preferred food source for grasshoppers but they will feed on pea plants if no other food source is available. Damage is most likely to be seen along ditches and road allowances. Usually, grasshopper infestations of 10/m2 and less do not cause economic losses in pea. Insecticide is registered for the control of grasshoppers on pea.
Wireworms: this insect tends to be more abundant in moist soils and in lower, damper areas of a field. They are the immature stage or larvae of “click” beetles. Although wireworms prefer grassy plants (cereal crops), wireworm damage has been noted in other crops including canola, chickpea, and potato. Wireworms tend to shred the plant tissue below the soil surface. Initially, symptoms may show up as wilting in the central leaves of the main stem, but can eventually cause death of the plant. Damage may not be noticed early enough in the season to reseed. There is no established economic threshold for wireworm in pea and there is no in-crop insecticide available to control wireworm. Controlling wireworm requires the use of an insecticidal seed treatment.