Food & Health
Food & Health
Pulse Crop: Pea, lentil
The tarnished plant bug, Lygus lineolaris, is one of the more common species and is known to feed on over 385 crops and weed plants. Adult Lygus bugs are about 1/4 inch in length, and pale green, light brown, or dark brown with a distinctive triangular marking on its back.
Lygus bugs overwinter as adults in weedy areas under debris along fence rows, ditches and roadsides. Adults emerge in early spring, lay eggs in the stems, leaves and flowers of host plants, and then die. Immature nymphs hatch from these eggs and look like aphids. Several generations occur each year with the second generation occurring in mid-July to early August. As with many other insect pests, warm, dry weather favors the buildup of Lygus populations and increases the potential for early season damage to peas or lentils. Both immature and adult Lygus bugs feed on developing pods and seeds of peas and lentils, and have been linked to "chalk spot".
Damage is caused by the piercing-sucking mouthpart, which punctures the pods and seed coats injecting a toxic substance into plant parts. Chalk spot is a pit or crater-like depression in the seed coat with or without a discolored chalky appearance. Damage seeds are smaller, deteriorate faster in storage, have poor germination, and produce abnormal seedlings as well as lower the grade and marketability. It is important not to confuse damage caused by Lygus bug to damage caused by rough harvesting or handling. For example, peas harvested at high moisture levels are also susceptible to bruising when harvested or handled roughly, resulting in damage similar to chalk spot.
Monitor for Lygus bugs using a 15-inch sweep net during bloom to pod development (until seeds within the pod have become firm). Make ten 180 degree sweeps at five sampling sites in a field during the warm sunny part of the day (2-6 PM). Lygus populations can increase suddenly. For example, when an alfalfa (preferred host) field is cut, Lygus will migrate quickly into nearby pulse crop fields and often in high numbers. No economic threshold has been determined for this region. However, in the Pacific Northwest, an insecticide treatment is recommended when
"10 Lygus per 25 sweeps"
are present. Spray a blooming crop when there is minimal bee activity, preferably during the evening hours (after 8 PM).
Economic thresholds may vary depending on the value of the crops and cost of control, as well as variation in potential seed weight caused by variation in precipitation and heat stress.
The economic threshold in peas at $5.71 per bushel and average control cost of $6.73-$9.25/acre is 2 to 3 aphids per 8-inch plant tips, or 9 to 12 aphids per sweep (or 90 to 120 aphids per 10 sweeps), at flowering. If the economic threshold is exceeded, a single application of insecticide when 50% of plants have produced some young pods will protect the crop against yield loss and be cost-effective. Cultivars of peas may also vary in their tolerance to feeding by pea aphids, thus economic injury levels may differ between cultivars. The economic thresholds presented above were developed using "Century" field peas.
The following table relates yield loss in peas for average aphid counts from 1 to 8 aphids per 8-inch pea stem tip when about 25% of the crop has begun to flower.
Research in Manitoba has shown that insecticides applied when pods first form protects pea yield better than earlier or later applications. Control at the early pod stage provides protection through the pod formation and elongation stages, which are very sensitive to aphid damage.
Lygus bug adult
Lygus bug nymph
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