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Lygus Bugs
Lygus bus are small, oval-shaped insects that feed on a variety of crops and weeds. Several species infest canola and alfalfa. In western Canada, Lygus lineolaris (the tarnished plant bug), L. borealis, L. elisus and L. keltoni have been observed destroying canola flower buds and seeds. All four species are equally destructive.
Adult lygus bugs are about 3 mm wide and 6 mm long. They vary from pale green to reddish brown to black in color and from fairly uniform to mottled. Lygus bugs share characteristics with all "true bugs" - the distinctive, triangular or "V"-shaped marking in the upper center of their backs, and membranous wingtips. Adults are active and fly readily when approached.
Immature lygus bugs(nymphs) are wingless and light green in color. Young nymphs are often mistaken for aphids, which are similar in size and shape. However, lygus bugs are much more active, are harder bodied, and lack the cornicles ("tail pipes") of aphids. Several black spots, usually five, become noticeable on the backs of nymphs as they mature through five instars or growth stages. Wing buds are evident in the fourth and fifth instar.
Host Plants
Lygus bugs feed on the sap of new growth and reproductive tissue. Host plants of lygus include alfalfa, canola, potato, strawberries, vegetable crops, tree fruits, and weeds such as redroot pigweed, stinkweed, wild mustard, and lambsquarters. The tarnished bug plant has more than 300 recorded host species.
Adult lygus bugs overwinter under plant litter at the soil surface in headlands, uncultivated areas and field margins. They emerge from hibernation soon after the snow melts in spring. Upon emergence, adults seek winter annuals and the buds of flowering shrubs to feed on. Overwintered adults can be abundant in fall-seeded and early spring-seeded canola, especially if these crops are in bud or flower and other hosts are not yet available. After mating and maturing eggs, females seek suitable host plants, such as budding alfalfa or canola, on which to lay their eggs. Egg-laying take place throughout May, June and July. The first nymphs appear by about the end of May. In the south, the new generation adults first appear by about the end of June.
Generations
The lygus pest species produced at least one generation.
In alfalfa, a new generation can mature about the time the alfalfa is cut for forage. The new lygus bug generation leaves alfalfa seeking another crop, often canola, on which to lay its eggs. Adults move into canola to feed and lay eggs when buds and flowers are developing in June through early July. In canola, the hatch of lygus eggs is complete by the end of July.
After hatching, lygus nymphs feed on new growth and reproductive parts, molting five times before becoming adults. In late summer, the new adults disperse from mature canola fields into later mating hosts such as alfalfa and continue feeding until hibernation.
Damage
New generation lygus bugs have been responsible for most of the reported crop damage in canola. However, with the trend to earlier seeding and fall seeding of canola, overwintered adults can also threaten canola production.
Lygus bugs use their needle-like mouthparts to suck plant sap from the developing flower buds and seeds. In canola and alfalfa, lygus bug feeding is most injurious to the flowers and developing seeds, and causes visible lesions to surfaces of stems, buds, flowers, and pods. Feeding on pods, flowers and young pods causes "blasting", in which buds turn white and fail to develop, flowers fall without forming pods, or pods fall without maturing. Seeds that have been fed on will collapse or shrink, as well as darken, and lose their quality and viability. Additional loss may occur if flowering is delayed by heavy feeding pressure or drought. Feeding punctures on the outside of pods and stems may ooze droplets of sap, causing an infested crop to become noticeably sticky. These droplets can harbor pathogens and create small, dark, circular patches on the pod surface and on the underlying seeds. Adults and the oldest (fourth or fifth instar) nymphs are responsible for most of the feeding injury.
Monitoring
Begin monitoring canola when it bolts and continue until seeds within the pods are firm. Because adults will move into canola from alfalfa, check lygus bug numbers in canola when nearby alfalfa crops are cut.
Economic Threshold
The economic threshold for lygus bugs in canola is applied between the end of flowering and the early pod ripening stages. At this time, if there are between about 10 and 30 lygus bugs per ten 180-degree sweeps of a standard 30-cm diameter insect net an insecticide should be applied. Fourth and fifth nymphs and adults are combined in this assessment. Sweep samples should be taken at several locations (at least 8-12) in a field. Once the crop is mature and seeds are ripening to a yellow or brown color, the cost of controlling lygus bugs may exceed the damage they cause.
An economic threshold for lygus bugs in canola at the bud stage has not been established.
Biological Control
Lygus bugs have several natural control agents. A tiny "fairy wasp" in the family Mymaridae parasitises the eggs of the lygus bug. In western Canada, a parasitic wasp, Peristenus pallipes, attacks lygus nymphs in alfalfa, and may do the same in canola. Nabid plant bugs, big-eyed bugs, and spiders occasionally prey on young lygus bug nymphs. A European wash, Peristenus digoneutis, has been introduced into alfalfa fields in eastern North America where it parasitises about 40 percent of the tarnished plant bugs. One of the few parasitoids of lygus adults is a tachinid fly, Alophorella spp.
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