Lygus Bug

Lygus bug adult
Figure 1. Lygus bug adult

Lygus bug nymph
Figure 2. Lygus bug nymph


Figure 3. Alfalfa plant bug adult


Figure 4. Alfalfa plant bug nymph

Lygus bugs have piercing and sucking mouthparts, with which they puncture plant tissues and suck the juices. The plants may react to the saliva that the insects inject when they feed.

Host Crops

Plant bugs in the genus Lygus feed on many plants; crops, weeds and wildflowers. Some of the crops that they potentially can be of greatest economical concern in Manitoba include alfalfa seed, canola, sunflowers, beans, buckwheat, and strawberries. Lygus bugs will also feed on flax and wheat, but feeding in these crops does not appear to be economical. Some weed hosts of lygus bugs include chickweed, dandelion, red clover, red root pigweed, lamb’s quarters, plantain, goldenrod, and asters.

Biology

Although there are 14 species of plant bugs in the genus lygus in Manitoba, 3 of these can be common on many crops in Manitoba. Lygus lineolaris, sometimes referred to as the tarnished plant bug, is often the most abundant of these 3 species of lygus bugs. The other 2 species that can be common on crops are Lygus borealis, and Lygus elisus, sometimes referred to as the pale legume bug. These species are all native to North America. In addition, the closely related alfalfa plant bug, Adelphocoris lineolatus, which was accidentally introduced from Europe to North America, can also be common in Manitoba but is relatively restricted to alfalfa. Although appearances can vary between and even within species of lygus bugs, below is a generalized description of lygus bugs.

Adult lygus bugs (Figure 1) are about 5 mm (1/4 in.) long and 2.5 mm (1/8 in.) wide, oval, and slightly flattened. They vary in colour from pale green to reddish-brown and have a distinctive triangle or "V" mark about one-third of the distance down the back, just in front of the wings. The legs and antennae are relatively long.

The young nymphs (Figure 2) are pale-green in colour and look like aphids except that they are more active. The older nymphs develop prominent black dots on the top of the thorax and abdomen.

The adults overwinter, hidden in protective hedgerows, fallen leaves and other plant debris. Emerging in mid-April to late May, they feed on host weeds. The eggs are laid on the stems and petioles of these hosts. About 10 days later, the eggs hatch and the nymphs feed.

The 1st-generation adults appear in mid-June to mid-July. They leave the host weeds, dispersing into adjacent crops. The 2nd-generation nymphs feed on the host plants during August, potentially causing economic damage. The adults produced by this generation emerge in late August and September.

May June July August September
Overwintering adults become active, feed, lay eggs Nymphs feed, new adults emerge New adults mate, feed, lay eggs, nymphs feed Nymphs feed, new adults emerge Adults begin to overwinter

Scouting Techniques And Economic Thresholds

Canola - Thresholds are based on the number of lygus bugs sampled per 10 net sweeps. Canola should be sampled as flowering ends (stage 4.4), particularly if precipitation is low. If densities are near but less than the threshold at stage 4.4, canola should be resampled at stage 5.1 (when seeds in the lower pods are full size, translucent). If densities are sufficiently high, control is still warranted at stage 5.2 (seeds in lower pods green).

Lygus bug densities should be determined from a minimum of 15 samples of 10 sweeps or 10 samples of 20 sweeps per field. Samples can be collected from along the edge or at right angles from the edge of the field. Research has shown that samples taken along the edge of commercial fields and at various distances into the field all gave similar estimates of plant bug density. Sampling along the edge reduces effort during years when thick crop growth impedes access to the field. For edge sampling, the area selected for sampling should be at a crop stage similar to that in the main part of the field.

At crop stages prior to end of flowering, feeding by lygus bugs on canola does not generally result in economic damage. Research in Alberta found that low levels of Lygus bug feeding from bud to early flower stage induced greater branching and thicker stems compared to plants without Lygus bugs, and may increase seed yield.

Economic thresholds at the end of flowering in canola are calculated based on an assumed loss of 0.1235 bu/acre for each lygus bug per 10 sweeps.

Application Cost End of Flowering (Canola Crop Stages 4.4 - 5.1)1
$ / ha $/ ac Economic Injury Level
22 8.00 11 8 7 5 5 4
25 10.00 13 10 8 7 6 5
27 12.00 16 12 10 8 7 6
30 14.00 19 14 11 9 8 7
32 16.00 22 16 13 11 9 8
35 18.00 24 18 15 12 10 9
Canola Price ($/bu) 6.00 8.00 10.00 12.00 14.00 16.00

Economic threshold at pod ripening in canola are calculated based on an assumed loss of 0.0882 bu/acre for each lygus bug per 10 sweeps.

Application Cost Pod Ripening (Canola Crop Stage 5.2)1
$ / ha $/ ac Economic Injury Level
22 8.00 15 12 9 8 7 6
25 10.00 19 14 11 10 8 7
27 12.00 23 17 14 11 10 9
30 14.00 27 20 16 13 11 10
32 16.00 30 23 18 15 13 11
35 18.00 34 26 20 17 15 13
Canola Price ($/bu) 6.00 8.00 10.00 12.00 14.00 16.00

¹Crop stages of Harper and Berkencamp (1975):

4.4 is flowering complete, seeds enlarging in lower pods; 5.1 is when seeds in the lower pods are full size, translucent; 5.2 is when seeds in the lower pods are green.

When precipitation is greater than 100 mm from the onset of bud formation to the end of flowering, the plant may partially compensate for damage by lygus bugs.

Alfalfa Seed - In fields of alfalfa for seed, make five 180º sweeps with a 15-inch (40 cm) insect net through alfalfa canopy at each sampling site. Record total number of plant and lygus bugs (both nymphs and adults) captured. Calculate average number per sweep.

Economic thresholds in seed alfalfa are:

8 lygus bugs/sweep (40 in 5 sweeps) or 4 alfalfa plant bugs/sweep (20 in 5 sweeps) or 5 nymphs/sweep (25 in 5 sweeps) of any or all species of plant bugs, when the alfalfa is in bud or in bloom.

Chemical application should be timed so that the majority of nymphs are third instars or older.

Research in Manitoba has found that applying an insecticide in August or September to control late-season plant bugs in seed alfalfa is not economical.

Control is not recommended in alfalfa grown for hay.

Control Tips

If insecticide is required during the bloom stage, early morning or late evening applications should be targeted to minimize possible bee kill. In canola, spraying is best done at "post-petal fall" to minimize the effects of the insecticide on pollinators.

Heavy rainfall may reduce levels of early-instar nymphs of Lygus bugs. A study in alfalfa found heavy rainfall reduced first generation nymphs of Lygus lineolaris by 50%.

Nymphs of Lygus bugs may be killed by parasitic wasps in the genus Peristenus (Hymenoptera:Braconidae); with parasitism being common in weedy alfalfa stands or uncultivated weedy sites but very low in canola.