Western tarnished plant bug,
Tarnished plant bug, Lygus lineolaris
Pale legume bug, Lygus elisus
Description. There are three species of lygus that predominate Texas cotton: the western tarnished plant bug, the
tarnished plant bug and the pale legume plant bug. The western tarnished plant bug is by far the most common species encountered in the western half of Texas whereas the tarnished plant bug dominates the eastern half of Texas. All three of these species are very similar in appearance and for management purpose are treated equally.
Adult lygus are about 1/4 inch long, approximately half that wide, and are somewhat flat and oval in outline. They have a conspicuous triangle in the center of the back, are winged, and vary in color from pale green to yellowish brown with white and reddish brown to black markings. Lygus adults are very mobile, readily flying from host to host. They tend to do most of their flying 1 hour before sunrise and 1 hour after sunset. Adults lygus may live up to 30 days. Adult females will go through a short pre-reproductive period and may not begin laying eggs for 5 to 10 days. A single female may lay as many as 30 to 70 eggs over her life time. Lygus eggs are very small, pale white, and cylindrical in shape. They lay eggs by inserting them into the plant tissue, leaving only the end of the egg exposed, thus making them very difficult to find. In cotton the eggs are usually laid in the leaf petioles, usually at the junction with leaf blade and will hatch in 6 to 14 days depending on temperature. When they first hatch, lygus nymphs are pale green in color and difficult to distinguish from later instar cotton fleahoppers. As they molt and mature, the lygus nymphs will take on a brighter color of green and will develop four distinctive small black spots on the thorax and one large black spot near the base of the abdomen; cotton fleahoppers have multiple tiny black spots on their abdomen. The nymph’s wings are not developed, but nymphs can move rapidly and are difficult to detect in cotton foliage. Lygus will go through 5 nymphal instars before molting to an adult. Depending on the temperature, it usually requires 10 to 15 days to develop from a 1st instar nymph to an adult.
Lygus bugs are also often confused with some species of scentless plant bugs, but scentless plant bugs tend to be thinner in appearance and have wider heads, relative to the thorax, than lygus. Scentless plant bugs many vary greatly in color and some scentless plant bugs have a scaly appearance whereas lygus are smooth and shiny.
Lygus prefer legumes to cotton and usually are found in large numbers in alfalfa or areas providing wild hosts such as clovers, vetches, mustards, thistle, pigweeds and dock. Lygus can also build up large number in many oil seed crops such as safflower. During June and July it is not uncommon to have very high lygus populations in alfalfa, but relatively low numbers of transitory adults in nearby cotton. However, beginning in August, lygus begin to disperse out of alfalfa into cotton in large numbers. In West Texas and in the High Plains, lygus tend to be more abundant in cotton beginning in August and continuing until boll opening. Afterwards, lygus will disperse back into alfalfa and weeds, enter diapause and overwinter as adults. Wild mustards appear to be a favored overwintering hosts for lygus in many areas.
Damage. Although both adult and immature lygus can feed and damage cotton, the nymphs are more voracious feeders and tend to cause the most severe damage. Lygus damage cotton primarily by feeding on the squares and small bolls. They feed by inserting their mouthparts into the tissue and sucking. When feeding on squares, lygus target the developing anthers. Small to medium sized squares will usually darken, shrivel and fall from the plant, while larger squares may remain on the plant. Flowers that develop from squares damaged by lygus may have tan to brownish colored markings and is referred to as a dirty bloom. These flowers will often have blackened shriveled anthers incapable of producing pollen. Bolls that develop from these blooms may be malformed and lopsided from unfertilized seeds and empty locules. These bolls are often called parrot beaked bolls, but not all parrot beaking is caused by insect feeding.
Lygus will feed on bolls as well, and when doing so are primarily targeting the developing seeds. Lygus feeding on bolls will cause small black sunken spots on the outside of the boll. However, external damage is not a concern, but it is when the lygus penetrates the carpal wall that economic damage occurs. Not all external damage equates to internal damage. On bolls 1/2 to 3/4 inch in diameter, approximately 50% of bolls with external lesion may suffer internal damage. Small bolls are most susceptible to lygus damage, while bolls that have accumulated 450 DD60 heat units (larger than 1 inch in diameter depending on the variety) are generally safe from internal lygus damage. However, this value may vary depending on water stress, variety, and shading. Bolls that are 1/2 inch in diameter or smaller will often shed due to heavy lygus feeding. Damaged bolls that remain on the plant will often have undeveloped seeds which results in reduced lint production, and misshapen or parrot beaked bolls. Additionally, lint that does develop will sometime have light tan stains.
Management and decision making.
Cultural management. Lygus problems in cotton can sometimes be averted by how surrounding lygus habitat is managed. Alfalfa is a significant source of lygus, and large populations of lygus may disperse into nearby cotton when the alfalfa is cut. Significant dispersal can often be eliminated by strip or rotational cutting of areas of the alfalfa. Lygus prefer alfalfa over cotton and if suitable alfalfa is available, lygus will primarily move into the uncut alfalfa rather than the cotton. Similar tactics can be used for weedy areas. Avoid mowing or plowing weedy areas infested with lygus, or leave portions for lygus to disperse into rather than the cotton.
Lygus prefer well shaded plant canopies, and will often not colonize cotton that is short in stature and less dense. Avoid cultural practices that promote rank, growthy cotton. Excessive irrigation and nitrogen will promote rank cotton, as will loss of early-season squares. Proper use of cotton plant growth regulators may help prevent rank cotton. Tight row spacing and volunteer cotton may also affect cotton canopy coverage and affect lygus colonization.
Biological control. There are a number of predators and parasitoids that prey on lygus. Undoubtedly the most significant biological control of lygus occurs in alfalfa and weedy habitats, but predation can also be important in the cotton crop. Big-eyed bugs, damsel bugs and collops beetles are notable predators of lygus eggs and small nymphs. Spiders will prey of nymphs and adults as well. Avoiding killing these natural enemies with broad spectrum insecticides will enhance lygus management and may prevent the development of damaging populations.
Scouting. There are several techniques that may be utilized to sample for lygus in cotton including sweep net, drop cloth, beat bucket and visual plant inspection. Refer to the section on sampling on page ?? to learn how to properly utilize these techniques. Prior to peak bloom it is recommended that a sweep net be utilized for sampling for lygus. A beat bucket also works well in this window, but our action thresholds are currently based on sweep net counts. Using a standard 15 inch sweep net and make 15 to 25 sweeps at a time concentrating on a single row. The number of sweeps you will be able to make is dependent on the amount of foliage that accumulates in the net. Avoid letting the net accumulate too much debris since it will adversely affect the ability of the net to effectively sample. Try to take at least 100 total sweeps from 4 to 6 locations in the field.
A drop cloth is recommended for sampling cotton post peak bloom. A black drop cloth tends to work better than white ones since the nymphs show up better against a dark background. When using a drop cloth, take 4 to 6 samples from 4 to 6 locations within the field.
Regularly monitor lygus populations in nearby alfalfa, safflower, weeds or other lygus habitat to determine the potential for developing a lygus problem in the cotton.
Chemical control and Action thresholds. The action threshold for lygus varies depending on the stage of growth of the cotton and the sampling technique used. Currently we are offering threshold based on sweep net and drop cloth sampling. Sweep net sampling is recommended prior to peak bloom and drop cloth sampling is recommended thereafter.
It is not uncommon for transitory adult lygus to move into a field but do very little feeding. Thus, make sure you check your square set to determine if the lygus are actually inflicting damage before deciding to spray. Once squares no longer have sufficient time to produce a mature harvestable boll, check bolls up to 1 inch in diameter for external lygus feeding marks as an indication of feeding activity. Once the cotton has reached 350 DD60s beyond cutout or 5 nodes above white flower, sampling and treating for lygus may no longer be necessary since bolls produced after this point will not fully mature. However, it is possible that this value may slightly shift due to factors such as water stress, variety, and shading.
You may find that portions of the field that are adjacent to alfalfa may have treatable levels of lygus whereas the remainder of the field is relatively clean. In this situation is may be advisable to treat only the portion of the field that is heavily infested.
Whereas lygus in most portions of the cotton belt are difficult to control due to insecticide resistance, lygus in Texas are still very susceptible. However, insecticide choice should still be carefully considered. Use of broad-spectrum insecticides such as pyrethroids and organophosphates may flare secondary pests such as aphids and mites. Be particularly careful if low levels of secondary pests are present when spraying for lygus. There are several insecticides available that are efficacious for lygus, yet relatively soft on many natural enemies.