Lygus bug adults are about 0.25 inch long and 0.1 inch across, and flattened on the back. They vary in color from pale green to yellowish brown with reddish brown to black markings, and have a conspicuous triangle in the center of the back.
Nymphs resemble adults, but are uniformly pale green with red-tipped antennae; larger nymphs have five black spots on the upper body surface. Nymphs do not have wings. Other insects may be confused with lygus, including beneficials such as the bigeyed bug.
Lygus bugs can threaten a cotton crop from earliest squaring through cutout and final boll set. They pierce squares and damage anthers and other tissues. When squares are less than 0.2 long, they shrivel, turn brown, and drop from the plant. Damage to larger squares may be to anthers, styles, and stigma, and may interfere with fertilization.
If many squares drop, the plant may put its energy resources into vegetative growth, resulting in tall, spindly plants and reduced yields. Lygus bugs also feed on and destroy terminal meristems, causing bushy plants. If bugs pierce the wall of young bolls (typically less than 10 days old) and feed on young seeds, these seeds may fail to develop. Lint around the injured seeds is stained yellow, and may not mature normally.
Lygus bugs migrate to cotton from other hosts, so management of this pest begins with assessing its populations outside the field. Check for them on weeds, in nearby alfalfa, and in other crops, and keep in touch with your pest control adviser (PCA). Proper management of alfalfa harvest can reduce damaging migrations to cotton. The need for insecticides in cotton must be evaluated carefully on a field-by-field basis, as treatments may result in secondary outbreaks of spider mites, aphids or other pests.
Other crops are more attractive to lygus than cotton. These include alfalfa (seed and hay), safflower, sugar beet, tomato, beans, and potato. As these crops are prepared for harvest, winged adults migrate out of the field in search of new hosts. Careful management of these crops can reduce the migration of lygus into cotton fields during cotton's most vulnerable period: mid-May through late July. Watch closely cotton fields that are downwind from these crops by sampling the cotton and surrounding fields often.
Alfalfa hay can be managed to minimize movement of lygus by staggering alfalfa cuttings to maintain a favored habitat. Avoid cutting all fields in an area within a short time period by leaving uncut strips along the border between alfalfa and cotton in order to slow migration. These uncut strips of alfalfa can be treated with an insecticide (if needed).
Many weeds, including Russian thistle, black mustard, London rocket, wild radish, tarweed and goosefoot are good lygus hosts. When weedy fields and orchards are located near cotton, the lygus population in these fields may migrate when the weeds begin to dry. Avoid such migrations by removing the weeds before the population of lygus reaches the winged adult stage. Before disking or mowing weeds, inspect them for the presence of lygus and the stage of population development. If the population is already in the adult stage, migration will occur. Where possible, apply an insecticide before disking or drying the field.
Planting trap crops can be helpful in managing lygus, but unless the crop has the same growing requirements as cotton, it can be difficult to maintain. Alfalfa is a good trap crop but can be difficult to grow under the same conditions as cotton. When used as an interplanted crop for lygus management, 20-foot strips of alfalfa are planted every 300 to 500 feet of cotton. (When two crops are growing in the same field, harvest restrictions, label restrictions, and crop destruction for both crops must be obeyed.) Other trap crops more compatible with cotton production include cowpea and lima bean. Generally the trap crop is not planted for commercial purposes and should be considered part of the pest control cost.
Monitoring, treatment decisions
Decisions about lygus treatment must take into account the measurements of fruit retention in the San Joaquin Valley, as well as the results of sweep net samples.
In the San Joaquin Valley, begin monitoring fruit retention when five fruiting branches are present. Take weekly plant measurements from four different areas of the field to assess plant retention of squares. Randomly select 5 plants in each area (for a total of 20 plants) and count the number of first-position squares present on the top five nodes. Starting at the top of the plant, count the first mainstem leaf that is at least the size of a quarter dollar as the first node. Also count the number of retained fruit in the first position on the bottom five fruiting branches. (Until 10 fruiting branches are available, there will be overlap between the top five and the bottom five nodes.) Finally, count the number of fruiting branches, that is, those branches above the vegetative branches. (After the plant has developed more than 10 fruiting branches, counting fruit on the bottom fruit branches can be discontinued after 2 weeks if the average boll retention remains constant, assuming that no boll damage or loss due to bollworm and stink bug occurs.)
Determine the percent retention of first-position fruit for both the bottom and the top five fruiting branches by dividing the number of retained first-position fruit by the number of fruiting branches examined (20 plants X 5 fruiting branches = 100 branches) and multiply by 100.
For example, if 60 first-position fruits were found on the bottom five fruiting branches of 20 plants, then 60 first-position fruits ÷ 100 fruiting branches ≡ 0.60. Multiply this number by 100 to get percent retention (0.60 X 100 ≡ 60%). Therefore, the percent retention for the bottom five branches is 60%. Do the same calculation to determine the percent retention of the top five fruiting branches.
Use the table in the online version of this guideline (http://www.ipm.ucdavis.edu/PMG/C114/m114sccalcretn.html) to determine the critical square retention based on the total number of fruiting branches and the percent fruit retention on the bottom five fruiting branches. Continue sampling for lygus bugs until monitoring nodes above white flower indicates the plants are no longer susceptible to their damage.
Sweep net sampling
Begin sweep net samples for lygus at first square, sampling twice a week in each field. Note that lygus populations may rise rapidly when they migrate in from drying weeds or safflower, harvested alfalfa, or other crops.
- Always use a standard net with a diameter of 15 inches. Take one sample in each quarter of the field in fields that are up to 8 acres. Take more samples in fields that are larger.
- Each sample consists of 50 sweeps across a single row of cotton. Walk briskly down the row and swing the net in front of you so that the lower edge of the rim strikes the plants at about 10 inches below the top. Keep the lower edge slightly ahead of the upper edge.
- Keep the sweeps far enough apart that you do not sweep plants that have already been jostled by the net. Sweeps that are too closely spaced may cause lygus to fly or drop from the plants and thus be missed. Keep the net in motion to prevent adults from flying out.
- After each set of 50 sweeps, count all the lygus bugs in the net including nymphs, and record the number on a monitoring form. Do not confuse aphids or big-eyed bug nymphs with small lygus bug nymphs.
Suggested thresholds are:
- Early Squaring (before 1st flower): 2-4 lygus/50 sweeps
- Mid-Squaring (1st flower - 1st boll): 7-10 lygus (at least 1 nymph) per 50 sweeps and expected or better fruit retention. If retention is higher than expected you may be able to wait and monitor again that week before making a treatment decision. If retention is lower than expected and lygus bugs are present, consider treating.
- Late Squaring (after 1st boll): 10 lygus/50 sweeps, including the presence of nymphs
There are two basic approaches to selecting an insecticide for lygus control. The first approach occurs during early fruiting when monitoring indicates lygus densities are low and square retention is only slightly off (5%). Under these circumstances, re-inspect the field again in 3 days before making a control decision. Upon re-inspection, if square retention continues to be slightly off normal and there is some migration in from surrounding areas, consider an insecticide that provides adequate control but has little residual effect on natural enemies.
The second approach is when population densities of lygus are high and there is the potential for repeated and sustained invasion, or there is evidence of widespread reproduction. In addition, square retention is below the expected level and reduced greatly from previous inspections. Use insecticides that provide quick and residual protection. Research has demonstrated the link between pyrethroid use and aphid population buildup, and this must be considered when planning to use one of these products.