Lygus Bug (Western Tarnished Plant Bug)

Lygus bug adults are about 0.25 inch long and 0.1 inch (2.5 mm) wide, 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 bugs, including beneficials such as the bigeyed bug.

Lygus bugs can threaten a cotton crop from earliest squaring through cutout and final boll set. Lygus bugs pierce squares and damage anthers and other tissues. When squares are less than 0.2 inch (5 mm) 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 these 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, Extension agent or Farm Advisor for area-wide information on lygus bug populations. 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 bugs than cotton. These include alfalfa (seed and hay), safflower, sugarbeet, tomato, beans, potato, and occasionally oats. 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 bugs 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.

As a preferred host, alfalfa hay can be managed to minimize movement of lygus bugs into cotton by staggering cuttings to preserve habitat.

• Maintain nearby alfalfa fields in a succulent condition.
• Avoid cutting all alfalfa fields in an area within a short time period. Leave an uncut strip or check at each cutting along the border between alfalfa and cotton to slow lygus bug migration.
• If lygus bug numbers get very high, uncut strips of alfalfa may be treated with an insecticide if needed, but sprays should be avoided where possible to protect beneficials.

As a preferred host, alfalfa hay can be managed to minimize movement of lygus bugs into cotton by staggering cuttings to preserve alfalfa habitat. Leaving small, uncut strips at each harvest is very valuable in limiting the movement of lygus into neighboring cotton. Alfalfa strips also serve as reservoirs for predators and parasites that will eventually move into cotton and help suppress spider mites, lygus bugs, and worm pests. It is important to maintain nearby alfalfa fields in a succulent and vigorous condition to prevent large-scale release of lygus bugs. Avoid cutting all alfalfa fields in an area within a short time period. Staggering cutting will provide a mosaic of alfalfa growth stages. Within a field, leave uncut strips at each cutting in several locations in a field and along the border between alfalfa and cotton to slow lygus bug migration. The border strip may be used as a trap crop if lygus bug numbers are very high and threaten to move into cotton. If needed treat with an insecticide. Broad-spectrum insecticides should be avoided where possible to protect natural enemies.

Interplanting alfalfa and cotton has been successfully practiced in the past, drawing migrating lygus bugs away from cotton and concentrating it in alfalfa. This practice requires additional planning and management to avoid spilling lygus bugs from interplanted strips to cotton.

Black-eyed beans, easier to integrate into cotton production, have also been used successfully as a border crop to intercept and trap lygus bugs.

Lygus bugs have a wide host range of over 200 plants including many weeds. Russian thistle, black mustard, London rocket, wild radish, and goosefoot are good lygus bug hosts. When weedy fields and orchards are located near cotton, the lygus bug population in these fields may migrate when the weeds begin to dry. Avoid such migrations by removing the weeds before the population of lygus bugs reaches the winged adult stage. Before discing or mowing weeds, inspect them for the presence of lygus bugs and the stage of population development. If the population is already in the adult stage, migration will occur. Where possible, apply an insecticide before discing or drying the field.

Most of the cultural controls detailed above can be used to manage lygus bugs in organically certified cotton with the exception of treating weeds with an insecticide.

Populations of lygus bugs from cotton, alfalfa hay, and alfalfa seed fields have developed resistance to certain organophosphate, carbamate, and pyrethroid insecticides. Pyrethroid resistance increased significantly in the late 1990s, shortening the residual period for lygus bug control following an application. To manage resistance in lygus bugs that are infesting cotton fields, spray as few times as possible and rotate between insecticides with a different mode-of-action group number. Remember that sprays applied for other pests such as aphids can select for resistance in lygus bugs if they are present, so these need to be considered in a rotation scheme when selecting an insecticide for lygus bugs.

Consider fruit retention (in the San Joaquin Valley) as well as the results of sweep net samples when making treatment decisions.

Measuring Fruit Retention

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). Count the following :

• 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.
• 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.
• 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 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.

Taking Sweep Net Samples

Begin sweep net samples for lygus bugs at first square, sampling twice a week in each field. Note that lygus bug 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 bugs 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 (PDF). Do not confuse aphids or big-eyed bug nymphs with small lygus bug nymphs.
• Stop monitoring lygus bugs when Acala has 5 nodes about white flower (NAWF) and Pima has 3.5

Thresholds

Suggested thresholds are sliding thresholds, because lygus bug densities increase at a steady pace between late May and the end of June:

• Early Squaring (before 1st flower, until early June): >1 lygus bug/50 sweeps
• Until June 15: <2 lygus bugs/50 sweeps
• June 15-June 30: >2 lygus bugs/50 sweeps
• Mid-Squaring (1st flower - 1st mature boll, beginning of July): 7-10 lygus bugs (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 mature boll): 10 lygus bugs/50 sweeps, including the presence of nymphs

The above thresholds are guidelines to be used with square monitoring, depending on the particular weather patterns. For example, during warm springs they are very reasonable, because cotton is setting fruit early and has high retention potential. Higher thresholds may also be applicable, if fewer samples than outlined above are taken.

In contrast, late plantings, vigorous cotton, and high plant populations promote lower fruit retention and therefore thresholds will be lower. Additionally, duration of fruit retention may vary according to the cotton cultivar present in the field. The longer the fruit is retained, the longer it will be attractive to lygus bug populations. Finally, success in retaining early squares will greatly determine the final yield; therefore protecting cotton during the early square formation period (June) is critical. Protection during the early season is very complex. Factors such as low lygus bug numbers, high susceptibility of cotton, and variability in sampling require the grower to be extremely vigilant and ready to act at an instant.

Insecticide Selection

There are two basic approaches to selecting an insecticide for lygus bug control.

First Approach

During early fruiting when monitoring indicates lygus densities are low and square retention is only slightly off (5%).

Under these circumstances, reinspect the field again in 3 days before making a control decision. Upon reinspection, 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. Examples of such insecticides include flonicamid (Carbine), novaluron (Diamond), indoxacarb (Steward), or oxamyl (Vydate).

Second Approach

Population densities of lygus bugs 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 greatly reduced from previous inspections.

Insecticides that provide quick and residual protection are required. These include the pyrethroids (bifenthrin [GCS Bifenthrin 2EC], beta-cyfluthrin [Baythroid], imidacloprid plus beta-cyfluthrin [Leverage 360], lambda-cyhalothrin [Warrior], and zeta-cypermethrin [Mustang]). 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.

• Calculate expected square/fruit retention table
• Lygus monitoring form (PDF)
• Manage lygus by managing nearby alfalfa
• Photos of insects confused with lygus