Lygus Bugs

The lygus bug adult is about 0.25 inches (6 mm) long and about half as wide. It is generally brownish but varies from green to straw-colored, tawny, or light brown. Its body is marked with a pattern of different shades of brown and occasionally yellow or red marks. A prominent V-shaped yellowish area is near the center of the body at the base of the wings.

Lygus bug eggs are laid within plant tissue so that only the oval-shaped cap is visible. These eggs are hard to locate, even with the use of a hand lens. Active green nymphs hatch from the eggs. Red coloration on the tips of the antennae in combination with their faster movements helps to distinguish early instar lygus bugs from aphids. Older nymphs may be recognized by distinct wing pads and by the five black dots on their backs, two on the first and second segments of the thorax and one in the center of the abdomen.

Lygus bugs are a highly destructive bean crop pest that can be found in the field from June through September. They have sucking mouthparts they use to pierce and consume plant tissue. The type of damage varies with plant age:

• During early bud and flowering stages, lygus bugs cause bud and flower loss, reducing yields.
• As young seedpods develop, lygus bug feeding causes pod distortion, pitting, and blemishes (stings) on table market beans and reduced germination in seed beans.

Lygus bug eggs are often parasitized and killed by a small parasitoid wasp, Anaphes iole. General predators, such as lacewings and damsel bugs, may prey on lygus bug nymphs. Plant habitat that attracts natural enemies and avoid the use of broad-spectrum pesticides known to harm natural enemies.

Lygus bugs are likely to migrate from weedy areas to crops when weeds such as pigweed, wild radish, and mustard become dry and unsuitable. Control weedy vegetation on field margins, especially wild mustard and radish. In addition to weeds, lygus bugs can migrate from newly cut alfalfa and safflower fields to nearby bean crops, causing considerable damage.

Alfalfa hay, a preferred host, can be managed to suppress movement of lygus into dry bean fields by staggering the alfalfa cuttings to preserve habitat. Leave a small, uncut strip at each harvest to help limit the movement of lygus bugs into neighboring beans. The use of habitat preservation does not work as well in beans as in cotton because lygus prefers legumes as a habitat. However, alfalfa strips also serve as reservoirs for natural enemies that will eventually move into beans and help suppress spider mites, lygus bugs, and caterpillars. When considering the use of alfalfa habitat preservation (strip or staggered cutting), be aware of the potential for quick movement and establishment in beans by lygus bug.

Consider using tolerant varieties when available. Large lima bean varieties show no differences in susceptibility to lygus bug. However, some baby lima varieties are more tolerant of lygus bug feeding damage than others:

• UC Beija-Flor bush type and UC Haskell vine type both have shown good lygus bug tolerance.
• Luna showed fair lygus bug tolerance, however, this variety does not have nematode resistance, thus should not be used in fields with a history of nematode outbreaks.

For more information on varieties, refer to Table 1 in Lima Bean Production in California, (PDF) Publication 8505.

Use lygus bug- tolerant varieties when available, biological control, and cultural controls for an organically certified crop.

Start sampling during the flower bud stage and continue through pod fill.

• Check fields twice weekly.
• Record observations on a monitoring form (PDF).
• Determine lygus bug numbers (adults and nymphs) by using a standard insect sweep net.
  • Take a series of five to ten 180° sweeps in four to six areas of the field.
  • Pass the net through the top of two rows of bean plants (one bed for double row plantings or two beds for single row plantings).

Treatment thresholds are available for bean yield; however, these thresholds may not reflect losses in bean quality. Treatment thresholds are averages of all sweeps:

• Blackeyes: An average of 0.5 lygus bug per sweep during bud through small pod stage; 1.0 bug per sweep later in season.
• Baby Limas: Luna, Beija-Flor, and UC Haskell varieties: An average of 1.0 to 1.5 lygus per sweep from bud to flowering; 1.5 to 2.0 per sweep later in season.
• Limas, all other varieties: An average of 0.5 lygus bug per sweep during early bloom; 1.0 to 2.0 bugs per sweep later in season.
• Common beans: An average of 1.0 to 1.5 bugs per sweep.

Note: Determining lygus bug numbers in vine-type varieties is difficult because of the large amount of plant biomass that makes it impossible to penetrate into the canopy with a sweep net. Therefore, when sweeping a vine-type variety, pull the canopy apart and visually inspect for lygus bug activity (e.g., presence of insects or damage). Mid-morning evaluations are more accurate than afternoon evaluations because hot temperatures cause lygus bug to retreat into the lower sections of the canopy.

• Lygus monitoring form (PDF)