Lygus may be in fields throughout the season and can be highly destructive to a dry bean crop, according to University of California entomologists.
Lygus 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 helps to distinguish early instar lygus 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.
Damage from the lygus bug’s sucking mouthparts varies with plant age. During early bud and flowering stages, they cause bud and flower loss resulting in reduced yields. Lygus bug feeding on young, developing seed pods causes pod distortion, pitting, blemishes on table market beans, and reduce germination in seed beans.
A small parasitic wasp, Anaphes iole, as well as general predatorslike lacewings and damsel bugs, may prey on lygus bug nymphs.
Lygus are likely to move when weeds such as pigweed, wild radish, and mustard dry and become unsuitable. Lygus are also known to migrate from newly cut alfalfa fields and safflower fields to nearby crops, where they cause considerable damage.
Alfalfa is a preferred host and lygus may be managed there, to suppress movement of lygus into dry bean fields by staggering cuttings to preserve habitat. Leave a small, uncut strip at each harvest to help limit the movement of lygus bugs into neighboring beans. However, habitat preservation does not work as well in beans as in cotton because lygus prefers legumes as a habitat. Alfalfa strips also serve as reservoirs for predators and parasites that will eventually move into beans and help suppress spider mites, lygus bugs, and caterpillars. If 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.
Lygus populations can impact vine and bush varieties of baby and large limas differently. In research trials, bush variety Luna showed better lygus tolerance and higher yields than other varieties. Yields were not affected up to 1.5 lygus/sweep with Luna variety at the bud through flowering stage. (This variety does not have nematode resistance, however, and should not be used in fields with a history of nematode populations).
Start sampling during the bud stage and continue through pod fill. Check fields twice weekly. Determine lygus bug numbers (adults and nymphs) by using the standard insect sweep net. Take a series of five to ten 180 degree 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:
- Blackeyes: 0.5 lygus bug per sweep during bud through small pod stage; 1.0 bug per sweep later in season.
- Baby Limas: Luna variety 1.0 to 1.5 lygus per sweep from bloom to flowering; 1.5 to 2.0 per sweep later in season.
- Limas, all other varieties: 0.5 lygus bug per sweep during early bloom; 1.0 to 2.0 bugs per sweep later in season.
- Common beans: 1.0 to 1.5 bugs per sweep.
Determining lygus populations 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. Pulling the canopy apart and visually inspecting for lygus activity is highly recommended along with sweeping. Mid-morning evaluations are more accurate than afternoon evaluations because hot temperatures cause lygus to retreat into the lower sections of the canopy.