Lygus pose a threat to cotton from earliest squaring through cut-out and final boll set.
The pests pierce squares and damage anthers and other tissues. When squares are less than 0.2 inch long, they shrivel, turn brown, and the plant sheds them. Damage to larger squares may be to anthers, styles, and stigma, and may interfere with fertilization.
If too many squares drop, the plant may go vegetative, resulting in tall, spindly plants and reduced yield. Lygus feeding also destroys terminal meristems, causing bushy plants. If lygus 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.
Sampling for lygus with a sweep net should begin at first square, according to University of California recommendations. Sample fields twice a week. Be aware that lygus populations can increase rapidly as lygus migrate from drying weeds, safflower, harvested alfalfa, or other crops.
Always use a standard net with a 15-inch diameter. Take numerous samples across fields.
A sample is 50 sweeps across a single row of cotton. Walk briskly down the row and swing the net in front of you so 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 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 be missed. Keep the net in motion to prevent adults from flying out.
After 50 sweeps, count all the lygus in the net including nymphs, and record the number. Do not confuse aphids or big-eyed bug nymphs with small lygus nymphs.
Early squaring(before first flower): 2-4 lygus per 50 sweeps.
Mid-squaring(first flower to first boll): 7-10 lygus (at least 1 nymph) per 50 sweeps. If fruit 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 are present, consider treating.
Late squaring(after first boll): 10 lygus per 50 sweeps, including the presence of nymphs.