Monitor to Make Lygus Decisions: Early Squaring to Bloom

Monitoring lygus bugs in cotton with a sweep net.

Decisions about lygus treatment must take into account the results of sweep net samples and measurements of fruit retention. Begin square/fruit retention monitoring at early squaring. Start sweep net samples for lygus at first square.

How to monitor lygus by sweeping:

Starting at first square, sample 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 sweep net with a diameter of 15 inches (37.5 cm). One sample consists of 50 sweeps across a single row of cotton. Take one sample in each quadrant of fields that are up to 8 acres (32 ha). Take more samples in larger fields.

  1. Walk briskly down the row and swing the net in front of you so that the lower edge of the rim strikes the plants about 10 inches (25 cm) from the top.
    • Keep the lower edge tilted 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 moving to prevent adults from flying out.
  2. After each set of 50 sweeps, count all the lygus bugs in the net, including nymphs, and record the total number on a lygus and fruit retention monitoring form (PDF). If you would also like to monitor lygus in your field over the course of the season, record your lygus counts on the lygus monitoring form (PDF).
  3. Stop monitoring lygus when:

Use these monitoring results in conjunction with results from square/fruit retention and treatment thresholds below to make a treatment decision.

How to monitor square/fruit retention:

Starting at early squaring, examine the 5 top branches and the 5 bottom branches on each of 5 plants in four areas of your field. For complete instructions, see the monitoring form (PDF).

  1. On each branch check for the presence or absence of first position squares.
    • Use the diagram of the cotton plant to help you.
    • After the plant has developed more than 10 fruiting branches, stop counting fruit on the bottom fruit branches if the average boll retention remains constant for 2 weeks in a row. You can use this retention level for the rest of the season.
  2. Now, find the expected square/fruit retention and compare it to your sampled value.

Use the sweep net samples and the square /fruit retention comparison, with the treatment thresholds, to decide about lygus treatment.

Treatment thresholds

  • Consider treating if square/fruit retention is lower than expected and if on average there are:
    • Early squaring: 2 to 4 lygus per 50 sweeps.
    • Midsquaring (bloom): 7 to 10 lygus (including at least 1 nymph) per 50 sweeps.
    • Late squaring (boll filling): more than 10 lygus (including 1 or more nymphs) per 50 sweeps.
  • If retention is higher than expected, you may be able to wait and monitor again later in the week before making a treatment decision.

Important links

Statewide IPM Program, Agriculture and Natural Resources, University of California
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