Texas West Plains Cotton: Bollworm Management Considerations

    Bollworm on cotton leaf. Photo: Texas AgriLife Extension

    Area producers need to be scouting conventional cotton for bollworms! Just this week the scouts and I have been finding cotton bollworms, not over threshold of 8,000 small to medium worms per acre, but at 0 to 3,500 small worms per acre. I have been finding an egg lay of around 2,000 to 20,000 eggs per acre and subsequent worm numbers for the past couple of days.

    Beneficial numbers are good and may be sufficient to stay up with these current infestations of worms. Fortunately, we are not seeing cotton aphids, which can be a tough expensive venture if you need to treat for both. But do be careful to scout for both and not to flare the aphids if you do end up needing to spray for worms.

    The peanuts and grain fields can be more attractive to egg laying moths possibly. But you must check every cotton field individually to be certain. This is the reason for a good consultant.

    Now I have talked mostly about conventional cotton. This does not excuse you from checking your other cotton acres with technology. We can find worms there as well. Now we are not finding as much damage or so far cause to treat. However, I would still closely watch/scout all cotton for all insects, period.

    Bollworm and Tobacco Budworm Management

    Bollworm and tobacco budworm larvae look very much alike and cause similar damage. Full-grown larvae are about 1½ inches long and vary in color from pale green, pink, or brownish to black, with longitudinal stripes along the back.

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    Bollworm and tobacco budworm moths are attracted to and lay eggs in cotton with lush new growth. Moths usually lay single eggs on the tops or undersides of young, tender, terminal leaves in the upper third of the plant. However, in transgenic Bt cotton, moths frequently lay eggs on blooms, drying petals of blooms (bloom tags), or other tissues deeper in the canopy.

    Larvae survive better because these Bt cotton tissues have lower concentrations of Bt protein toxins.

    Tobacco budworm and bollworm eggs are pearly white to cream color and about half the size of a pinhead. They can be confused with looper eggs (which are flatter), can have a blue greenish-white tint, and are usually laid singly on the undersides of leaves.

    Bollworm and budworm eggs hatch in 3 to 4 days, turning light brown before hatching. Young worms usually feed for a day or two on tender leaves, leaf buds, and small squares in the plant terminal before moving down the plant to attack larger squares and bolls. Small worms are most vulnerable to insecticides when they are in the upper third of the plant.

    Moths sometimes deposit eggs on squares, bolls, stems, and lower parts of the plant when cotton plants are stressed and making little new growth, or during periods of high temperature and low humidity. Detecting eggs and controlling small worms is more difficult when the eggs are deposited in the lower plant canopy.

    Budworms are typically less numerous than bollworms, and, in recent years, have rarely reached damaging levels. With the increased adoption of transgenic Bt cotton, insecticide sprays for bollworm and tobacco budworm control have become much less common in Texas, particularly in West Texas.

    Scouting and Decision Making

    In Bt cotton, search the entire plant for tobacco budworm and bollworm larvae and injury. A proper sample includes squares, white blooms, pink blooms, bloom tags, and bolls. Reduce the scouting intervals to 3 to 4 days during periods of increasing bollworm egg laying, especially during peak bloom. The presence of eggs alone should not trigger treatment since hatching larvae must first feed on the cotton plant to receive a toxic dose.

    Terminal and Square Inspection Method

    • Divide the cotton field into four or more manageable sections, depending on the field size.
    • Examine 25 plant terminals (upper third of the plant), selected at random from each quadrant, for small larvae and eggs. Also, from each quadrant, examine 25 half-grown and larger green squares as well as small, medium, and large bolls for bollworms and bollworm damage.
    • Keep track of the number of undamaged and damaged squares and bolls. Select fruit at random and do not include flared or yellow squares in the sample.
    • Pay attention to bloom tags and petals stuck to small bolls; they will often hide larvae that burrow into the tip of the boll.

    Whole Plant Inspection Method

    • Divide the cotton field into four or more manageable sections, depending on the field size.
    • Make whole-plant inspections of five randomly chosen groups of three adjacent cotton plants in each section. Look in every square, bloom, and boll.
    • Thoroughly inspect dried blooms or bloom tags attached to small bolls.
    • Count the number of undamaged and damaged fruit and calculate the percentage of damaged fruit.

    Chemical Control and Action Thresholds

    Thresholds in Bt cotton fields are based on how many worms survive to late first- or second-instar larval stage, not on newly hatched larvae or the presence of eggs. Since newly hatched larvae must feed on the plant for the Bt toxin to be effective, base treatment decisions on damaged fruit and the presence of larvae.

    Budworms are more resistant to certain insecticides (for example, pyrethroids) than are bollworms, but more sensitive to the Bt toxins in transgenic cotton.

    Aphids and other secondary pests may increase when broad-spectrum insecticides targeted at budworms or bollworms disrupt natural control. When secondary pests are present during a budworm or bollworm outbreak, use a selective insecticide to help prevent a secondary pest outbreak.

    Insecticides in the diamide, oxadiazine, and spinosyn classes are more selective than the pyrethroid and carbamate classes.

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