 |
Field and Forage Crops |
Weekly cotton insect update tapes are available in MP3 format on the Cotton Insect Corner web site at: http://ipm.ncsu.edu/cotton/insectcorner/radio/index.html.
Although many areas of the state are still dry, cotton is generally growing well and the threat of thrips is now over in many fields. Even with lower levels of migrating thrips, it would still pay growers to check later-planted or later-germinating cotton fields in the cotyledon to 4 true leaf stage for adult and immature thrips and associated damage to buds and newly-forming leaves.
From now until the beginning of our bloom period, typically the only insect pests of potential concern are tobacco budworms, plant bugs, cotton aphids, and spider mites. Although tobacco budworms have not yet been able to find a way to damage Bollgard cotton, they can become established in conventional cotton in significant numbers in mid to late June in some years. Fortunately, budworms need to remove over 1/2 of both the squares and terminals in pre-bloom cotton before any maturity delays or yield losses occur. After bloom begins, it is a different situation. But that is another 3 to 4 weeks away in most cotton fields. At this time, if one does find this high degree of budworm infestation, either Tracer or Steward is recommended for control. Our early budworm threshold of 15 percent live budworms in terminals or 10 percent live budworms on squares is probably overprotective.
Plant bugs are not an economic problem on pre-blooming cotton if the retention of small squares is 80 percent or greater. Monitoring the retention rates of small squares is the easiest way to keep abreast of potential plant bug problems in pre-blooming cotton. If the square retention rate drops below 80 percent, then sweeping with a standard net is recommended. The finding of small, blackened squares is often an indication of recent of plant bug activity. Sets of 25 rigorous sweeps down and through cotton rows in 8 to 10 randomly-selected areas in a cotton field should provide some idea if treatable levels of plant bugs are present. Observe cotton field edges for indications of plant bug migration and the possible need for shorter monitoring intervals, but do not oversample these areas. In North Carolina, we typically treat only about 5 to 8 percent of our cotton fields for plant bugs at this time of year, though plant bugs can cause heavy damage, particularly in some of our far eastern counties. Weekly assessments of square retention should be regarded as the minimal monitoring approach for the initial detection of potential plant bug damage.
The cotton aphid is a pest that is often best left to natural control. At this time of year, an aphid-mummifying wasp parasite can greatly limit aphid levels, and by mid-July a fungus can eliminate cotton aphids over wide areas. Predators such as lady bird beetles can also help growers. Additionally, unless conditions are very dry and the cotton plants are under drought stress, treatment for cotton aphids is not usually justified. However, if aphids are present on most of the plants in a dry cotton field, honeydew is easily detected on many plants, and no parasites or aphid-parasitic fungi are present, then treatment may be indicated. One of the new chloronicotinoids such as Centric or Trimax will usually do a good job of both controlling cotton aphids and sparing beneficial insects. Because of cotton aphid resistance to organophosphate insecticides, avoid using insecticides such as Orthene and Bidrin.
Spider mites typically leave cotton alone in most years. However, with our dry weather in many areas of the state, mite outbreaks are more likely. As is the case with cotton aphids, a combination of high mite levels, mite damage to the point that affected plants are losing yellowish to bronzy lower leaves, and no detectable fungus may indicate the need of one (or occasionally two) foliar application for mites. If rainfall is predicted, hold off on applying insecticide. Significant rainfall both relieves the dry plants of stress and helps create conditions favorable for the parasitic mite fungus.
Employment and program opportunities are offered to all people regardless of race, color, national origin, sex, age or disability. North Carolina State University, North Carolina A&T State University, U.S. Department of Agriculture, and local governments cooperating.
Last modified on June 14, 2004 by Stephen J. Toth, Jr.
