Oklahoma Cotton: Weekly Scouting for Thrips a Priority

Thrips damaged cotton.

Thrips will be one of the first pests to scout for in cotton during the growing season. Each field needs to be scouted on a scheduled weekly basis to be sure no pests are present.

Thrips generally are not considered a serious pest of cotton, except in years when favorable growing conditions permit early planting. Heavy thrips infestations will occur if plants have emerged before wheat or other small grains mature.

Mature thrips often move into stands of succulent cotton seedlings, causing curled and misshapen leaves. Thrips infestations vary from field to field and from year to year and should be dealt with accordingly.

Thrips are small, approximately one-sixteenth inch in length. The color varies according to species. It may be similar to the color of wheat straw, yellow or light brown.

The adults have two pairs of long, narrow, fringed wings which enable them to fly from one crop to another. The life cycle contains several stages: egg, larva, pupa, and adult. Larvae and adults will over winter in debris and trash near the field.

In the spring, the adult females lay eggs by inserting them into the plant tissue. The creamy white eggs hatch into small larvae which begin to feed on the plant. After a short time, they burrow into the soil and transform into a non-feeding stage called the pupae.

They emerge as adults and continue to feed on the plant. Thrips damage cotton by using their rasping-sucking mouthparts to feed on the plant epidermis. Ruptured cells release plant fluids which are sucked up by the insects.

Injury first appears as dark brown spots which assume a silvery appearance several days later. Feeding occurs on the lower side of the leaf and may injure the terminal bud so that new leaves fail to develop and growth is retarded. Leaves will be crinkled and cupped.

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Weekly scouting is the only way to monitor a seed treatment performance. Expect damaging populations of thrips to materialize first in fields where no seed treatment insecticide was used. Windy conditions will impact your ability to accurately assess thrips numbers.

In-field detection becomes nearly impossible as the wind picks up. Take a composite sample pulling at least 30 plants across the field placing them in a plastic bag or bucket. Waiting to examine plants until you return to your vehicle will take a little longer, but will be a lot more accurate.

Besides looking on the undersides of cotyledons and true leaves, be sure to examine the terminal bud. Both adults and immature thrips feed and lounge around there and are easily overlooked unless you carefully inspect this region.

Also don’t forget to count and record the numbers of dislodged thrips running around on the inside of the baggie.

Crop demographics play a large role in thrips pressure. Wheat is widely known as an early season habitat for thrips. However, alfalfa is another thrips nursery that can produce large numbers. With each cutting thrips migrate from the field in search of a food source.

Cotton fields in close proximity to alfalfa meadows may experience huge influx of thrips overnight that might even rival the exodus from adjacent wheat fields. Also, with the amount of spring rainfall we have encountered in some areas, other alternate hosts have provided considerable habitat for thrips populations to buildup.

Finding adult thrips in protected fields is normal and is expected as long as the thrips migration continues. Remember that thrips blown in from adjacent areas may not feed immediately and feeding is required for the insect to pick up a lethal dose of a systemic insecticide.

Various foliar products are available and have also been recently evaluated in Texas.

Orthene/acephate is the standard foliar thrips control product, and when used properly can provide good thrips control. At the 4 oz/ac rate, acephate will generally provide about 5 days control.

Bidrin (dicrotophos) has long been used for aphid and stinkbug control, and in the past used more frequently for thrips. At 3.2 oz/acre it performs comparably to acephate, but based on limited data appears to provide slightly less residual control.

Things to consider when using foliar applications for thrips control:

  1. Timing is critical. Controlling thrips during the first 2 weeks post crop emergence appears to be the most important period; especially under cool conditions. You need to be “Johnny on the spot” with these applications when thrips are numerous; even a few days delay can be detrimental.
  2. Avoid automatic treatments. Automatically adding a foliar thrips material in a Roundup application may not be necessary or may be poorly timed. Often either the weeds aren’t present when the thrips are or vice versa.
  3. Scout for thrips. Go out and visually assess if thrips are present. Pull up plants and thoroughly search for them or beat the plants inside a plastic cup.
  4. Don’t spray based on damage. The damage you see today happened 3 to 5 days earlier and the field may have already suffered yield loss. Spraying based on damage is essentially a revenge treatment.
  5. Spray based on thresholds. Use an accepted action threshold to help you determine whether or not you should treat.

It is easy to spot when the insecticide performance begins to fade by keeping track of the plant’s physical condition related to thrips numbers. As protection fizzles, visual leaf damage should increase along with a rise in thrips numbers.

Cool temperatures will result in lack of vigorous early cotton growth, and will in turn increase susceptibility to thrips damage. Quick action will prevent maturity delays associated with infestations that reach or exceed three thrips per plant.

Over-the-top sprays can be used in fields planted to glyphosate-tolerant (Roundup Ready Flex and GlyTol) varieties. This strategy of tank mixing an insecticide with glyphosate is cost effective.

Acephate (Orthene) has been a standard foliar thrips treatment for many years. For the application rate, refer to the specific label for your product of choice, as several products containing acephate are now available.




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