Aphids and Thrips

Western Flower Thrips Adult
Photo: Kate Harrell

Howdy,

  I am currently keeping an eye out for thrips. This is a small (about 1/15") light tan or straw colored insect with a punch and suck type mouthpart and asymmetrical mandibles. They punch a hole with one side, then siphon the juice out with the other. They feed one plant cell at a time, and march along punching and sucking as they go. The adults are winged, can travel short distances on their own, and can be carried by a breeze for a fair distance. Larvae hide on the underside of the leaves, often close to the leaf veins. Feeding damage for this insect causes the leaves to crinkle and curl, and often looks silvery when examined. Thrips feeding can cause delays in plant maturity and eventual yield reduction.

Thrips Feeding Damage
Photo: Kate Harrell

  While the insects are visible to the naked eye and scouting can be done just by examining the plant, it is easy to miss some of the smaller larvae. Smacking a cotton plant around on the inside of a cup will knock them off and can make them easier to count. This video Blayne Reed put together has techniques for scouting thrips as well. Cotton with a neonicotinoid seed treatment is usually safe from thrips for about 2-3 weeks after emergence. Seedlings in a sandier soil will lose the effects of a seed treatment more quickly than those in heavier clay soils. Rainfall can also impact how long the seed treatments are effective, the more it rains the shorter the amount of time the seed treatment stays effective. Our weather has been particularly uncooperative up until most of this week, and we have a good deal of 1-2 leaf cotton that has already been in the ground long enough to have lost seed treatment protection.

  The economic threshold for thrips is one thrips per true leaf of the plant until the 5th true leaf stage. Once the plant reaches this stage, treatment for thrips is rarely justified. Check out the cotton insect guide at this website for more information.

Aphids on Cotton
Photo: Kate Harrell

The threshold for cotton aphids is 50 aphids per leaf, and if you see the aphid mummies in the field, that's a good thing. Parasitoid wasps lay eggs in the aphids, and the aphid forms a mummy while the wasp larvae pupates inside. These wasps, lady beetles, and lacewings all can make a dent in aphid numbers. Treatment for aphids is very rarely justified since the numbers need to be so high before they can cause an economic problem. If you do decide to treat for aphids, do not use a pyrethroid. Pyrethroids are non specific, and kill predatory insects as well, but aphids will bounce back quickly due to their high reproductive rate. 

Aphid Mummies on Cotton
Photo: Kate Harrell

  Lately I've heard some talk about spider mites in cotton, but those tend to prefer hot and dry weather. The humidity here normally keeps their numbers from getting too high. If you are concerned about a field, there is more information on this website.

  I've still been seeing sugarcane aphids on Johnson grass on the edges of fields. A&M is participating in a nation wide mapping project for the sugarcane aphid movement again, and I check very few sorghum locations. If you see sugarcane aphids moving into sorghum, I would greatly appreciate it if you would contact me. Your input will help a great deal with this mapping project.

  Please feel free to contact me if you have any questions or concerns, and check out the links below for more information.

Sincerely,

Kate Harrell

For More Information:

Aphids in Cotton

Cotton Growth and Development overview

Cotton Growth and Development

Plant Population Evaluation

Replant Decision Making