This planting season on the gulf coast has ranged from a little dry to fantastic so far. Hopefully we’ll catch a few rains where we need them and get everyone off to a good start. It’s been windy, and I’ve seen cotton with damaged stalks and leaves from the wind, as well as some stand reduction.
This year I’ve seen more stalks getting whipped around by the wind, then seedlings wilting from introduction of plant disease, like rhizoctonia, from the injury sites at the base of the stalk. If you’ve got stand reduction, fields can still be considered viable with stand counts as low as 13,000 to 26,000 plants per acre, as long as they are fairly evenly spaced.
There have been a few thrips, aphids, and spider mites out in the fields as well. I’m a little more concerned about thrips, as the numbers of spider mites and aphids have not been at levels of concern.
Thrips are 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, as well as in the terminal of the plant.
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.
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. A video Blayne Reed put together (see below) 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.
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.
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.
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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.
The spider mite populations I’ve seen have been pretty low. The humidity here normally keeps their numbers from getting too high, but it has been a bit less humid here lately. If you are concerned about a field, there is more information on this website.
If you have any questions feel free to contact me. Stay safe and wash your hands!