Thursday, July 14, 2016

Southern Plains of Texas: Time to Look for Lygus

Suhas Vyavhare, Texas A&M AgriLife Extension Service

With the daytime highs over 100 degrees almost each day, cotton is squaring with some fields starting to bloom. Although we have received light and spotty showers, we need more over the next couple of weeks for the cotton to continue to grow. Insect pressure remains very low in most places. I know only of a couple of fields that needed to be treated for cotton fleahoppers (Swisher County) so far. Cotton fleahoppers are generally considered a pest in early squaring cotton. As plants increase in size and fruit load, larger numbers of fleahoppers may be tolerated without yield reduction.

With the fields starting to bloom, the next insect pest we
should look for is a lygus bug. Both adult and immature lygus can feed on cotton with their piercing and sucking mouth parts. The damage occurs primarily by insect feeding on the squares and small bolls.  As a result of lygus feeding, small to medium sized squares usually darken, shrivel and fall from the plant, while larger squares may remain on the plant. Flowers that develop from squares damaged by lygus may have tan to brownish colored markings and are referred to as dirty blooms.
Lygus feeding on bolls causes small black sunken spots on the outside of the boll.  Small bolls are most susceptible to lygus damage, while bolls that are larger than 1 inch in diameter are generally safe from lygus damage. Bolls that are 1/2 inch in diameter or smaller will often shed due to heavy lygus feeding.  

Alfalfa is a significant source of lygus, and large populations of lygus may disperse into nearby cotton when the alfalfa is cut.  Significant dispersal can often be eliminated by strip or rotational cutting of areas of the alfalfa.  Lygus prefer alfalfa over cotton and if suitable alfalfa is available, lygus will primarily move into the uncut alfalfa rather than the cotton.  Similar tactics can be used for weedy areas.  Avoid mowing or plowing weedy areas infested with lygus, or leave portions for lygus to disperse into rather than the cotton.

We are seeing a good numbers of big-eyed bugs, damsel bugs and collops beetles in cotton that are notable predators of lygus eggs and small nymphs.  Spiders prey on nymphs and adults as well.  Avoiding killing these natural enemies with broad spectrum insecticides will enhance lygus management and may prevent the development of damaging populations. We have just produced a new video on scouting for Lygus in cotton. (https://www.youtube.com/watch?v=gfSM8jF_Rqs )

Lygus Action Threshold

Sampling method
Crop stage
Drop cloth
Sweep net
1st two weeks of squaring
1-2 per 6 ft-row with unacceptable square set
8 per 1oo sweeps with unacceptable square set
3rd week of squaring to 1st bloom
2 per 6 ft-row with unacceptable square set
15 per 100 sweeps with unacceptable square set
After peak bloom
4 per 6 ft-row with unacceptable fruit set
15-20 per 100 sweeps with unacceptable fruit set