Thursday, August 25, 2016

Cotton on the Texas High Plains: Watch out for Bollworm Activity

Suhas Vyavhare, Texas A&M AgriLife Extension Service

Insect pressure remains light in most parts with cotton ranging from 5 nodes above white flower to hard cutout. We are seeing conchuela stink bug population reaching economic threshold in few fields in Crosby County. However, the infestation is much localized and it is unlikely that we will see economic stink bug infestations in cotton in other areas of the High Plains. I often encounter a few lygus adults and nymphs but the numbers remain well below economic threshold. At this point, one insect that is on our radar is the cotton bollworm. Although much of our cotton has cutout hard and is becoming non-attractive to worms, there are still enough suitable cotton fields out there to worry about.

Earlier this week, Brad Easterling, IPM-agent in Glasscock, Reagan, and Upton counties reported above threshold levels of bollworms in Bt cotton fields near Garden City. Blayne Reed, IPM-agent in Hale, Swisher, and Floyd counties reports that he is seeing increased numbers of bollworm moths in his pheromone traps (http://halecountyipm.blogspot.com/2016/08/late-august-2016-bollworm-threat.html). With the corn and sorghum maturing, we may see increased bollworm movement out of corn into cotton in the next few weeks which makes regular scouting for bollworm larvae essential.
Please report any signs of higher than normal worm damage in cotton (especially Bt cotton) to me at 806-723-8446. We can visit a field and collect insects for resistance and/or old world bollworm screening.

Bollworm larva
When scouting, make sure you do whole plant inspections (squares, white blooms, pink blooms, bloom tags and bolls) for bollworm larvae and injury. Make sure to inspect at least 100 randomly selected plants covering all major areas in the field. Bt toxin is not well expressed in the flower tissues, and as a result bollworm larvae can often be found associated with pink blooms and bloom tags. One should be careful about not oversampling bloom tags while scouting Bt cotton fields. Also, remember bollworms must feed on the cotton plant before they ingest a lethal amount of the Bt toxin, so 1st instar larvae (<1/8-inch) should never be used as a trigger point to spray.

It is often hard to control bollworms with foliar insecticide application once larvae grow larger than ½-inch long. Therefore, it is important to spray for larvae when they are still smaller. If treating a bollworm population that is actively feeding on bolls, consider using a long residual contact insecticide that the worm is more likely to become exposed to when moving from one boll to the next. When targeting bollworms, pyrethroids with good cov
Bollworm damage
erage can still do the trick. However, if fall armyworms are present, the product choices may differ as pyrethroids are weak against fall armyworm, especially larger larvae.

Once cotton plants have an average of 3 nodes or fewer remaining above the uppermost first position white bloom or when the upper bolls that will be harvested have become difficult to cut with a pocket knife, they are normally safe from bollworm injury.

Here is a link to our new video on scouting for bollworms in cotton. (https://www.youtube.com/watch?v=vJhTJ9doDSw

Whole plant inspection method: bollworm action threshold based on number of larvae per 100 plants

Cotton type
Cotton stage
Worm size
Before bloom
≥30% damaged squares and worms are present
After boll formation
≤1/4 inch
10-15 worms per 100 plants
Do not treat
>1/4 inch
8-12 worms per 100 plants
8-12 worms( >0.25 inch) per 100 plants with >5% damaged fruit
Fields that have accumulated 350 DD60s beyond 5 NAWF are no longer susceptible to first or second instar bollworm.