Managing late-season insects in cotton

 As we approach the end of August, one question that has been received lately more often than any other is at what stage do I stop worrying about insects in cotton?  

 

The major insect activity that I am noticing in South Plains cotton at this stage is around lygus bug and stink bug.  

Stop sampling and treating for lygus bug when cotton accumulates 350 DD60 (degree days 60) beyond five nodes above white flower (NAWF). The window of vulnerability for developing cotton bolls to lygus bug damage is significantly less after bolls accumulate 250–300 heat units or 8 days post-anthesis when the carpel wall has become sufficiently thick and mature that probing activity is less damaging. Lygus bugs will continue to feed on squares and small bolls in the plant terminals even beyond 5NAWF+350 DD60 but those fruiting structures will not contribute significantly towards the yield (not to the extent to offset the cost of an insecticide application).  Same applies to bollworms. Fields that have reached 5NAWF+350 DD60 are no longer susceptible to small (≤ 1/4 inch) larvae. This time frame may widen for larger worms but if you are not picking any bollworm activity at this stage, it is very unlikely they will appear from somewhere and reach damaging levels anymore.   

 

Stink bugs, on the other hand, will have to be scouted for a bit longer. Although they favor medium-sized bolls, they can feed on any size bolls. Stink bugs may feed on bolls 25 or more days old; bolls of this maturity are relatively safe from yield loss. In larger bolls, stink bug feeding often results in dark spots about 1/16 inch in diameter on the outside of bolls. These dark spots do not always correlate well with the internal damage—callus growths or warts. Once the cotton reaches 450 DD60 beyond 5NAWF, sampling and treating for stink bugs may no longer be necessary since bolls produced after this point will not become fully mature or contribute significantly to the crop yield. Now, it is possible that this value may shift slightly due to factors such as boll shedding, variety, irrigation, presence of pathogens (e.g., boll rot).  Action threshold for stink bugs is based on percentage of bolls with internal injury. For instance, the stink bug threshold during 5^th, 6^th, 7^th, and 8^th week of bloom is 10% 20%, 30%, and 50% internal boll damage (lint staining, boll warts), respectively.

Link to updated cotton insects guide: https://lubbock.tamu.edu/files/2022/07/managing-cotton-insects-in-texas.pdf

Update on the Mozena leaf-footed bug in Texas High Plains cotton

 Suhas Vyavhare and Kerry Siders

Leaf-footed bug, Mozena obtusa (fig. 1) is being spotted on a variety of plants across the Texas High Plains. We have gotten a few inquiries about how damaging this insect is to cotton and what insecticides would be most effective. In cotton, we have spotted this insect in multiple counties across the South Plains region. The numbers remain very low in all the fields where we have seen it except one field in Hockley County where it was found to infest cotton in large numbers (15-20 immatures/plant). In this particular field, insects appeared to be moving from the adjacent fallow field with mesquite trees which are preferred host of Mozena bug. Even the immature stages of this insect are quite agile and can travel some distance in search of new hosts. Fig. 2 shows results from the insecticide efficacy trial being conducted near Sundown, TX. Data collected at 3 and 7 days after treatment application indicate pyrethroids to be the most effective insecticides. We are also collecting data on the impact of Mozena bug on square retention in cotton. Thus far, we have not observed any square loss that can be attributed to this insect. Overall, square retention across the trial area remains over 90%.

Fig. 1. Mozena obtusa adult and nymphs

We are beginning to notice a few egg masses and small immatures on cotton plant indicating its suitability as a host for Mozena bug. With the absence and/or reduction in the availability of preferred hosts such as mesquite and other legumes, this leaf-footed bug can become an issue in cotton. Leaf-footed bugs in general prefer to feed on fruits and seeds and therefore their populations need to be monitored closely as our crop enters the boll development phase.  I would treat them more like stink bugs which feed in the similar manner with their piercing and sucking mouth parts and are mainly boll feeders in cotton. When scouting fields monitor both insect population density and the extent of damage to the crop (fruit retention, external and internal boll injury) and base the treatment decisions accordingly.

Here is a link to our recently updated cotton insect management guide: https://lubbock.tamu.edu/files/2022/07/managing-cotton-insects-in-texas.pdf

  

Fig. 2. Insecticide efficacy against leaf-footed bug, Mozena obtusa in Texas South Plains cotton. Means showing the same letter are not significantly different [Tukey’s HSD; P = 0.4226 (3 days after treatment (DAT)); 0.0302 (7 DAT)].

*Insecticide products were evaluated for research purpose only. When using an insecticide read and follow label directions for safety precautions, rates and preharvest intervals.

 

 

Highest Fall Armyworm Numbers in 12 Years of Trapping

The title of the article pretty much says it all; for the week ending today on June 22 I caught 1,242 fall armyworm moths per trap at Lubbock. The prior record, 912, was set in 2021 when FAW caused extensive damage as far as the upper Midwest. 

Thanks to some genetic work done by Ashley Tessnow, a post-doctoral researcher at Texas A&M, we know the vast majority of May moths were "corn strain" fall armyworms. However, she found that our summer flight might be as much as 35% "rice strain" mixed in.  This is not to say the majority of moths from this week won't go to sorghum or pasture grasses, for they surely will - the host preference between strains is not that strong. 

The bottom line is that any of our crops that are susceptible to fall armyworm should be scouted. Our Bt corn will provide good control of these larvae on whorl stage plants. However, sorghum, especially small sorghum, is at significant risk, as are any of the other host crops including vegetables. 

In looking back on my 12 years of data it is apparent that FAW numbers have increased on average over the years. I don't have an explanation for this, but I know what I am seeing. I am using the same pheromone lure year after year, and the traps are in the same place, so I think it is fair to say that increasing fall armyworm is a real phenomenon. 

 

Insect pest guesses for this strange growing season

As we begin this growing season where most of us have not had appreciable rain in over 200 days, it might be useful to review what things look like on the pest insect front. Of course there is no way to know what will happen and I don’t have a crystal ball. Here is what it looks like through the bottom of a Dr. Pepper bottle. 

Overwintering

Our winter was fairly mild, so we did not get high insect mortality from deep and sudden cold. Winter insect kill is greater when there is rain or melted snow and we had neither. Both things taken together mean that more insects successfully overwintered than in a “normal” year (whatever that is).

Early season host plants

Entomologists think in terms of early season “non-crop host plants”, meaning anything in the system that can support pests until the crop hosts become available. A common example would be tansy mustard that serves as an excellent false chinch bug host. Years with rain and lots of tansy mustard almost always mean we will have more false chinch bugs. Of course the lack of rain has meant a lack of early season hosts, so many of the insects emerging from overwintering will have little to no food. Populations of pests that overwinter here will start the growing season at lower overall levels than in wetter years. 

Fewer insects but fewer crop acres

Starting at lower pest levels is good, but fewer crop acres mean mobile insects will concentrate on those acres. If we cut the number of restaurants in Lubbock by 1/3, then those restaurants still in business would have around 1/3 more customers. Most overwintered insects that find themselves with no food or poor food host plants will leave, either by using their wings or catching a ride on the wind. They are able to detect healthy plants either by color or odor or both, so they will navigate to healthy crops. 

Immigrant pests

Crop conditions in the southern part of Texas are not as bad as ours here, so it is reasonable to expect a somewhat typical northern movement of cotton bollworm (corn earworm), fall armyworm and other migrants. These too will seek out, and concentrate on, the reduced number of healthy crop acres. Moth eggs are fairly susceptible to drying out and dying before they hatch, so high temperatures with low humidities will reduce the next generation to some extent. Those same conditions will affect the health of their crop hosts, especially dryland crops, which might in turn also mean less survival of their larvae. We can expect higher concentrations of migrant pests on our reduced acres, but how well their offspring survive is a tossup. 

Biological control

Most of our beneficial insects overwinter here and build up on our pests slowly at first, then faster as we get more pests. Given that we are starting with fewer pests, it is likely that our beneficial insects will get a much slower start this year. There will be fewer of them around to deal with any large influx of immigrant pests arriving from the south. 

Wildcards

Insects have many ways to survive hard times, and some of them are really good at it. When one species is heavily favored by some climate/host plant condition, it is common that other species that would normally compete with the favored species will be at a big disadvantage. Any insect that is wildly successful on the few weeds and non-crop vegetation that we have in our system can be expected to be at higher levels this year than in a “normal” year. 

Future years

What is happening now will likely affect our insect situation in future years. A good example might be Mozena obtusa, one of those wildcard pests. This big true bug hit us hard in 2013, but in fact its populations started building up in the drought of 2011. Mozena thrives on mesquite, which, all things considered, was not as heavily affected by the drought. Mozena has tough skin and is relatively free from biological control agents, so it got way ahead in 2011, those populations kept getting bigger and bigger on mesquite in 2012, and then they overflowed to our crops in 2013.  

That is what things look like through the bottom of a Dr. Pepper bottle. Field scouting will be very important this year; our overwintered pests might concentrate on the healthy crop acres, and the immigrant pests might be worse than normal.