An Insect with Many Names
Helicoverpa zea is an insect with many names. Its most common names are the corn earworm and cotton boll worm. It really depends on where it is feeding which can cause some confusion. These caterpillars can also be found feeding on sorghum where they are called headworms. We have been seeing quite a few in the field while scouting sorghum heads. Learn more about their effects in sorghum here.
Hereafter Helicoverpa zea will be referred to as bollworm to avoid confusion, but I just wanted you to keep in mind this insect has a number of hosts.
Bollworm in Bt Cotton
Last year numerous reports of economic bollworm populations in occurred in Widestrike, Bollgard II and Twinlink cotton. Collecting efforts last year revealed bollworms feeding on cotton bolls and flowers, but the numbers were very low for the amount of cotton we inspected. The images of the bollworms we found in Widestrike cotton are included in this newsletter. Reports of bollworm in south Texas cotton are similar this year with an unusually large number of cotton acres being treated with insecticides.
It is common knowledge that the proteins expressed in Widestrike cotton are less effective against cotton bollworm than other Bt technologies. It was somewhat concerning that cotton bollworm are damaging Widestrike cotton across a much wider area than we saw last year with many acres in Kleberg county treated once or twice with insecticides. This suggests that resistance to some Bt proteins may be a growing issue among cotton bollworm. Bollworm resistance to certain Bt proteins (mainly the cry 1 proteins) is well documented in the mid-south and southeastern cotton regions in the U.S. and in other countries around the world. It is not too much of a stretch to have similar issues in Texas cotton.
Bollworm insecticide treatments in Bt cotton consisted of several insecticides but most fields have been treated with a pyrethroid (a lot of bifenthrin has been applied to fields). Most applications are providing an acceptable level of suppression (60% or greater) while some pyrethroid applications were considered unacceptable necessitating a second insecticide application. I will talk about pyrethroids toward the end of this letter.
How Do You Explain These Boll Worm Populations in Bt Cotton?
The following is a list of some of the reasons why we might have seen bollworms feeding on Bt cotton and some important principles to consider.
- More bollworms than we generally see. There is a history of years where large bollworm populations have appeared in Bt cotton and causing economic damage to all Bt technologies. These years seem to be related to large influxes in bollworm populations. In other words those years in the past between 1995-96 and 2016 there seems to have been a much larger number of bollworms. One reason for the recent issues with bollworm in cotton is could be attributed to the growing number of corn acres in south Texas including the Rio Grande Valley. Cotton bollworm populations begin building in corn with subsequent generations moving to sorghum then cotton. Migration is another consideration with moths flying northward during each generation. Over time within a season, bollworm populations can be very large once they move to the upper Gulf Coast and Winter Garden regions of south Texas. Large bollworm populations (including a larger population of bollworm resistant to some cry proteins) can overwhelm Bt technologies. If you dump five million worms into a field with Bt traits and dump five thousand worms into a neighboring Bt cotton field, you will see a big difference in survival. In the first case some are much more likely to survive because of the shear number present.
- Bollworms are gaining resistance to Bt proteins. This is looking like a key factor for increased damage in south Texas Bt cotton fields. Keep in mind that jumping to this conclusion without documentation can be very dangerous…it creates tensions, distrust, and doubts around the technology. There is work being done at Texas A&M to document bollworm resistance to specific Bt proteins so we should have a better handle on this issue next season. Regardless of the outcome of this research, the best solution is working together to find answers to the issue at hand and developing remedial plans to rectify control failures.
Overall I feel like the answer is some sort of combination of the reasons listed above. Regardless an important thing you need to remember when dealing with pests is that you should always have multiple management techniques ready to go in your plan. Never rely on just one type of control to give you 100% management. The following video about integrated pest management discusses this principle and why it is important to use multiple types of control methods for the best long term results.
Pesticide Treatment Options for Bollworm
One type of pesticide that is commonly used to treat boll worms are pyrethroids. This in part because they are inexpensive and usually offer an acceptable level of suppression. The challenge we have found with pyrethroids and bollworms goes beyond the actual suppression. When a group of bollworms survives a pyrethroid treatment they are more likely to pass on resistant traits to their offspring. Over-spraying across the region caused genetic pressures to favor pyrethroid resistant bollworms.
Bollworm resistance to pyrethroids is somewhat reversible between seasons. This only occurs when spraying is limited so non-resistant moths can breed with resistant moths.
The fact that pyrethroids are the most cost-effective means of managing bollworm is as true today as it was in 2003, when we saw the first signs of resistance. This is particularly true considering that certain pyrethroids can be used for simultaneous control of stink bugs, plant bugs, and bollworms in cotton. Use of best management practices (IPM) are important to limit control failures with pyrethroids. Some of the proposed practices offered in 2003 remain excellent options to manage pyrethroid resistance in bollworm. These management suggestions included:
- Thresholds Scouting for eggs of a combination of eggs and worms is not recommended. Remember, the worms has to ingest plant material to be exposed to the proteins expressed in the plant. My suggestion is to consider treatment when 2 to 3 percent of the bolls show signs of bollworm damage AND live worms are found on the bolls (or other reproductive parts of the plants) and live bollworms are found in plant terminals. If worms are not in plant terminals consider treating when 5 to 6 percent of the bolls have bollworm damage and live worms are found on damaged bolls. If possible, treat when worms are small (1/4″ to 3/8″ long) and before they bore into bolls. Once the worm is inside the boll they will be almost impossible to kill.
- Insecticide Applications Penetration of the insecticide through the canopy is extremely important. As bollworms grow they will move to the lower part of the plant and they must come in contact with the insecticide for the insecticide to kill the worm. Consider a final volume of 10 to 15 gallons per acre for the application and slow the ground rig down as you move through the field. A minimal volume of 5 gallons per acre should be considered when making aerial applications of an insecticide.
- Use the highest labeled rates of the pyrethroid
- Consider tank mixes of pyrethroids with a low rate of an alternative chemistry to pick up stragglers. Previous studies have shown that pyrethroid resistant caterpillars only comprise about 20% of the population.
- Use alternative modes of action. The modes of action are the pathway that the poison affects the insect. Some pesticides act in different ways. If you want to try an alternative to pyrethroids consider the diamides (Prevathon and Besiege) or spinosin (Blackhawk). Stay within the recommended application rate range for each product (i.e. 17 to 20 oz/a for Prevathon, 9 to 10 oz/a for Besiege, or 2.4 to 3.2 oz/a for Blackhawk).
I also wish to note that identifying the larvae properly is an important step. Fall armyworms a different species tends to have a higher resistance to pyrethroids. You can read a bit more about how to tell the difference between the two in the headworm section of my last newsletter here.
Thanks to Jason Thomas for helping write and contribute ideas, video footage and content for this newsletter. All videos, graphics and images were created by him unless otherwise noted in the image caption.
Robert Bowling, Ph.D.
Assistant Professor and Agrilife Extension Entomology Specialist
Texas A&M Agrilife Research and Extension Center at Corpus Christi
10345 Hwy 44
Corpus Christi, TX 78406