Texas Cotton, Corn: Bollworms, Bt Resistance, and Field Scouting

Transgenic crops are an ever-evolving technology dealing with pests in the corn, cotton and sorghum fields in Hale and Swisher counties of Texas. Farmers began relying upon these technologies in the 1990s. Bt and the newly released Viptera (VIP) have really changed the game for agriculture, but still haven’t eliminated the need for humans to scout fields for insects and damage to crops from other sources.

Scouting is one of the best ways evaluate economic populations of pests and to detect early signs of resistance to Bt or Viptera plant traits.

Bt comes from the bacteria Bacillus thuringiensis which has crystal-proteins (Cry). When worm pests ingest the Cry, their digestive system is destroyed thereby killing the worms without harming other beneficial insects and in many cases eliminate or reduce the need for chemical treatments of crops for control of these worms.

Another bacterium was used to make the Viptera trait which is commonly used and is usually stacked with a Cry trait to kill even more types of harmful worms. These traits are inserted into crop genes, which allows the crop to produce the toxins that kill the pests and ultimately make the crop resistant to the worms.

Bt and Viptera technologies help crops protect themselves from harmful pests, but also save farmers from having to apply pesticides, dramatically cutting annual costs for growers and aiding in environmental protection.

Like all forms of control measures though, pests develop ways of dealing with our technologies. Any caterpillar pest may express a previously unimportant gene that would make it resistant to Bt or Viptera traits. While very few insects will initially express a resistant gene when control measures are first introduced, over time the insects that contain the resistant gene will reproduce through a now increased selection pressure.

That’s when you begin to see a higher portion of insects that are resistant to the control measure. That’s also why stacked traits are commonly used to help fill holes of resistance by creating a “backup” control and by targeting pests that may not be affected by some traits (Porter 2018) or express two genes controlling resistance.

Bollworms are one of the primary pests in cotton as it starts producing fruit. Although Bt wasn’t originally designed to target bollworms, it did show some level of control against them. This is where bollworm resistance to Bt traits started as a larger portion of the bollworm population was surviving the Bt trait than the target pests.

While control of the other targeted pests remains good, bollworms soon became a focus for additional Bt traits. The bollworms already had an advantage on some of the other Bt traits targeting it in that they were already surviving the original Bts easier. So, some of the staked traits behaved on a substantial portion of the bollworm population as a single trait.

The more recently released Vip3A is one of the only traits now proven in field and labs to work against bollworms. Unfortunately, many seed companies are putting the Vip3A trait in corn and cotton.

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Prior to Vip3A’s entry into corn to control corn earworms, what bollworms are referred to in corn, the bollworm was not considered an economic pest in corn. The ear-tip only damage that bollworms cause in corn was considered light while the amount of chemical treatments and effort to keep bollworm out of the ear was astronomical.

Bollworms have cannibalistic characteristics; you may find several earworms at the tip of the corn ear, but soon they will attack each other until there is only one earworm left. With this characteristic in mind, controlling the worm in corn was deemed not economical by most, bad IPM by entomologists by killing off beneficial insect populations and flaring secondary pests, and dangerous by environmentalists by pumping so much insecticides into a field.

However, with Bt traits working so well on the originally targeted caterpillar pests, bollworms became the only ear damaging pest of note in corn. It was not long before the new Vip3A was evaluated and found to be a great defense against bollworms in corn allowing producers to fully harvest 100% of the ear, boosting yields higher.

Now we are faced with a new dilemma. Since the Vip3A is one of the greatest defenses against bollworms, using it in both corn and cotton puts an extraordinary and successive generational selection pressure on this species that has already shown to have ra propensity to overcome the preexisting Bt traits.

As bollworms prefer corn as a primary host and cotton as a secondary host, moths will always choose corn first. As corn has a lower soil planting temperature, corn is typically planted first by growers. The bollworms then first attack corn. As this corn is maturing and finishing out, the next generation of bollworms then moves to cotton, just then setting bolls.

This repetitive selection generation after generation is very likely to cause Vip3A resistant bollworm populations (which also happen to be able to survive other traits too) to increase dramatically and quickly. So far, there isn’t any sign on the High Plains of this resistance. In some of the southern areas of the cotton belt where corn is also a major crop, lab results show some cracks are already forming in the Vip3A trait in bollworm control.

All is not doom and gloom in dealing with the bollworm. Field trials and sentinel plot monitoring of bollworm control via the varying Bt traits from across the Cotton Belt do show that all Bt types offer reductions in bollworm populations. Most Bt traits are also still offering good control on most of the originally targeted pest species. But, in many cases and under heavy bollworm pressure, these traits have proven to not be sufficient alone in controlling the bollworm any longer.

A return to mandatory and scheduled weekly field scouting with the possibility of treatment to maintain field profitability is a must. As of today, Vip3A should be the only trait on the market that has a chance of keeping bollworms from reaching economic levels that require chemical treatment to maintain economic profitability.

With this trait in both corn and cotton, we do not know how long this will last on the High Plains. We also need to understand that the Vip3A toxin is not expressed as strongly in cotton as it is in corn, so we expect to see resistance first in cotton.

Unfortunately, cotton producers are likely to need to spray more as VIP trait starts to fail, while corn producers will not see any affects for a while longer. It’s important to understand the management and use of transgenic crops so that we do not help increase insects’ resistance to the traits we are developing in our crops. It might be possible to delay the onslaught of bollworm resistance by simply not planting corn with the VIP trait early in the growing season so a generation of selection pressure is missed.

Ultimately it is up to the producer to decide what traits they will plant in their fields. It is also up to the producer to determine what is happing in their fields on a timely basis. While company development new Bt technologies can help slow down pests’ resistance to Bt and Viptera traits, it’s important to watch for the first signs of resistance by scouting fields for harmful pests. Field scouting of all Bt types in cotton is now recommended by Texas A&M AgriLife Extension.

Scouting of transgenic crops and treating once research proven established economic thresholds are reached, will ensure that our technologies like Bt and Viptera remain beneficial to growers by keeping yield damage down and yield production high. Ag Researchers believe that Bt and VIP traits have caused many growers to become too reliant on the protection offered by transgenic crops. They see the practice of scouting decreasing by farmers as they seem to think of scouting as an outdated practice no longer needed since they plant Bt and Viptera crops.

Yet, we know scouting is one of the best ways to monitor and detect early signs of pest resistance to our technologies and is extremely useful for decision making related to pest control. Without scouting, growers do not realize there is resistance until it is too late to do anything about it to save their crops.

When there is a large amount of yield loss, many farmers try scrambling to seed companies hoping to resolve their issue. But full resistance is difficult to definitively prove after the fact. By scouting and reporting early signs of resistance, farmers can stay ahead of insect resistance to Bt technologies and maintain control of the economics in their field.

In this current environment, blind overreliance on Bt technologies such as Bt and Viptera could prove disastrous and we truly do not know when any additional technologies might be released to help alleviate the issue. A return to good scouting techniques remains a must for all cotton fields.

When you are learning or preparing to scout cotton fields, we do recommend you consult the latest Cotton Insect Management Guide from Texas A&M AgriLife Extension which can be found here.

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When we scout cotton fields in the Plains Pest Management Program, we utilize the 8,000 to 10,000 bollworm per acre threshold and long-time established scouting techniques that look for multiple pests. For each field we normally get 4-10 data sets taken across the field where at least 2 whole plant inspections are conducted and a drop cloth are taken (drop cloths are more helpful for detecting other cotton pests).

Larger fields require a higher number of data sets and a longer walk across the field. We conduct the whole plant inspections by scanning the tops and bottoms of leaves for bollworm eggs and the squares and bolls for damage. Sometimes, fruiting bracts must be carefully manipulated for a good 360˚ view. Any fruit that has been dropped or shows bollworm damage is popped open and scanned thoroughly which includes under the boll or square petiole.

All bloom tags from each plant, both old but still moist and new, are torn open and all flower parts inspected for small worms. The number and age of each egg and bollworm found is recorded at each data set along with overall health, fruit load, and status of each plant is recorded along.

Next, we move on to the drop cloths at each data set. Bollworms can be recorded via drop cloth too, but mostly this would only confirm what you are seeing on the whole plant inspections. All fruit that has fallen from these plants both on the drop cloth and on the ground along the drop cloth are evaluated just like during the whole plant inspections. Many pest populations are best recorded quickly and easily with the drop cloth but they should never be used for bollworms without conducting whole plant inspections.

Sometimes the best field scouts can detect very, very low populations of bollworms with lots of practice but an inexperienced scout using drop cloths alone will likely not find the accurate bollworm population without inspecting plants. When in these fields, we will also gather information on plant stage, weed species and pressure, PGR measurements, irrigation needs, and fertilizer needs.

After finishing each field, the number of bollworm eggs, small, medium, and large bollworms are calculated against the number of plants inspected and then the final plant per acre population to arrive at the needed bollworm per acre figure.

For producers or researchers only interested in bollworms and not any other pests or plant health, irrigation, weeds, or cotton PGR needs can quickly conduct the newly developed percent harvestable fruit damage for a clear bollworm population pressure measurement tool. For this method, simply collect 100 harvestable cotton fruit (what is harvestable will vary depending upon crop stage and time until last possible boll set and nearness to average freeze date) from at least 4 areas in a field with no more than 2 fruits being collected from each plant.

Then simply evaluate the fruit for bollworm damage. By using this method, 6% of the harvestable fruit damaged by bollworm feeding is the economic threshold. For those now asking, to reach this 6% level, you need about 8,000 to 10,000 bollworms per acre, cotton stage depending. Both methods of scouting cotton for bollworms are interchangeably accurate.

With the per acre method, many pests and crop issues can be evaluated at once. With the percent damage method, you can get an accurate picture of the bollworm population quite quickly.

Scout your fields and if you have questions or concerns, reach out to the local county IPM agent and attend informative field days sponsored by Texas A&M Extension Service.


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