During Roger Leonard's tenure as an LSU AgCenter research entomologist he's heard many consultant stories. The stories often recall a treatment recommendation and application followed by an alarming, pest-heavy field evaluation, and end with suspicions of insect resistance.
But while it always lurks, a bevy of questions must be addressed prior to a resistance diagnosis. Among them: Did the farmer actually spray the field? Was the chemical put into the tank by a clueless loader at the aerial applicator's headquarters? Was enough water used? Was the application mistimed because some of the worms appeared to be four or five days old?
“If it was a repeat treatment, did you wait too long between intervals?” asked Leonard, who spoke on integrated resistance management at a recent DuPont-sponsored consultants meeting in Memphis. “Was the rate cut? Are you evaluating (product efficacy) too soon?”
Was the right pest being treated? “In many instances, because we're targeting pests at a smaller stage, we still have scouts misidentifying fall armyworm/beet armyworm with corn earworm/tobacco budworm.”
Was the pest pressure high enough to require a second application immediately to control them? Were the larvae too big? And, finally, was there rainfall within a few hours after treatment causing all residual control to be lost?
After the list
If, after going through the list above, “you're convinced none apply, resistance could be associated with the control failure. However, we have to validate that population as resistant in the lab to rule out many of the other possibilities.
“But just the presence of insecticide resistance in a population doesn't guarantee a failure. I know that sounds like a contradiction. But, in fact, it's true in many instances — including in the Mid-South with pyrethroids and corn earworm.” Two factors contribute to this.
The gene frequency is in the population — but at a low level.
There haven't been high numbers in recent years even though there's a portion of the population expressing a resistant “gene frequency so control failures haven't been observed. It's being masked by the relatively low populations of tobacco budworm and bollworm.”
Several examples of this can be illustrated by looking at two population scenarios: one pest per 10 plants for a 10 percent infestation versus a high population at a level of one pest per plant (100 percent infestation). Realistically, said Leonard, the best control for most contact activity insecticides “is around 80 percent against susceptible individuals.”
Treatment against a low-level population will leave about 2 percent of the pests — or a larva every 50 plants. In such cases, a producer probably wouldn't call for a re-treatment.
With 80 percent control of the high population, however, “there are still about 10,000 live insects in the field. If there's an egg-lay, the population will probably increase. That leaves about a 20 percent infestation, even with completely susceptible insects. And you will probably need to re-treat…There was good control, but not satisfactory.”
Things are very different when viewing the same population levels but with a resistant population. The low population will only see 40 percent control — a pest every 15 plants.
In the worst-case scenario, there's high pressure and a high frequency of resistant insects. With one insect per plant and 40 percent control, the crop still has two insects for every three plants. “That's a problem. So what do you do? First, call your chemical salesman out and tell him, ‘We need to re-treat. We have a control problem. Everything was put out the way it should (have been).’”
In such circumstances, here's the first rule of thumb: “If you know it's a resistant population never re-treat with the same product or products within a class. In other words, if you're putting our one pyrethroid, never re-treat with another one just because it has a different name.”
History and selection
Insecticide resistance is a genetic change in any pest.
“When pyrethroids were first introduced in the mid-1970s, they were extremely active at low rates on tobacco budworms. In recent years, those products have gone by the wayside. Newer, more active pyrethroids were developed until they began to fail against budworms in the 1980s.”
Against beet armyworms, however, the first pyrethroids — and the most recent ones — have never expressed good activity. “Populations of beet armyworm have always been tolerant — not resistant — (to pyrethroids. ‘Tolerance’) isn't a genetic change. These insects were predisposed to that tolerance.”
There are two principal components involved in resistance development, said Leonard: gene frequency and selection pressure. “How often do you spray? What rates are used? The more frequently you spray, the faster the resistant individuals are selected out.”
A similar situation is currently being observed with the increase in glyphosate-resistant weeds. “It's taken a lot longer to see weed resistance develop because there are only one or two applications on weeds that, for the most part, have one generation a year. With insects that have multiple generations in a year, you're selecting 4X or 5X more.”
There are three groups, or classes, of factors affecting resistance. The first two are “biological and genetic. These are associated with an organism in the environment and we can't do much about them.”
The third class is referred to as “operational” and involves things “we control in the management of an insect and how we can reduce the potential for resistance development. These include selecting the right product, using the correct rate, formulation, use of refuges, etc.”
Can resistance be managed? Not really, said Leonard.
Delaying the widespread occurrence of field failures due to resistant populations is possible, however.
If producers had an alternative as effective as Orthene on plant bugs a decade ago, “we wouldn't be talking about tarnished plant bug resistance. That's the bottom line.”
When organophosphates quit working on tobacco budworm, pyrethroids were used. When pyrethroids quit working, producers moved to Bollgard cotton. If Bollgard ever becomes ineffective, Leonard said, there will likely be a switch to a new trans-gene.
Alternative insecticides, “or alternative toxins,” are one strategy for managing resistance. A mix of products is another.
“In many cases prior to Bollgard, high rates of product mixtures were used to manage populations. Although it's called a ‘resistant management strategy’ for mixtures to truly be effective, they must be used at the same time populations are susceptible to both insecticides — not after resistance occurs.”
Rotation of products has become the most common integrated resistance management philosophy. This involves rotations among classes as well as modes of action.
Leonard said the “temporal” or “windows” approach has been most commonly used.
Leonard showed a slide illustrating the seasonal rotation of insecticides divided into four application windows.
“Tarnished plant bugs are rarely a significant pest on seedling cotton up to first square. At that time, most treatments are going out are for thrips. Dimethoate isn't that effective against plant bugs, but it's fairly effective against thrips. Low rates of Bidrin are also excellent against thrips.”
Moving into the second window (first square to first flower) “we'd use the neonicotinoids such as Centric, Intruder, or Trimax; a carbamate such as Vydate; and the new product, Carbine. You could band those products and see satisfactory management at an economical cost.”
In the third window, Leonard has listed Orthene and Diamond. “Later in the year, because you haven't used Orthene, it will be much more effective.”
Moving into the last window, “There's been some success mixing Centric with other products at this late stage. Diamond in combination is also used in this stage. Bidrin and Orthene also fit this window — partially because this is a time when stink bugs can be a major problem.”
Such a rotation sequence in cotton “makes logical sense and also fits in the sequence of treatments for other pests as well.”
A refuge for Bollgard, Bollgard II, or WideStrike cotton refers to a structured area of non-transgenic cotton. But, depending on the type of technology being used, it can actually be non-crop hosts or even an unsprayed area of other field crops.
Producers need to recognize that yield losses can be associated with structured cotton refuges. For a refuge to be effective, in many cases, “there can and will be yield loss. The goal of a truly effective cotton refuge is to produce insects, not yield. If you're going to allow insects to complete development on, for example, a structured, non-Bt cotton refuge, they must survive and feed. That means the crop will be damaged.”
The goal, said Leonard, is to produce enough insect numbers in the refuge to “literally outnumber” those in the transgenic crop. That makes it less likely the rare, resistant insects will find each other “because you've overwhelmed them with the numbers migrating from the refuge. That's the premise behind the strategy.”
Asked his opinion on doing away with refuges when registration for Bollgard II is approved, Leonard was reluctant to endorse such a move at this time. “I've had an opportunity to review Monsanto's data. In fact, I was a member of a scientific team that helped generate a lot of the numbers that went into it. We still believe there are a few questions and issues in the Mid-South database.
“I admit the data for the Southeast is much clearer than what we have for the Mid-South. The data is clear in showing that there are instances during the season where cotton is still important in producing these insects. There are other periods, though, when cotton isn't a primary host — they're coming off native plant hosts and we aren't even sure of what those are. I need to look at the data a little deeper and discuss my concerns with experts in this field of IRM.
“In addition, I'm concerned…if we make that change and something happens, we won't be able to go back. The risk, to me, may outweigh the benefit. But this is a risk that must consider the needs of our clientele, the Mid-South cotton industry.”
The future and costs
Leonard's last slide focused on future resistance concerns and the cost of resistance to cotton producers in the Mid-South. When considered in these terms, “it emphasizes the value of IRM strategies.”
If Bt cotton is lost, products like Steward, Tracer and Denim will be used for control of tobacco budworm. Producers will spray three to five times.
Lose pyrethroids for bollworm control and producers will likely spray one to four times with those products.
Lose both the above control measures — “and that's a possibility. Remember: resistance will eventually occur” — and producers could spray up to eight times.
And why, Leonard asked, “would you plant Bt cotton if you're going to spray it with alternatives as many as eight times?
“Right around the corner (due to developing resistance), we're looking at losing organophosphates for tarnished plant bugs. Presently, there are no products as effective as the organophosphates, Bidrin and Orthene, on tarnished plant bugs late-season. Keep that in mind when looking at maintaining the effectiveness of these products in the future.”