Just because growers currently have a fairly well stocked arsenal of miticides doesn't mean they should throw caution to the wind. That's the way Larry Godfrey, University of California, Davis Cooperative Extension entomologist summed it up for growers and pest control advisers at a recent Bayer CropScience-sponsored cotton conference. Godfrey knows how quickly resistance can develop in mite populations and what incredible problems it can create.
It wasn't too long ago that San Joaquin Valley cotton growers knew first-hand what those problems could entail. Kelthane, Comite and even Zephyr have all had resistance issues in some areas at some point. Those problems, while sometimes sporadic and not always permanent, often created a tremendous headache in the absence of effective alternative materials to control mites. Fortunately, that's no longer the case. Growers now have a relatively wide range of choices to battle mites. Even so, they should guard the treasure trove with caution, according to Godfrey.
“Mites can build resistance very quickly,” he says. “There are three species of spider mites that are important in San Joaquin Valley cotton — two-spotted, Pacific and strawberry. Of those, the two-spotted spider mite does the most damage.”
Mites are particularly troublesome due to several factors, according to Godfrey. They are found on multiple crops. They're capable of overwintering as adults in multiple crops, weeds and even in the soil.
“Because they can overwinter in the soil, you'll often find the same problem in the same place year after year,” he says. “However, they are also very mobile which can make the problem even worse. They can rapidly damage a crop.”
During the heat of the season, spider mites can quickly cycle through a generation — sometimes in as little as one week. That propensity to reproduce and do it so quickly only fuels the possibility of resistance.
“There are over 300 cases of documented resistance in two-spotted spider mite,” Godfrey says. “During the time we relied heavily on Zephyr, we were fortunate to get through it without running into problems. Now, we're fortunate to have a lot of new materials with different modes of action.”
Rotating between those materials is critical to preserve the efficacy of the entire arsenal. Now that Zephyr is off patent and its price has dropped, there is concern that overuse of the material could lead to resistance problems.
“That situation always seems to result in increased use,” says Tim Krueger with Bayer CropScience. “It's not always about price. You have to think about the bigger picture. If we don't steward these products, we're shooting ourselves in the foot.”
Oberon from Bayer is one of the newer entries to the market. In trials conducted at UCCE's West Side Field Station at Five Points, Calif., by Godfrey since 2001, it has proven to be a consistent performer. In 2006, Godfrey evaluated numerous treatments including Fugimite, Zephyr, Zeal, Denim, Oberon, Onager, Comite, Kelthane, Acramite and Ecotrol.
“At three weeks after treatment, Oberon is as good as, if not better, than everything else,” he says. “Like some of the other newly registered materials, it works faster than Zephyr which doesn't have any knockdown. Oberon at the higher rate of 16 ounces provided better knockdown than Oberon at the lower rates.”
Ecotrol, the organic compound in the trial, was the only material that did not perform close to par with the other miticides in the cotton trial. However, Godfrey noted that Ecotrol has measured up fairly well in other trials in other crops such as almonds that UCCE Kern County Farm Advisor David Haviland has conducted.
From a cultural standpoint, outbreaks of spider mites are more likely to occur on water stressed plants, in dusty conditions or improperly irrigated fields. Utilizing good IPM practices, monitoring for pest and treating at threshold levels are always recommended to help stave off resistance issues.