Spider mites are the insecticide “budget busters” in Mid-South cotton because of the costly — and often unexpected — sprays growers have to make to control them, according to Mississippi Extension entomologist Angus Catchot.
As recently as five years ago, in 2004, spider mites were not included as a major pest of cotton in Mississippi. Spider mites popped up as the No. 3 most damaging pest in 2005-08. “They are definitely one of our top pests now, and we are definitely starting to think about them every year.”
In 2008, 181,000 acres were infested with spider mites in Mississippi, compared to nearly a million acres in 2005. But that doesn’t tell the whole story, according to Catchot. “We lost 46 percent of our cotton acres from 2006 to 2007 and another 46 percent last year, so we’re down 70 percent, from 1.2 million to 360,000 acres of cotton. On a percent of acres treated, we’ve been the same since 2005. In 2008, we were on track to have one of our worst mite years ever before some storms came through with significant rainfall and got things back into shape.”
Catchot says problems with early-season thrips can contribute to spider mite control. “If you have a thrips problem and you happen to have spider mites in that field at a low level and you apply acephate, you will make your spider mite problem worse in a hurry. You can go from a pretty field of cotton to one with dead spots in a seven-day to 10-day period. It’s unbelievable how fast spider mites move across a field in the early season.
“We’re also starting to see a lot more spider mites in corn. When corn starts to dry down, there’s not a field you can walk into in the Delta and not see spider mites to some degree.”
Catchot said that winter vegetation such as henbit can be a “great host” for spider mites “and when it dies out, we’ll start getting coverage of primrose, which is another great host.”
Catchot and others have started a series of studies to determine the extent of yield loss to spider mites “to help us make better management decisions later.”
In one study, parts of a dryland field showing symptoms of spider mite damage were compared to parts of the same field not showing symptoms. “In a dryland situation, the hot spots where mites were present had a 51 percent yield loss. Basically, we went from 1,000 pounds to 500 pounds. Since this was a late-season mite infestation, the biggest chance of yield loss was in the top of the canopy.”
GPS-reference aerial imagery of the field indicated that the hot spots comprised about 15.8 percent of the field. Researchers calculated the per acre loss across the entire field at around 80 pounds.
On irrigated fields that were studied, “we still had mites, but our cotton plants were healthy, and that seemed to keep the symptoms down. We saw a 9 percent decrease in yields in irrigated fields that had just as many mites in it.
“We know spider mites are very opportunistic,” Catchot said. “They like plants that are stressed. The one thing we couldn’t tease out from the test was whether or not those hot spots were weak anyway and yielding less.”
Researchers are also looking at some burndown options for winter weeds “to see if we can burndown earlier and reduce the number of mites.”
Phillip Roberts, Extension entomologist at the University of Georgia, says 25 percent to 30 percent of the Southeast cotton growing region is infested with spider mites. “But percent treated acres and yield losses still remain very low. We hope that trend continues.”
Roberts says the pest is mostly a localized and sporadic problem. He noted that the Tennessee Valley area of northern Alabama is an area frequently infested with the pest.
Since control of spider mites “can be very expensive,” Roberts recommends management approaches to avoid spider mite problems or reduce the risk of spider mite outbreaks occurring. “We recommend to our growers that they burndown their winter weeds and cover crops in a very timely manner, conserve natural enemies and avoid any unneeded sprays.
“We also have some innovative growers who, anytime they see mites beginning to infest a field from a source such as a waterway or an adjacent crop, will aggressively treat these small spots before the pests move to the remainder of the field.
“Our primary educational message in Georgia is that if you have spider mites in a field, it should influence management decisions you make for other pests. We do know the potential is there for them to blow up, and we want to avoid those problems. We absolutely have to use thresholds, scout and treat on an as-needed basis. In fields infested with spider mites, depending on the time of year, perhaps we can elevate thresholds and avoid spraying unless we absolutely have to.”
Roberts recommends that when Southeast growers spray for other pests like bollworms or stink bugs, “try to use the least disruptive compound if possible. Or perhaps we could use a material that would offer some suppression of spider mites. In the Southeast, we’re not having to use a lot of broad spectrum sprays until later in the season for stink bugs. But maybe that’s one of the reasons why we’re not blowing up these populations.”
Roberts also advises against making automatic applications for thrips. “When we’re going over cotton at the five-leaf stage with glyphosate, it’s very easy to add an insecticide to take care of thrips. We believe that has negative consequences in terms of spider mites.”