While spider mites resurged in Mid-South cotton fields in recent years due to environmental conditions and changes in cultural practices, they remain an inconsistent pest not approaching the status of stink bugs and lygus.
According to a Beltwide survey of insect losses for the 2005 cotton crop compiled by Mike Williams, Extension entomologist at Mississippi State University, spider mites infested 5.23 million acres of cotton in 2005, enough to be U.S. cotton's fifth most damaging pest. Over 122,000 bales of cotton were lost to the pest in 2005, including 45,500 bales in California, 24,000 bales in Mississippi and 21,000 bales in Arkansas.
According to University of Tennessee Extension entomologist Scott Stewart, spider mites are not a true insect, “and are more closely related to spiders.” Stewart spoke during a panel discussion, called “The Next Generation of Insects,” at the 2006 Beltwide Cotton Conferences in San Antonio.
“The type of spider mite that we deal with in the majority of cases is the two-spotted spider mite,” Stewart said. “It has a very wide host range, including many broadleaf plants and grasses, field crops and ornamentals.
“These insects feed with a piercing, sucking mouthpart. In cotton, you find them on the underside of leaves, and they will spread to other parts of the plant when infestations become severe.”
Injury to the cotton plant occurs from the loss of photosynthetic ability in the plant. In severe cases, plants can become defoliated, and shed excess fruit. “Spider mites have a very rapid life cycle, as little as five days, but as many as 10 days during normal summertime temperatures.”
Stewart says infestation tends to occur most commonly during hot, dry weather. “In the far West, there are resistance management plans for spider mites because they are known to be resistant to some of the common insecticides and miticides used to control them. But even without the occurrence of resistance, spider mites have been, at times, very difficult to control.”
Treatment thresholds for spider mites vary across the Cotton Belt and are not very specific, Stewart noted. “Farmers tend to pull the trigger on spider mites when 30 percent to 50 percent of the plants are infested.”
According to studies by University of Arkansas entomologist Don Steinkraus, pitted morningglory and Palmer amaranth are host plants for spider mites.
Stewart noted, “A fungal pathogen is a very important biological control agent in many instances. The pathogen tends to occur in high humidity conditions and heavy rainfall will precipitate a population crash.”
Stewart says some insecticides will actually flare spider mite populations.
“One significant change that has occurred in our production environment in the Mid-South and Southeast is that the use of Temik as an at-planting, in-furrow application for thrips control has been replaced by seed treatments. These products don't have spider mite activity at all. In fact, they can actually flare early-season spider mite populations.”
Current recommendations are for two applications of insecticide on a four-to-five-day interval to control the pest, Stewart noted. “This is probably more important if we're using insecticides rather than true miticides, but it can be applicable for the miticides in some circumstances.”
The significant problems with the pest in 2005, particularly in the Mid-South, “was unusual because most of the problems we had were on seedling and pre-flowering cotton. Areas affected included the Missouri Bootheel and northeast Arkansas, much of Mississippi and parts of Louisiana. In Tennessee, we had isolated, severe problems.”
Stewart noted that true miticides provide the best control. “Another point is that a lot of insecticides and miticides don't provide a long residual. The data shows after eight to 10 days after most applications, spider mite populations start to build.”
Stewart noted that while the adoption of non-miticidal seed treatments has flared populations of early-season spider mites, better control of in-field and margin weeds would better control infestations.
“We do see a pattern with spider mites in reduced tillage fields, because they are allowed to persist in the field a little longer. Also pyrethroids applied at planting for cutworm control are notorious for flaring spider mite populations. The occurrence of herbicide-resistance weeds such as (potentially) Palmer amaranth might actually increase the occurrence of spider mites.”
Stewart doesn't believe that early-season spider mites will become a wide-scale problem. “Spider mite losses haven't been very high historically. There was a blip in 1995 and in 2005. The point is that spider mites have occurred erratically, but have never been a Beltwide problem. We shouldn't put them in the same class as lygus and stink bugs as the next generation of pests.”