Two species of mites commonly feed on cotton plants in Texas: the twospotted spider mite and the carmine spider mite. These two species are difficult to distinguish from one another. Carmine spider mite females are red; twospotted spider mites are greenish. When conditions are suitable for initiating diapause (dormancy), female twospotted spider mites may also be red. Because the damage they inflict and their biology and ecology are similar, we will discuss them as one pest for this guide
They feed by piercing plant cells with their mouthparts and suck- ing the liquid contents of the cells. Damaged leaves develop white or yellowish specks, called stipules. Specking damage is Phase I damage. As feeding increases and the mites persist, the damage spreads, leaves develop a reddened appearance (Phase II damage), and eventually turn brown. This damage affects cotton photosynthesis, which appears to decline sharply at Phase II, or about 20 mites per leaf.
Spider mites also infest bracts of squares and bolls, causing the bracts to desiccate (dry up). Heavy and prolonged spider mite infestation can prematurely defoliate cotton plants. Spider mite injury can reduce cotton yield, fiber quality, and seed.
Management and Decision Making
Hot, dry weather favors spider mite infestations. Conversely, high relative humidity and precipitation deter spider mite infestations by washing them off leaves and creating conditions that favor disease outbreaks. Since spider mite outbreaks tend to develop on field borders adjacent to spider-mite-infested weeds or other crops, manag- ing weeds along field margins often prevents the mites from migrating into cotton. Avoid excessive nitrogen fertilizer because spider mite outbreaks occur more frequently in fields fertilized with higher than necessary nitrogen rates. Spider mite populations often develop where dust from roads blows onto cotton plants, possibly interfering with natural enemy efficiency.
The spider mite treatment threshold has not been fully worked out, but the following formula provides general guidelines for treatment. Before bloom, scout cotton for leaf damage to protect it from spider- mite-induced defoliation. After bloom, protect leaves responsible for boll filling from spider mite damage until 650 to 750 DD60s beyond cutout or NAWF + 5. Because spider mite populations are often clumped together, particularly along field edges, spot treating infested areas often prevents spread and can be cost-effective. Consider alternatives to pyrethroids for managing pests such as bollworms and lygus. Also, consider selective insecticides such as neonicotinoids for aphid control when mites are present. Maximize insecticide coverage when treating for mites. Drop nozzles and high spray volumes can significantly enhance mite control.