We have received several reports of spider mites showing up in corn throughout South Dakota. The two species of spider mites that may be present in corn are the two-spotted spider mite (Figure 1) and the banks grass mite (Figure 2). Both of these pests are more common during hot and dry conditions, which were common throughout much of the state this summer. However, the samples we have been observing actually don’t have spider mites on them anymore. This may be the result of recent precipitation in some areas of the state. Both species of mite pests are susceptible to pathogenic fungi that is capable of wiping out their populations. These fungal pathogens require humidity to thrive, which is one of the reasons why spider mites are more of an issue during hot, dry conditions.
Fig. 1. Two-spotted spider mites. Courtesy: F. Peairs, CSU, Bugwood.org
Fig. 2. Banks grass mite.
Courtesy: F. Peairs, CSU, Bugwood.org
Corn that has been infested with spider mites will have leaves that have a sand-blasted appearance. The leaves will have small yellow or white spots on them, which are the result of spider mite feeding. This type of feeding injury is called stippling. In addition, corn leaves may also have webbing on them, which is an indicator for two-spotted spider mites.
There are several predators that can be effective at managing spider mite populations. Spider mite issues can often occur after an insecticide is applied for a different pest. Insecticide applications may reduce the number of spider mite predators, which allows spider mite populations to increase rapidly and potentially cause yield loss.
Although there may be spider mites still in the field, management decisions should be made carefully. Management at or after the dent stage in corn is not economically effective. Typically, spider mite management needs to occur between the pre-tassel and soft dough stages of corn.
At this point in the season, we don’t recommend managing for spider mite populations in corn, especially if only the feeding injury is being observed and not the mites themselves.