Twospotted Spider Mites
Over the last few weeks, we received reports of two spotted spider mite infestations in alfalfa. The twospotted spider mite is capable of feeding on a wide variety of plants including soybean, alfalfa, and corn. Twospotted spider mites are generally not an issue in alfalfa; however, dry conditions similar to those that have been experience in South Dakota for the last couple of weeks can allow for infestations to occur. Recent rains may help reduce the twospotted spider mite populations, but it will be very important to scout alfalfa fields to ensure that infestations do not cause yield reductions.
Behavior & Appearance
Drought conditions in the early spring encourage the movement of the twospotted spider mites from grasses into other green plants such as alfalfa. Typically during the summer, cutting alfalfa during dry conditions can cause twospotted spider mite issues to occur in adjacent crops such as soybean as the spider mites migrate to new hosts. Populations of twospotted spider mites are not observed during periods of adequate moisture due to naturally occurring fungal pathogens that attack the mites and reduce their populations. The activity of these pathogens are encouraged by cool and humid conditions.
Twospotted spider mite nymphs and adults are very small and often require a hand lens for identification. The nymphal stages of twospotted spider mites have six legs, while the adults will have eight legs. The adult twospotted spider mites get there name due to the two black spots present on each side of their bodies (Figure 1).
Twospotted spider mite populations are rarely an issue for alfalfa production, and as a result South Dakota specific recommendations are currently unavailable. Our current recommendations are to monitor alfalfa for stippling caused by twospotted spider mite feeding and webbing associated with their populations. If alfalfa is close to being cut, consider waiting and observing the new growth before making any chemical applications for management. However, if it is several weeks from cutting and conditions are favorable (i.e., hot and dry) for twospotted spider mite population growth chemical management may be necessary.
If the alfalfa is harvested with twospotted spider mites it is important to bale it and remove it from the field as soon as possible to reduce the likelihood of re-infestations occurring from the cut hay. If chemical management is required please refer to the current edition of the South Dakota Pest Management Guide: Sunflower & Oilseeds/ Alfalfa & Range. If a different pest is being managed with a known infestation of twospotted spider mites consider avoiding pyrethroid class insecticides as some active ingredients are known to flare twospotted spider mite populations after application.