Managing mites in raspberries and blackberries

Monitor and manage mites to protect cane health and yield. Predatory mite populations may be down, providing less suppression of pest mites.

''Most caneberry growers in Michigan have had little need for mite management because of the abundance of predatory mites that keep pest mite populations in check. However, the current increased level of insecticide against spotted wing Drosophila is starting to cause some outbreaks of two-spotted spider mites, and these can compromise raspberry cane health and lead to reduced yield. This is especially likely inside high tunnels that tend to block the immigration of predatory mites.

Two spotted spider mite (TSSM) is the main species of pest mite encountered in Michigan caneberries, and this pest can quickly reach high abundance if the predator mites are not sufficiently abundant to suppress their populations. TSSM can be monitored through the season using a hand lens on 10-leaf samples taken weekly. Look on the underside of the leaves for the small spherical translucent eggs and the stationary/slow-moving immatures or adults of TSSM that have two dark spots in their bodies. In contrast, the predatory mites are light colored and they do not have the dots, and tend to move quickly across the leaf surface. These mites will require a hand lens to see, as the mites are less than a millimeter diameter. A general rule of thumb is that if the predator to pest mite populations are 1:10 or higher, then the predators should keep spider mites in check.

Treatment for two-spotted spider mite is considered unnecessary unless populations reach a threshold of one or more TSSM on 50 percent of the leaves. If predator mites are not present, the pest mite populations can far exceed this threshold. If that happens, growers will notice stippling damage on the leaves as the pest mite populations build. If it gets out of control, there can be severe leaf bronzing. Canes will typically recover from this damage eventually and put out new leaves, but the goal of mite management is preventing that situation from happening in the first place. This can be done through inundative release of predatory mites, but this approach has not yet been well-tested in Michigan farms and the releases are best done when the TSSM population is low and has not yet reached damaging levels.

Maintaining some broad-leaf weed/ground cover can also provide some habitat for predator mites, and this can also provide food for them to persist on. Fields with clean cultivation and completely weed-free management are more likely to experience mite outbreaks.

If chemical control is needed, caneberry growers have a number of miticides registered for use against TSSM. These can be grouped into those products that have activity on the immature and adult mites (Acramite, Vendex, Kanemite, soaps) and those with activity primarily on eggs and immatures (Savey, Zeal). For growers producing fall red raspberries, it may be important to highlight that Savey can be used when honey bees are active, although we still recommend that applications are done in a way that does not lead to direct application to bees, and so early morning or late evening application is suggested. The insecticidal soaps such as M-Pede, Safer, and other formulations are potassium salts of fatty acids, with activity on eggs, immatures, and adult mites. They have 0 day PHI restrictions and 12 hour re-entry. Soap products require thorough coverage, including on the undersides of the leaves to be effective. Miticides for use in raspberry have 0-3 day preharvest intervals.

Dr. Issacs' work is funded in part by MSU's AgBioResearch.

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