As the weather begins to warm, mite activity is likely to increase in high tunnels and greenhouses. Due to their small size and cryptic habits, it is usually easier to initially scout for signs of their activity, rather than trying to see the mites themselves. There are several pest species to watch for on vegetables in high tunnels, with two-spotted spider mite and tomato russet mite the most common.
While spider mites are large enough to see with the naked eye, they are often on the undersides of leaves. Their feeding with piercing-sucking mouthparts results in noticeable tiny pale spots on the leaves, which is referred to as “stippling” (Figure 1). When this symptom is noticed, flip over a leaf and search for spider mites with a hand lens. Spider mites produce silk and webbing between leaflets and this webbing is often associated with heavy mite populations. It is always important to confirm that active mites are present before applying miticides, as the stippling damage will remain long after the mites disappear.
Russet mites are too small to see, even with a typical hand lens. Infestations often begin at lower portions of plants and work upwards. The symptoms to look for include tomato stems beginning to take on a bronze color (rather than green) and small buds on stems that have withered and died (Figure 2). When these symptoms are observed, the presence of active russet mites on leaves can be confirmed with a microscope. As with spider mites, the damage from russet mites remains long after the mites have been controlled. To determine if controls have been effective, use a pen to mark the edge of bronzing on several damaged stems in order to see if the damage is still continuing.
Spider mites and russet mites belong to different mite families. Generally, miticides will be effective against spider mites, but only a few vegetable miticides will also control russet mites. So if russet mites need to be controlled, check pesticide labels to make certain that chemical will be effective. Recommendations in Vegetable Production Guide for Commercial Growers (ID-36) specify which miticides control russet mites, broad mites, and/or spider mites.
By Ric Bessin, Extension Entomologist