We have received a number of reports of outbreaks of spider mites, primarily in watermelons and in high tunnels. The problems in high tunnels are not unexpected because one of the primary causes of mortality in mite populations is rainfall washing them off the plants and, of course, that is lacking completely in high tunnels. With all the rain we have had, it’s a little surprising that we are seeing problems in watermelons, but the older I get, the less I’m surprised by how infrequently arthropods behave the way we expect them to.
In both scenarios, we don’t really have treatment thresholds for mites. Generally speaking, if populations are increasing, they need to be controlled. Once the decision to treat has been made, that’s where things get very different. In watermelons, we have a variety of pesticide choices. See page 115 of the Midwest Vegetable Production Guide (ID56) for the list of options. Most of these miticides will provide good to excellent control of spider mites.
In high tunnels, the choice of a pesticide is a little more complicated. First, you need to find the products available for use on your crop. If, for example, tomatoes is the crop of concern, you can find 5 options to choose from on page 135 of ID56: Acramite®, Agri-Mek®, Oberon®, Portal®, and wettable sulfur. However, since you are growing in a high tunnel (defined as a greenhouse by the Office of the Indiana State Chemist), some of those options are not available to you, namely Oberon® and Portal®. So, you need to choose from the other three options. Sulfur is much less effective than the other two choices and is primarily for organic growers. If you are a conventional grower, your choices would come down to Acramite® and Agri-Mek®, both of which will work well.
Spider mite damage on watermelon may be recognized by the chlorotic pattern that occurs between the veins.