Spider mite management in high tunnel cucurbits

by Alan Eaton, UNH Cooperative Extension

Among the attendee feedback from the January 2017 cucurbit school were comments that growers wanted me to cover mite management, something I skipped because of our limited time that day. So I’ll cover it now.

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Mite-induced stippling on the leaves of a cucumber plant in a high tunnel.

Inside high tunnels, it is usually warmer and drier than outside, and the growing season is longer. These are especially conducive to problems with twospotted spider mite (TSSM). Cucumbers are the cucurbit where I see the most TSSM problems, but they can be on other cucurbits as well.

Growers typically think of pesticides as their first line of defense for this problem, but I suggest first looking at preventative measures. If possible, begin with a totally clean, bare greenhouse. This includes the floor area. That is difficult for a high tunnel where the plants are grown directly in the soil. In that situation, at least eliminate all traces of the previous crops. That means removing and disposing of all crop residues. If the tunnel or greenhouse has been given a good opportunity to completely freeze out for a couple of weeks or more, that makes it less likely that you are starting the season with a significant number of live TSSM’s. Cucumbers, legumes, roses, and sometimes petunias are among the crops where I see lots of TSSM problems.

Biological controls are an option to consider, especially for an enclosed greenhouse. I almost never recommend them for an outdoor crop, because the predators or parasites just disperse once released. One spot where you can look at a table of the various predator species and their needs/advantages/disadvantages is in the Greenhouse Manager’s Guide to Integrated Pest Management in Northern New England. We have run out of hard copies of this spiral-bound publication, but if you don’t own a copy, you can look at a copy on line. Here is a link to the UVM Entomology Research Lab’s web page on greenhouse IPM.

The guide is on the left side of the web page. Part of the biological control section (actually pages 65 and 66) show that information very concisely for spider mites, including optimal temperatures for each species to work. Dr. Michael Brownbridge of the Vineland (Ontario) Research and Innovation Center tells me that most Canadian greenhouses have switched to biological controls, rather than struggle with spraying chemicals and fighting pesticide resistance and re-entry rules. The biological controls can work very well if 1) you start with a relatively clean, pest-free greenhouse or high tunnel 2) you select the proper species for your conditions and 3) begin very early in the crop cycle, when mite numbers are very low. Unless you nuke them with a pesticide application, they can provide season-long control. Usually it takes several applications (of predators!) to do so.

Pesticides: Avoid using the same material repeatedly. That leads to resistance. Instead, rotate between materials with different modes of action. Many pesticide labels will direct you to make 2 applications, about 5 to 7 days apart. That is fine, but if another treatment is needed later, switch to something else in a different group. In the New England Vegetable Management Guide, we have included which group each miticide is in, so you can rotate easily. Under cucumber & muskmelons, I see 14 products listed, including two (Trilogy and M-Pede) that are OMRI-listed. Coverage is very important to get good control. That is difficult, because there are usually more TSSM’s living on the undersides of the foliage than on the upper surfaces. Monitor the levels of TSSM’s in your crop before and then a few days AFTER you have applied a spray. That allows you to evaluate how effective (or ineffective) a treatment was. In many cases, effectiveness is in direct relation to how well you got the crop covered (including the undersides).

Thresholds? The threshold is that size pest population where it is worthwhile to apply controls. There are no specific thresholds established for mites in greenhouse/high tunnel cucurbits, in part because each situation is a little different. I strongly advise regular checking of the crop. Look for the fine white speckling that these mites cause, and then pull out a hand lens or magnifier to confirm that the pests are present. Don’t have a magnifier? Check out my publication on that subject. It explains how to use them, what advantages and shortcomings the options have, and where to find some.

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