It didn’t take long for spider mites to attack tomatoes and attack with a passion. I have had a number of calls about tomatoes that are turning yellow and in most cases it is spider mites that are the culprit. The two spotted spider mite is responsible for most of our tomato problems. They are very small at 1/32 of an inch or less. If you turn over a tomato leaf you will see the webbing characteristic of spider mites and if you look closer you may see the actual mite moving around. Spider mites overwinter as adults and even continue to breed on host plants in mild winters. Spider mite adults lay a clear to yellow egg suspended in a fine web of silk. 6-legged nymphs emerge from the eggs and go through 2 molts before they emerge as 8-legged adults. A generation can last from 5 to 20 days depending on the temperature, the hotter the quicker. When the host plant begins to decline, the mites spin silk threads and use these strands to “fly” or “balloon” in wind to disperse to other plants. This is how they get to your tomatoes in the first place.
Scouting is essential to control. If you see spider mites early you can wash them off with hard streams of water or use an insecticidal soap. Sulphur has long been used as a preventative for mites as well as a fungicide for diseases. Garlic has been promoted but my experience has not been good. Unfortunately most gardeners do not notice infestations until they are severe and control is difficult. The chemical Malathion is labeled and tomatoes can be picked after waiting one day. Other chemicals are much better for killing the mites but unfortunately you must wait anywhere from 3 to 14 days before harvest. I like to recommend that gardeners remove the plants that are infested. This may seem drastic but letting populations explode doesn’t seem healthy either.
Lastly let me add that spider mites love plants that are stressed, especially from water. I was recently running a greenhouse experiment with tomatoes and marigolds. I had many pots of each and I had inadvertently left some of both plants almost outside the area that was sprinkled. This meant that 2 tomato plants and 2 marigold plants were getting just enough water to live but not much else. I then went on vacation for a few days and when I came back the only spider mite infested plants were those stressed for water. To back up my hypothesis, I water my vegetable garden every day for several hours and so far no spider mites!