The leaves on my tomato plants look like they have tiny light-colored dots all over them. Now they are turning brown. I also see something that looks like spider webs. What’s going on? WT

The problem you are describing is likely caused by spider mites. This is pretty much confirmed by your mentioning the presence of webbing.

One challenge associated with spider mites is that they are too small for us to see easily. To confirm, you can hold a white sheet of paper under the affected area and jiggle the branch. If there are spider mites, you should see some tiny black dots on the page.

As their name implies, spider mites are members of the arachnid family which connects them to their relatives: scorpions, ticks, and daddy-long-legs. If you can see webs, it means you have a pretty serious infestation of spider mites on your tomato plants.

Spider mites are a good reason to give your plants a visual inspection every day since spider mites can go from a small problem to a bigger problem quickly. The reason for this is that female spider mites can generate about a dozen eggs every day for a couple of weeks and spider mites reach maturity in about a week. With this rapid reproductive cycle, it’s easy for a spider mite infestation to get away from you.

When on the lookout for spider mites, keep your eyes out for leaves that look like they have been sand blasted with tiny, light-colored specks all over them. Spider mites have what we call piercing-sucking mouth parts which means they pierce the leaf so they can suck nutrients out. This damages the leaf and reduces its ability to produce food for the plant via photosynthesis. Eventually, these damaged leaves will turn brown and fall off. If they damage enough leaves, the plant will not be able to sustain itself and will die.

Jen Dominiak-Olson, of the OSU Extension Plant Pathology Lab says that “the number one tomato problem in the summer without a doubt is spider mites.” One thing that works against us here in Oklahoma is that spider mites love hot, dry weather, and we typically have plenty of that. In addition, plants that are stressed by hot, dry weather actually have a change in their chemistry that makes them more attractive to spider mites. So, the first defense against spider mites is to keep your tomato plants watered appropriately.

If you are starting to see signs of spider mite damage on your plants, you can begin by giving the plants a good hosing down. Since spider mites are so tiny, oftentimes a strong stream of water will wash them right off your tomato plant. Be sure to spray under the leaves since this is a favorite spot for spider mites to hang out.

Another issue that can contribute to your having a problem with spider mites is an overuse of pesticides. It’s often tempting to reach for what I call the nuclear option of carbaryl (Sevin) since it is very effective at killing insects. However, it does not discriminate and will kill of both the bad and the good insects. Good insects in this case would include lady beetles and predatory thrips that would love to dine on your spider mites. For this, and a lot of other reasons, I recommend the use of organic pesticides. Organic pesticides tend to be targeted at specific problems which help to minimize collateral damage. They also tend to have shorter pre-harvest intervals which is the time between when you use a product, and you can safely harvest.

Fortunately, we have several organic pesticide options that are effective against spider mites including insecticidal soap, neem oil, and horticultural oil. On the internet, you will see a variety of recipes for making your own insecticidal soap. This would seem to make sense but, dish detergent is not soap. Dish detergents contain a variety of chemical compounds that serve as de-greasers, sudsing agents, coloring, and scents, and these chemicals have the potential to damage your plants.

True insecticidal soap has two ingredients: water and soap (potassium salts of fatty acids). If you check the ingredients of your dish detergent, you probably won’t find any potassium salts of fatty acids. Ok, I’ll climb back down off my soapbox.

For insecticidal soap to work, you’ll need to apply this product directly on the spider mites. Since they like to hide under the leaves, you’ll need to give your plants a good soaking. Be sure to do this early in the morning or late evening when pollinator insects are not as active.

Neem oil is another good one for spider mites, but this time of year, be sure to test the neem oil on a portion of the plant first. Since this product is a horticultural oil, it has the potential to damage your plants with the kind of heat we have been having.

The bottom line is to pay attention to the plants in your garden. Early detection is always one of your best defenses. See you in the garden!

You can get answers to all your gardening questions by calling the Tulsa Master Gardeners Help Line at 918-746-3701, dropping by our Diagnostic Center at 4116 E. 15th Street, or by emailing us at mg@tulsamastergardeners.org. Photo: Walt McDaniel