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Master Gardener: Plant-zapping spider mites thrive in the summer
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Master Gardener: Plant-zapping spider mites thrive in the summer

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The leaves on some of my plants have small spots on them and are starting to turn brown. I also see some webbing. What is causing this? — D.H.

From your description, it sounds like your plant issue is caused by spider mites. Oftentimes we would suggest you do a visual inspection to confirm a specific pest, but spider mites are very small and can be hard to see with the naked eye. However, with the damage you are suggesting along with the webbing, spider mites rise to the top of the list.

To confirm, you can hold a white sheet of paper underneath the infested plant and jiggle the branch a bit to see if any tiny little dark specks appear. Granted, this may cross the line for the more timid gardeners.

Spider mites are in the arachnid family making them relatives of spiders, ticks, daddy long-legs and scorpions. The webbing you are seeing can occur in large infestations and serves to protect the mites and their eggs from predators.

Spider mites have a quick life cycle. After mating, female spider mites can produce a dozen eggs every day for two weeks and this new generation of spider mites can mature into adults in as little as a week. With this rapid rate of reproduction, it’s easy to see how small populations of spider mites can get large rather quickly.

Spider mites have what we call “piercing-sucking mouth parts.” They feed by puncturing leaves and sucking out the sap or juice of the plant. Naturally, this damages the leaves, making it hard for them to produce food for the plant via photosynthesis. Leaves with spider mite damage have kind of a sand-blasted or speckled look, but eventually, they tend to turn brown and fall off. In large infestations, this damage can essentially defoliate your plant, which can cause the plant to die.

Spider mites thrive in hot, dry weather — basically “summer” here in Oklahoma. One of the reasons for this is plants that are stressed by drought experience changes in their chemistry that make them more attractive to spider mites.

Because of that, one of the best natural defenses against spider mites is to keep your plants hydrated. While watering, you can also hose infested plants down with a jet of water which helps to relocate existing spider mites. You can’t get much more natural or organic that a strong stream of water.

Another reason spider mites can become a problem is the over-use of insecticides in the garden and the type of insecticide we use. Chemicals such as carbaryl (Sevin) are very effective at dispatching many different types of insects, including the natural predatory insects we like to have in our gardens like lady beetles and predatory thrips. Carbaryl is kind of the nuclear option for total insect destruction. The best practice is to use natural, organic pesticides that are at the lower end of the toxicity scale and target specific pests.

In addition to hosing down your plants in the morning, there are several organic pesticides that can be effective against spider mites including insecticidal soap, neem oil and horticultural oil. Be sure to apply these treatments in accordance with the directions and when beneficial insects are not around. The best times would be early morning or evening.

There are many recipes on the internet for making your own insecticidal soap using dish detergent. On the surface this seems like an economical approach. However, dish detergents contain many ingredients such as de-greasers, coloring, sudsing agents, etc., and these chemicals can damage your plants.

Insecticidal soap typically has two ingredients: water and soap (aka potassium salts of fatty acids). When applied properly, there shouldn’t be a lot of collateral damage to the other insects in your garden. Insecticidal soap must be applied directly onto the spider mites, so you will need to essentially soak your plant with insecticidal soap because spider mites tend to hide under the leaves. Unless you spray under the leaves, you will likely not solve your spider mite issue.

Neem and horticultural oil are good solutions as well, but in this heat, spray some on a test area as oils and heat tend not to play well together and can ultimately damage your plants. Neem oil is a pesticide made from the neem tree and is very effective when sprayed on pests. It also has a very low toxicity to humans and non-target pests.

Spider mites can also become a problem on house plants. Not only can you take these plants outside to give them a spray with the hose, but insecticidal soap and horticultural oils are appropriate for indoor use.

Whether indoors or outdoors, due to the quick reproduction cycle of spider mites, you may have to repeat applications for effective control. Good luck.

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 St., or by emailing us at mg@tulsamastergardeners.org.


Interview with Tulsa World's Jimmie Tramel from July 2021

 

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