JoAnne Skelly: Vegetable leaves looking bad

I’ve been watching my lemon cucumber leaves turn speckled, yellow, and then brown. Initially, I thought squash bugs were sucking the juices out. However, when I tapped a damaged leaf over a white sheet of paper, and looked through my trusty magnifying glass, I saw moving dust particles on the paper. Those “particles” are actually tiny spider mites.

Spider mites thrive in a dry, dusty environment. I should have been spraying the leaves off top and bottom on a regular basis. With severe infestations, hosing off might be daily. As the leaves look stronger and greener, it may only need hosing a couple of times a week.

Some people want to use pesticides to control mites. I choose not to use chemical pesticides on veggies. Insecticides/miticides should only be used as a last resort, and then preferably only as a spot treatment instead of as a total plant application. Hosing usually works well. If hosing off doesn’t work, insecticidal soap is an option. With eight legs, these arachnids are related to spiders and scorpions, so even though they are not technically insects, their soft bodies do succumb to soap sprays.

Spider mites can become a recurring problem when their predators, such as lady beetles, predatory mites (yes, there are good mites, too), minute pirate bugs, big-eyed bugs and predatory thrips, are killed with insecticides/miticides.

According to Penn State Extension: Although all soaps are made of similar basic materials, “not all soaps are good for killing insects.” In fact, some soaps actually kill weeds. “Insecticidal soaps, however, are specifically formulated to kill insect pests while having few adverse effects to people, plants and the environment.”

Insecticidal soaps must contact the pests’ bodies while the material is wet to “wash away the protective coating (the cuticle) on the surface of the insect’s body. Once inside, the soap will break the cell membrane and the cell will die. Insecticidal soaps are most effective on small, soft-bodied insects such as aphids, spider mites, thrips, whitefly and mealybug.”

Insecticidal soaps have a short residual so there is less destruction of beneficial insects after the application is complete. Soaps are usually non-toxic to mammals and birds and can be applied to food plants. Both sides of the leaves have to be covered and multiple applications are often necessary. Insecticidal soaps should not be applied when temperatures are expected to be 90 degrees or above or in full sun. Morning or evening applications work better when the material will take longer to dry out. Be sure to test the product on a small area first to see if it burns the leaves.

Or, just get out your hose.

JoAnne Skelly is Associate Professor & Extension Educator Emerita at the University of Nevada Cooperative Extension. Reach her at skellyj@unr.edu.