Spider Mites Can Occur after Applying Hard Pesticides

Q. I have a fruit tree with leaves that are brown on the edges and a dust that covers the leaves. Some of the leaves are turning gray green on branches and the tree does not look healthy.

Spider mites our famous for causing damage to Italian cypress. They cause Browning and branch death, usually there is webbing seen as well. That's why these critters are called "spider" mites. But they are more closely related to spiders than they are insects.It's always a good idea to wash off Italian cypress with a hose when they get dusty.

A. Most likely the tree damage was caused by spider mites. Spider mites are a summer pest problem during hot, dry weather.

            Another telltale sign that you mentioned is the “dusty” appearance of the leaves. This “dust” results from dead spider mites left behind as the population grows.

            Look for feeding damage to the leaves caused by spider mites. This feeding damage causes tiny yellow dots scattered all over the surface of the leaf and accompanied by tiny black dots the size of this period. By the way, the tiny black dots is mite “poop”. Even though mites feed on leaf bottoms, the yellow dots can be seen on the top of leaves.

            A common misconception is that webbing must be present if mites are the problem. Spider mites leave behind “webbing”, like the weak form of a spider’s web. But not all mites that cause plant damage spin webs.

Leaf damage and webbing scene on tomatoes caused by spider mites.

            So, finding webs when there is damage is not a “dead giveaway” that the damage is from mites. It could be one indicator, though. There are real spiders who are “good guys” that spin webs and hang out in plants. These spiders can be beneficial.

            Mite attacks to plants may come after the application of a hard pesticide. Bad mites are always present on plants but their numbers are controlled by predatory insects and even “good mites”. So, applying a hard pesticide to control borers, for instance, could lead to an outbreak of spider mites because the predatory “good guys” were killed.

            What can you do? Confirm that spider mites are the problem. Use a white paper test. Shake or slap an infested branch on a white piece of paper or paper plate. Closely look at the white surface for tiny dots, the size of a period, crawling along the paper.

            Smear them with your fingers if you aren’t sure. If you see lots of them, along with plant damage that I described, you have confirmed mite damage.

            Multiple applications of soap and water sprays do a good job getting small outbreaks under control. Hosing the leaves of plants monthly, or after a dust storm, removes dust from the leaf surface which can increase spider mite populations.

            Severe infestations of spider mites may require a pesticide application. Whenever spider mites were problem during the year, be sure to apply two dormant oil sprays during the winter months.