It was July 1991, and I had just arrived in Columbus to work with trees and landscapes in the care of city government. One of the first tasks was to address a problem with a large public planting of honeylocusts that were growing bare by the day, shedding yellow leaves.

Crews had been watering them generously, thinking that the trees — planted the year before — were under water stress. While the water certainly didn’t hurt, it turned out the trees were infested with red-spotted spider mites, which causes leaves to turn yellow and drop in summer.

Damage and loss from spider mite infestations can be severe. In mid-summer, the only control option is what entomologists call a “rescue treatment,” using an insecticide. As soon as we could line up the right chemical and the right conditions, we suited up at dawn, fired up the sprayer and treated the trees.

This was, incidentally, the occasion of my first acquaintance with the late Mayor Bob Stewart, who was out for his morning walk. “Hey, I thought I’d find you behind a desk,” he boomed, “not out spraying trees!” This was my kind of compliment.

Least toxic better

Hopefully, you only have to apply a mid-summer rescue treatment once, because there are preventive options that work better, with lower toxicity. Dormant oil, applied before growth begins in spring, can suppress spider mites and scale insects effectively, and at lower cost and risk than can a summer insecticide application.

Some advantages of dormant oil:

  • The material is simply a highly-refined oil, rather than an insecticide that works on nervous or other systems, so there is less risk to humans and other “non-targets.”
  • It works by physically smothering overwintering insects occupying cracks or surfaces.
  • It’s applied only to dormant branches and trunks, so you use less.
  • It’s applied at a time of year when beneficial insects are largely absent.

Should you miss the late-winter dormant oil window, and wind up in summer rescue mode, the fight is more toxic, more expensive and less sure in outcome. Does an extreme infestation sometimes warrant both? Yes, and more. Entrenched infestations of scale insects on magnolias and tulip poplars have been known to take all measures to get the trees back to health — including soil drenches in May and summer oil applications in late summer. Once control is reached, management can be ramped back down to dormant oil only.

Right tree, place

Some of our landscape plants — tulip poplar, magnolia, serviceberry, honeylocust — are more prone to scale or spider mites when they are planted near sources of reflected heat such as pavement, buildings and cars. This is why trees such as these might never see an infestation when planted in a rural or suburban setting.

Put one of those species in an urban setting and certain pests become standard equipment. The upshot? Match the plant selection to the growing conditions and you can avoid a number of problems.

Dormant oil also is one of the single-best pest prevention measures for fruit trees, and the time is now — before a tree leafs out. The temperature must be above 40 for an application, and always follow label instructions.

Kris Medic is Purdue Extension Bartholomew County’s educator for agriculture, natural resources and community development. She can be reached at 812-379-1665 or