Insecticide application needs care

KENNEWICK -- In the "old days" 30 years ago, gardeners usually were more concerned about killing any insect they found on their garden plants.

The adage that the "only good bug is a dead bug" prevailed.

Thankfully, times have changed.

Most of the old chemicals have been banned because of the harm they caused to the environment or to humans. They were applied to plants as a spray and killed a broad spectrum of both harmful and beneficial insects.

Today, there are new environmentally friendly chemicals available.

One relatively new material is imidacloprid (IC). There are spray and granular formulations of IC available, but it also comes in a soil drench used for treating trees and shrubs. Because it's a systemic, IC is absorbed by the roots and moves throughout the plant. It primarily kills insects that suck on plant sap, such as aphids, but can poison some leaf feeding insects, such as root weevils and leaf beetles.

First registered in the U.S. in 1994, IC introduced a new chemistry in pest control. It's a neonicotinoid. It disrupts an insect's nervous system by inhibiting certain nervous system receptors, causing paralysis of an insect's mouth parts and leading to death by starvation. It kills by contact (spraying the insect) and by ingestion (either by sucking of plant sap or eating leaves). While IC is considered only moderately toxic to humans, it's very toxic to bees and some small birds and slightly toxic to fish.

Both foliar and drench formulations of IC are available to home gardeners under the "Bayer Advanced" label. (Bayer CropScience holds the patent for IC.) The drench formulation is a boon to gardeners because it allows them to treat tall trees for aphids and some leaf beetle pests without having them sprayed by a commercial firm or trying to spray the trees by themselves. Controlling insect pests on tall trees is difficult because it requires special equipment to reach the top branches and disperses pesticide throughout the area, killing many nontarget insects. The only equipment required for applying an IC drench is a watering can. The product is mixed with water and applied to the soil around the base of a tree's trunk.

While typically called a systemic "insecticide," IC's chemistry is different than past systemic insecticides. It moves in the xylem, the water transporting vascular cells in woody plants. Because it has to move from the roots to the top of a tree, a drench application of IC doesn't provide immediate protection from insect attack. It can take four to eight weeks for IC to reach its way to the top of a small tree and eight to 12 weeks to become fully effective in larger trees.

When targeting spring feeding insects, such as wooly ash aphids on ash trees, an IC drench should be applied in September or early October. If you missed a fall application, try for very early spring. IC has a long residual and can stay effective in the plant for up to a year, so a fall application doesn't mean that it won't be effective in the spring when applied in the fall.

IC shows promise in controlling bronze birch borer and root weevils, two difficult to control pests of local landscape plants. If you have a problem with one of them, you just might want to be ready to treat your plants in late winter or early spring. If using an IC drench, follow the label directions for when and how much to apply.

* Marianne C. Ophardt is a horticulturist for the Washington State University Benton County Extension Office.