New Pesticide for Control of Shade Tree Insects

New Pesticide for Control of Shade Tree Insects

Marianne C. Ophardt
Washington State University Cooperative Extension
Area Extension Agent

There continues to be concern about our dependency on chemicals for pest management in yards, gardens, and agricultural crops.  Chemicals that we have relied upon for years have recently come under scrutiny for one reason or another.  Many of these "old" chemicals have one or more objectionable characteristics, such as they're very hazardous to the applicator due to their extreme toxicity in a concentrated form, they don't break down very quickly in the environment, they pose a threat to groundwater due to their leachability, or they may lead to long term health effects like cancer.

It's been said that, "Necessity if the mother of invention."  This is true when it comes to pesticide chemistry.  With concerns about old pesticides and their chemistry, the pesticide industry has worked hard at formulating new and safer chemicals which will be more environmentally friendly and thereby more acceptable to consumers.  Of course, these new chemicals must also do a good job of controlling the pest... whether it's a weed, insect, or plant disease.

One fairly new chemical on the commercial market is an insecticide known by the trade name of Merit.  Its common chemical name is imidacloprid.  Merit has been quite popular for several years in areas where lawn grubs are a problem.  However, since lawn grubs are not usually a serious problem in this area, you've probably never heard of Merit.  Merit is also labeled for use on trees and shrubs. Local pest control operators have started applying Merit to area landscape trees with fairly good results.

Let's take a look at this new chemical.  Bayer Corporation, who manufactures Merit, indicates that it's a "broad-spectrum insecticide."  This means that it kills a variety of different insects.  The label lists numerous sucking insects and some chewing insects which can be controlled with Merit.  These include aphids, adelgids, elm leaf beetles, lacebugs, leafminers, leafhoppers, pine tip moth larvae, scale insects, and thrips.

Bayer also indicates that Merit is a systemic insecticide which means that it's taken up into the plant sap and moves to different parts of the plant.  Bayer indicates that it can be applied as a foliar spray to the leaves or applied to the soil for root uptake.  They point out that soil application is more efficient and effective for long-term insect control.  

Merit uses "new chemistry" and is very low in toxicity to humans, mammals, birds, and fish.  It's used at extremely low rates and there's no odor or phytotoxicity problems (damage to plants.)  Soil treatment also eliminates the chance of spray drift, reducing the potential exposure of humans and pets to this chemical.  Most beneficial insects are also safe with soil applied Merit.

Merit can be applied to the soil as either a drench or by injection.  Commercial applicators often opt for soil injection because it's the easiest to apply when they have the proper equipment.  Injections can be placed right at the base of the trunk; in a grid pattern below the tree's dripline; or in a circle at the dripline of the tree.  Bayer research indicates that all of these methods of placement are equally effective.  Many commercial pest control applicators choose the base treatment because it's most efficient.  Injections are placed a foot or less away from the base of the tree at a depth of six to twelve inches.  If the soil is dry, it's important to follow the application with adequate irrigation to thoroughly moisten the soil.  The soil in the application area should be kept moist for seven to ten days after treatment.  The number of the recommended injection sites increases with the size of the shrub or tree.

Now here's one of the interesting characteristics of Merit...  it will provide season long control if it's applied in late fall or early spring.  It takes about two to three months after application for it to become fully effective in the control of foliar pests on large trees.  It takes about a month after application on small trees and shrubs to become effective.  With this time lapse between application and full effectiveness, it's obvious that it must be applied prior to the time that the anticipated pests will be at their peak.  Foresight is much better than hindsight when using Merit. Fall application would logically be the best time to treat shade trees for aphid problems. 

Merit is now available to home gardeners so that they can treat their own trees and shrubs.  Merit is living up to it's manufacturer's promises... as long as it's applied early enough to become fully effective... and enough water is available to keep the soil moist for at least a week.  As Merit gains in popularity we'll probably not even miss some of those older chemicals that controlled the same pests, but were not as safe to use or as friendly to the environment.  Necessity made it happen.