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I use Merit in my lawn for grub control, and wondered if this can also be used for insects that attack my trees and shrubs. If so, how is it applied and when?

A few years ago, a new insecticide, imidacloprid, also known as Merit®, made its way into use in lawns for control of white grubs and other lawn insects. The benefits of this new chemistry include effectiveness, safety of use with low toxicity, a fairly long residual, and fewer adverse effects on beneficial insects. Merit® is now also available for control of some insect pests in ornamental trees and shrubs. Some research has been done on timing the application of Merit® as a soil drench in either spring or fall for control of insects. Merit® has to be applied to woody plants well in advance of when the injuring population of insect pests arrives, and moves systemically through the plant.

Dr. Dan Herms, an entomologist at the Ohio Agricultural Research and Development Center, recently reported results from a very successful spring-applied Merit® research trial for managing scale insects; treatments provided excellent control of magnolia scale late this summer. Two products were tested in the research trial; Merit® 75WP, which is for commercial landscape use, and Bayer Advanced Garden Tree & Shrub Insect Control, labeled for homeowner use. Both were applied at full label rates as a soil drench to the base of trees. To assure good infiltration to the root zone of the trees, the leaf litter and/or mulch was removed prior to application.

The success of the spring application of Merit® to manage magnolia scales can be added to a growing, verified list of successful fall application treatments to manage birch leafminers, European pine sawflies, lace bugs, spruce gall adelgids, holly leafminers, boxwood psyllids, and soft scales. The fall application should be made mid-October to late-November in order to manage these insects the following spring and early summer. Merit® is mixed with water in a sprinkling can or bucket and poured around the base of the plant, so no spraying is involved. Note that in dense clay soils, soil may need to be mounded to form a shallow basin around the treatment area to hold the drenching solution in place until it infiltrates the soil. Read and follow label rates and directions.

There are a couple of notes regarding the use of Merit® as a soil drench for scale insects. First, it is most effective against soft scale insects and is less effective against armored scales (those with a hard covering). Soft scale insects insert their mouth parts into the plant's vascular system to feed, where they suck large quantities of sap. In the process, they also acquire large doses of Merit®, which moves through the plant's vascular system. The armored scale insects feed by sucking up the contents of individual cells, and so are not exposed to large doses of the insecticide.

Merit® is also labeled for managing some borers. The borers listed on the label are species of beetles, the roundheaded and flatheaded borers, such as bronze birch borer. However, Merit® will not manage the lepidopteran borers (moths) such as lilac ash borer, the greater peachtree borer and the banded ash borer.

The Bayer Advanced Garden product containing Merit® for home use can be found at many garden centers and home improvement stores. You will find that this product is fairly expensive to use; for shrubs, treatment is based on the height of the plant(s), and for trees, it is based on trunk circumference. However, this is a new and useful tool in managing several insect pests in woody plants.

The Master Gardener Offers horticulture advice and tips, focusing on current issues by volunteers who are with The Ohio State University Extension, Union County Master Gardener Program. For answers to gardening questions, call 937.644.8117 or e-mail gmcvey@ag.osu.edu. Hourse are 8 a.m. to 4:30 p.m. Monday through Friday