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Q. What is the difference between a systemic insecticide and one you spray directly on the plant?

A. Although both systemic and contact spray insecticides can be applied directly to plant foliage, they function in different ways. The systemic chemical solution is absorbed by the plant's vascular system and circulated so that the entire plant contains the insecticide. When an insect then feeds on any part of the plant, it ingests some of the insecticide. Systemic solutions are more commonly applied as a drench to the soil surrounding a targeted plant where they are absorbed by the plant's roots and further circulated through the plant's vascular system.

Contact sprays are intended to kill insects on contact and are effective only when the insects are present on the plant. Most offer little residual effect and need to be repeated according to the label directions.