A common insecticide active ingredient, malathion remains toxic to insects only for a short period of time, usually about one or two days. On spruce trees, malathion can control various insect pests, including aphids, adelgids, bagworms, caterpillars, sawflies and beetles. The appropriate timing for a malathion spray depends on the type of pest you have because various insects become vulnerable at different times. It also depends on your location and climate.
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Identify the type of insects that attack your spruce trees. Common insect pests of spruce trees include adelgids, aphids, armored scales, bark beetles, foliage-feeding caterpillars, pine weevils, sawflies and spider mites. Check that malathion can kill the insects you identify.
Research the symptoms of damage by the insects in question and their timing for the best results from your malathion application. For example, you need to wait for the galls to turn green and the insects to emerge from the galls for gall adelgid control.
Call your local cooperative extension office to find out the exact time when the insects will become most vulnerable to malathion spray in your area. The timing of insect development stages differs depending on your local climate and your local extension office will be able to give you an estimate for your particular location. This saves you from having to check for the signs of insect development too frequently.