Herbicides treat weed problems. Insecticides treat insect problems. Fungicides treat disease problems. A fungicide won't control an insect problem and an insecticide won't control blight on your tomatoes. If an insect is causing the problem, identification is of utmost importance.
Insects are vulnerable at different stages in their life cycle.
If you spray an adult insect that is only vulnerable in its larval stage, little or no control will be achieved. It is also important to know when the insects are causing the most damage. While white grubs are present in the soil in spring, they are not causing serious damage to the turf, so treatment in spring is a waste of time and money. The most appropriate time to treat them is late July, when there is the most potential for damage to the lawn. Not all problems warrant chemical treatments.