If aphids are not included in the list of insects the insecticide will control AND tomatoes are not included in the list of plants on which it can be used, select another insecticide. Always select the least-toxic pesticide possible that will control the pest. For pesticides to work best, they should be used when the pest is most susceptible to the product. The label should specify the best time for application. For example, some weeds are controlled better by fall applications of herbicides, but others may be more susceptible in spring, or just before flowering.
Insects are often vulnerable at one stage in their life cycle and relatively unaffected in another.
When using herbicides, make sure the weeds are not drouth-stressed because the physiological processes of plants slow down when deprived of water, which in turn inhibits the effectiveness of the herbicide. So read the label to determine the best time to apply the pesticide. The amount of pesticide used can also determine how effective it will be. Use specified on the label. Too much pesticide can damage the plant and leave toxic residues with which children, pets, and wildlife can come in contact.