There are both kinds of insecticides. Systemic insecticides are absorbed by the plant and stay inside the plant and work from the inside. They are especially good for borers that get inside the plant. Those are better suited for trees and shrubs, I would not use them in a garden because the insecticide remains in the food you eat.

Most garden insecticides are non-systemic as you described. They just sit on the leaves until they are washed off or break down chemically, they do both. You are correct, that insecticide can only be used five times between May and September.

littleblu87,

It depends on the insecticide as mentioned above.

Pest management is complex, if you wish to use chemical products, do so responsibly which goes further than reading the label. If you want really to learn, a good place to start is here:

Integrated Pest Management
This lecture is presented in two parts. Each part is 90-minutes in length. Recorded in Sacramento County in California's Sacramento Valley, this lecture is by Mary Louise Flint, Ph.D., Director, IPM Education and Publications, UC Statewide IPM Project and Extension Entomologist & Cooperative Extension Specialist.

Education:
B.S. Plant Science, University of California, Davis
Ph.D. Entomology, University of California, Berkeley

Appointment:
100% Cooperative Extension

Research Interests:
Integrated pest management of landscape, agricultural and garden pests; biological control of arthropod pests; alternatives to pesticides; adoption of alternative practices by practitioners; innovative delivery of pest management information.

Topics discussed in the Integrated Pest Management Lecture:

* IPM references and resources
* Preventing pest problems
* Natural common enemies
* Making less toxic pesticide choices
* Controlling aphids, scales, caterpillars, coddling moths, tree borers, snails and slugs, and lawn insects.

You can watch the programs now online:

Just make sure you have Real Player installed or download it free.

Integrated Pest Management Part1 90 minutes

Integrated Pest Management Part 2 90 minutes

You'll want to bookmark the following link to Professor Flint's Lab Research on:
Controlling Pests in Gardens and Landscapes: Vegetables and Melons

I promise you'll learn one or two things to put in your gardening bag of pest management arsenals.

;)