Water. Nothing fancy here, just plain old water. Keeping plants clean helps prevent problems with spider mites before they start, and a strong spray of water can knock aphids, spider mites, and other pests off plants. Spray in the morning, so the leaves will dry quickly.
Bt (Bacillus thuringiensis) is a microbial insecticide that kills only moth and butterfly larvae and is harmless to most other insects and animals. However, don't spray throughout the garden or excessively. It won't discriminate between pest caterpillars and those of desirable moths and butterflies.
Horticultural oil is a more highly refined version of traditional "dormant" oils applied to leafless trees and shrubs in winter. Horticultural or "summer" oils control a wide variety of rose pests, including rose scale and whitefly. These oils also control soft-bodied pests, such as aphids (and their eggs) and spider mites. Don't use horticultural oils when you expect temperatures to rise above 90°F.
Insecticidal soaps. These specially formulated soaps are a key element in any least toxic pest-control strategy. They are effective against a wide range of mite and insect pests, particularly soft-bodied insects such as aphids, immature scale, leafhoppers, mites, thrips, and whiteflies.
Neem oil extracts contain the substance in whole neem oil that is believed to be the most insecticidal. This product provides effective control of insect pests, and, unlike many other pesticides, synthetic and organic, neem is not harmful to most beneficial insects.