Insecticidal soap and horticultural oil can be mixed to help the soap stick to the insects. Of these, azadirachtin and pyrethrum are the most common and useful. Azadirachtin, from the neem tree, has several actions on insects, but most notably disrupts molting, with some activity as a stomach poison. As such, it does not kill instantly. It must be ingested to poison, so it is nontoxic to birds, fish and beneficials.
Pyrethrum is extracted from the flowers of chrysanthemum.
It is a contact poison that affects the nervous system, paralyzing the insect. The insects produce an enzyme to detoxify the pyrethrum, so organo-phosphates, carbamates or synergists are sometimes added to slow the enzyme action. Piperonyl butoxide is a synergist found in many formulations. Pyrethrum is extremely toxic to fish, toxic to bees and slightly toxic to birds. Rotenone, a stomach and contact poison derived from the roots of two South American legumes, is sometimes added to pyrethrum, and is toxic to fish.