excerpt from “Pyrethroids: Not as safe as you think” at Melissa Kaplan’s Herp Care Collection, last updated January 1, 2014:

…Some insects have developed ways to detoxify the naturally occurring pyrethrums encountered when feeding on the nectar of feverfew and chrysanthemums, a not uncommon adaptive response. Unfortunately, while insects and plants have had millions of years to work out these survival pathways, we humans haven’t.

An increasing number of insects have developed high levels of resistance to pyrethroids, such as cockroaches, head lice, and tobacco budworm, pear psylla, fall army-worm, German cockroach, spotted tentiform leafminer, diamondback moth, house fly, stable fly, head lice, and tobacco budworm. Many of these species are resistant to more than one pyrethroid. Because insects reproduce – and adapt – far more quickly than do vertebrates, they are far better able to evolve defenses against the toxins we throw at them, resulting in an ever expanding range of poisons developed and thrown into our environment.

Pyrethroids, like all toxins, are indiscriminate: they affect all the organisms who come into contact with them in the air, on plants, on the ground, in the soil, and in the water. While your local grower – or you – may be applying it to deal with a specific pest, the products affect everything around it. And, since particulates are easily airborne, they travel, often great distances, from the actual point of application….