Even though Bt is one of the safest insecticides on the market, treat all insecticides the same way--with great respect. When applying any insecticide, including Bt, always wear gloves. To be truly safe wear a long sleeved shirt and a pair of protective safety glasses over your eyes.
This may seem a bit extreme for such a safe product, but not only is protective clothing a good habit to acquire for all pesticide applications, it is one of the best ways to demonstrate the importance of being careful to others, especially any children who may be watching. When using the powdered form of Bt, avoid breathing the dust, as it may cause lung irritation.
What About Butterflies?
Because Bt targets caterpillars, the bacterium will kill the larvae of desirable butterflies if they eat it. However, because most desirable species such as monarchs, swallowtails, and fritillaries seek out very specific plants on which to lay their eggs and host the feeding caterpillars, you can spare butterfly caterpillars by avoiding their food plants when spraying.
Butterfly larva tend to be solitary. They do not travel in hoards and infest a plant. Spray Bt on only those plants infested with caterpillars that you wish to control. Do not use it as a general preventative spray on all plants.
Preferred plants of the larvae of some favorite butterflies
Black swallowtail: Parsley, carrot, Queen Anne’s lace
Tiger swallowtail: Wild cherry, tuliptree, ash
Painted Lady: Thistle, burdock, sunflower, hollyhock
Red Admiral: Nettle, pellitory, hops