Never combine Bt with insecticide or fertilizer. Use all mixed spray promptly; discard any remaining material. It is ineffective after 12 hours.
Avoid hard, or alkaline (pH over 7.0), water when diluting Bt for spraying. It reduces its effectiveness. If you add insecticidal soap as a sticker, add it to the Bt mix at the last minute. Because it tends to be alkaline, it may reduce the effectiveness of the Bt.
Bt breaks down in sunlight, so spray in the late afternoon or on overcast (but not rainy) days. Use it on vegetables and fruit up to the day of harvest. It will not harm people, even if they ingest a trace amount.
Liquid concentrate or powdered Bt stored in a dark room that does not go below 35°F or above 90°F, will stay viable for at least two years.
Getting the wettable Bt dust to stick to foliage is not always easy, especially when treating cabbages. Adding one tablespoon of dish detergent helps as an adherent. The BT formulated as a liquid concentrate adheres well.
Timing of your Bt application is critical. BT has no residual effect like most chemical pesticides, you must be sure to apply it when feeding larvae are actually present. Observe your plants closely, and spray when you see insects beginning to feed. Applications of BT are most effective in the spring and again in the late summer, when larvae feeding activity is greatest.