Powder or Spray?
Both powdered and liquid forms of Bt effectively control pest insect larvae. Whether the powder is dusted on foliage or mixed with water and sprayed as a wettable powder, the visible residue left on the plants is a handy indicator to determine if another application is necessary, especially after a rain. Liquid Bt is best for dealing with lawn-dwelling caterpillars such as sod webworms, and for treating tall trees and shrubs that may be easier to cover with a liquid spray than a powder.
Overall, Bt powder is the best buy. It stores better and, when mixed with water for a spray, is cheaper per gallon than liquid formulations. If you buy the liquid form, the concentrate will be cheaper in the long run than a ready-to-use liquid.
Using Bt Powder
When applied to plant foliage with a duster [781 400], Bt powder remains potent for 3 to 7 days. If a second application seems necessary, wait for 5 to 7 days before dusting again. To assure that Bt powder sticks to the undersides of leaves, dampen them first with water. The powder that adheres to the undersides of the leaves will remain potent longer since it is protected from sunlight, which rapidly degrades Bt. Always reapply the dust after a rain.
To facilitate mixing the wettable powder with water, make a paste first with a few tablespoons of water and the required amount of powder for a gallon of spray. Then add the paste to the gallon of water and fill a compression_sprayer , backpack_sprayer , or sprayer_bottle. Keep agitating the reservoir as you spray to keep the Bt powder thoroughly mixed in the water.
Using Bt Spray
When spraying liquid Bt, wet the tops and the bottoms of all the leaves you can reach. Saturate them until they drip. To help the wettable powder mix adhere to plant leaves, add a "spreader-sticker" to the mix. A single tablespoon of fish_emulsion [661 200], light_horticultural_oil [721 200], or insecticidal_soap [721 100] added to each gallon of spray mix will help the liquefied powder stick to leaves. The Bt that comes as concentrated or ready-to-use liquid adheres well to plants and needs no additional sticker materials. When sprayed on plant surfaces, a liquid form of Bt is potent for only 24 hours. Spray again in 3 to 5 days if caterpillar problems persist.
Read the label: When diluting Bt powder or liquid concentrate with water to make a spray, be absolutely sure you follow label instructions on how much Bt to use per gallon of water. NEVER disregard product label directions.