BTC (bacillus thuringiensis) is strictly professional and there are strict regulations about using it, especially when bees are flying. Dusk is good when there is no risk of rain (frost is fine).
"Spraying twice a year is simply incorrect. BTC It is a fungus which the caterpillars ingest. It is not toxic to any other insect nor stage of its lifecycle. When caterpillars stop feeding then it is not effective (4th seti pine processionary caterpillar for example).
Now is the correct time to use it on hedge munching caterpillars, they are a different [non toxic] species to processionary caterpillars which stopped feeding in January.