You may apply Monterey B.t. RTU up to and on the day of harvest.
Monterey B.t. RTU is not labeled for citrus fruit, including lemons.
We would recommend using Natural Guard Spinosad Landscape and Garden Insecticide instead. It is labeled for use on lemons and will kill caterpillars on the tree. One bottle will treat approximately 5300 square feet.
Yes, Monterey B.t. RTU should kill all caterpillar types.
If you are treating edibles you will need to let them dry out for a while as one of the biggest contributors to fungus gnat population growth is over watering/too wet of soil. You may need to contact Monterey Lawn & Garden to see if orange oil would interfere with anything in their specific formulation. If you are treating non edible plants you can try Talstar P Professional Insecticide and Gentrol IGR Concentrate to the soil to get some IGR going to stop the reproductive cycles.