I don't think I would use neem oil or any product that could be detrimental to predatory insects and other predators like spiders.
You are never going to have a "pristine" bug-free compost area, nor would you want to since they are part of the composting process, so it stands to reason that you would not want to harm the predators/beneficials that would control the unwanted population. My compost piles are locatÃ¨d next to the brush pile and the entire area is full of orb weaving spider webs, and the general compost pile area is home to centipedes and ground spiders (including Rollie pollie/sow bug killer = wood louse spider), ground beetles, and other predators.
I suppose if fruit flies are a serious problem, you could make traps for them. I've never considered using them outside, but I do make them when there is a fruit fly invasion/explosion in the house.
- A wide mouth jar with a piece of spoiled fruit and bit of vinegar and dribble of dish soap
- a paper cone made of card stock rolled into a funnel with a tiny hole at the cone tip
- securely tape the cone to the mouth of the jar with the hole of the cone inside the jar and about 1" above the bait
- shake the soapy solution in the jar to kill off the trapped fruit flies, wash out and and replace every 3-4 days
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