Pest in soil??

Toss some h2o2 into next irrigation or 3, unless you do a living soil... if so, use BTi product - like mosquito bits top-dress or mosquito dunks in your water reservoir or watering can. That will kill them without destroying your soil's flora. It may take a while to do so... some that are nearly mature may fly out, but their offspring won't survive. Some sticky traps may be a good idea if you know it is coming. put'em right near plants as best you can. near soil but not in it of course and you'll catch a lot more on them. Definitely see something crawling around there... assuming not wind-adided speck. If you are not allowing top layer to dry between irrigations, allow it to dry out more in future. It will reduce chance this happens.