BTI Mosquito Dunks will be a great choice to apply in french drains for mosquitos. We recommend using Bifen IT and Pivot 10 on the exterior of the home as well as other non-chemical methods for eradication. I have included some great articles below that you might find extremely helpful!
No, BTI Mosquito Dunks are not labeled for use in ponds or other bodies of water. Only in bird baths, old automobile tires, rain barrels, abandoned or unused swimming pools (particularly above ground types), tree holes, roof gutters for collecting rainwater, flower pots, animal watering troughs, elevator shafts, basements that flood, sump pumps and any drainage areas within buildings.