Hi Bonnie,
I've never had them but a ponder tried the Mosquito
Dunk against them and it worked. I'd try somehow
attaching the Dunk in the water before it reaches
the spillway. Hang it like a Christmas ornament?

Let me know how it works.

I've done some more reading on how the stuff in
the dunk works. The larvae injests it and then
it damages their digestive tract and they starve
to death.

(And for new ponders, it does not harm fish, frogs,
birds or pets)

kathy :- )