I have a farm pond plagued with crawdads that undermine the banks, causing them to collapse. The resulting shallow water warms up enough to permit algae to grow and allow cattails to take over.
The county conservation people advised me to pour a mixture of fuel oil and mothballs down the crawdad holes, but I felt that action would contaminate the pond (besides, there are hundreds of holes!).
Can you suggest a nonpolluting way to eradicate the creatures?
Do you really want to eradicate the crayfish (or crawdads), or do you just want to control them? It sounds to me as though your problem isn't crayfish, but rather too many crayfish.
To improve the situation, you'll first need to see whether your water level is stable. Crayfish reproduction is favored by a fluctuating water level (which in itself can undermine pond banks). If there are measures you can take to stabilize the water level, do so. This won't eradicate the crustaceans, but it will reduce the population.
Crayfish, in moderation, are actually an asset to a pond. They help control weeds and are an important fish food. Bass, in particular, are fond of the critters. You haven't mentioned what your pond is stocked with, but if it's like most farm ponds, it's probably overstocked with bluegills and understocked with bass. An effort to build up the bass population might help control the crayfish.
Of course, the most straightforward method of “crawdad control” is to catch the crustaceans and eat them. All North American species are edible and, in my opinion, delicious! Crayfish can easily be caught in an ordinary minnow trap that has openings of a suitable size and is baited with meat scraps or a punctured can of dog food. Umbrella nets also work welt. If you want to get rid of the crayfish quickly and your pond bottom is free of obstructions, use a weighted seine. Keep in mind, though, that all methods of crawdad capture work best at night.
I urge you to view the crayfish as a resource, or at least as a mixed blessing, rather than as a pest. However, if you feel you must eradicate them, you are right to fear that the fuel oil-mothballs mixture could contaminate the pond (it's also quite flammable). Other chemical poisons are even worse. An old-fashioned, nonpolluting technique involves making up a solution of eight teaspoons of lye to one gallon of water, pouring the mixture into each burrow, and sealing the holes. This method won't contaminate the pond or kill other organisms. [EDITOR'S NOTE: Bear in mind, though, that lye is a powerful chemical, and take the proper precautions when using it.]
You could eradicate the crayfish with rotenone, but this would also kill the fish, perhaps all of them (since crayfish are found primarily along the shoreline, the fish in the middle of the pond might survive). The standard rotenone dosage to kill fish is 1 pound of 5 percent rotenone, or its equivalent, per 300 feet of shoreline. I imagine it would take a higher dosage to eliminate the crayfish. Mix the rotenone with twice its volume of water and apply it parallel to the bank, just under the water surface. Unlike treatment in the burrows, which requires low water levels, this process is best undertaken when the water is high.
— Bill McLarney, founder of the New Alchemy Institute