Q: Last summer I called your radio show and asked how to get rid of caterpillars on several river birch trees in my back yard near my goldfish pond. Your answer was to get an organic product, and spray it on at intervals until all the caterpillars were dead. You also mentioned that I could possibly buy a liquid systemic product. Well, I have forgotten the name of the insecticides that you mentioned. I need to get busy and try to stop the caterpillars before they get real bad.
A: I am so glad you wrote back….because I was WRONG last year!
The river birch “caterpillars” are not caterpillars at all. They are sawfly larvae. They LOOK like caterpillars to the uneducated eye but they are not. A sawfly is a primitive wasp-like insect. The females have a saw-like blade at the tip of the abdomen that is used to cut slits into plant tissue into which they deposit eggs. The resulting larvae, since they are not caterpillars, are not affected by the organic caterpillar insecticide, Bt, that I recommended.
The contact insecticide carbaryl (click for sources) offers good control if sprayed on the entire tree. Imidacloprid (click for sources) is a systemic insecticide which can be applied to the soil around the tree before feeding activity is noticed. Both insecticides, however, are moderately toxic to fish.
In your specific situation consider using insecticidal soap (click for sources) every seven days during the few weeks in which the sawfly larvae are present.