Oakworm Excrement Staining Patio

Q: We have oak worms in a large tree that overhangs our concrete patio. Their excrement stains our entire patio a dark brown. Is there a pre-emptive spray?

A: This is a tough one. It is difficult to prevent caterpillars because they develop from eggs and nothing you do will prevent oakworm moths from laying eggs. The organic caterpillar poison B. t. (Caterpillar Attack, Caterpillar Master, etc) is very effective for stopping feeding but you have to get it on the leaves while small caterpillars are eating. Mature caterpillars are unaffected. B.t. is quickly broken down by sunshine and oxygen. You can’t spray it more than a couple of days in advance. Oak caterpillars vary widely in their occurrence so next year you might have no problem. Wait a few weeks and hire someone to bleach and power wash your patio.