It's widely known that earthworm castings (droppings) are a great fertilizer (especially for container plants) and can suppress soil diseases. Now, research from Germany has found that not only do earthworm castings prevent soilborne diseases, they actually help repel aboveground insects as well.
Researchers found that plants with worm castings added to their growing medium had higher nitrogen content in their leaves than control plants, but fewer aphids feeding on the leaves. They speculated that the earthworm castings altered the plant sap chemistry, making it less appealing to the aphids.
For more information on this research, go to the article in the Journal of Chemical Ecology.