How Do I Stop Caterpillars Eating My Radish Leaves?

Radish leaves that are disappearing rapidly, with large irregular holes or entire sections eaten away, are most likely being attacked by caterpillars. The main culprits in most gardens are the caterpillars of cabbage white butterflies (Pieris brassicae and P. rapae) — green or yellowish caterpillars that feed on brassica and radish leaves voraciously. In a small radish patch with only three or four weeks of growing time, heavy caterpillar feeding in the final week can significantly reduce the leaf area available for photosynthesis and stress the developing root.

Identifying the caterpillars

Check the undersides of leaves for egg clusters — cabbage white eggs are small, yellow and arranged in neat groups. The caterpillars are initially small and black-headed, becoming larger and more distinctively patterned (bright green with yellow stripes for large white; uniformly pale green with a yellow line for small white). The velvet-soft, bright yellow-green caterpillars of the diamond-back moth (Plutella xylostella) also attack radish — these are smaller and more active, wriggling and dropping on a thread when disturbed.

Hand-picking eggs and caterpillars

For small patches, daily inspection and hand-removal of eggs and caterpillars is highly effective. Squash egg clusters on sight. Drop caterpillars into soapy water. This is time-consuming but requires no products and has zero side effects on beneficial insects. Given radish's short growing season, a few days of hand-picking may be all that is needed.

Bacillus thuringiensis (Bt)

Bt kurstaki is a naturally occurring soil bacterium that is toxic to caterpillars when ingested but completely harmless to humans, pets and other wildlife. It is OMRI-listed for organic use and available as a spray. Apply to affected leaves, coating undersides particularly. Repeat after rain. This is the most effective organic spray specifically for caterpillar control.

Row cover

Fine insect mesh applied from day one of sowing prevents butterfly access entirely. The butterflies cannot reach the leaves to lay eggs, and no caterpillars develop. This is the simplest, most reliable approach for a crop as fast as radish.