What Caterpillars Are Eating My Rhubarb Leaves?

Large, irregular holes in rhubarb leaves are a common sight in summer and are usually the work of caterpillars or, less visibly, slugs and snails feeding overnight. Rhubarb leaves are not the most desirable food for most caterpillar species, but a few generalist feeders — including the elephant hawk-moth caterpillar and various noctuid moth larvae — will visit them opportunistically. Since we do not eat rhubarb leaves, the practical impact on the harvest is almost always negligible.

Does leaf damage affect the harvest?

Unless the damage is so severe that the majority of the leaf surface is destroyed on multiple leaves simultaneously, caterpillar feeding on rhubarb leaves does not meaningfully affect stalk production. Even heavily eaten leaves still contribute some photosynthesis, and the plant is large and vigorous enough to compensate for moderate leaf loss. Removing caterpillars by hand is an effective and proportionate response if populations are large.

Identifying the culprit

Check leaves in the early morning or at dusk when feeding is most active. Large, ragged holes with no slime trail suggest caterpillars. Regular ragged holes with slime trails point to slugs or snails. Small, neat circular holes in young leaves may be flea beetles (unusual on rhubarb but possible). The presence of frass (black or green pellets of caterpillar droppings) on or beneath leaves confirms caterpillar feeding.

When to act

For small caterpillar populations, hand-picking in the evening is the most practical control. For very large infestations that are stripping multiple leaves from the plant, a biological control such as Bacillus thuringiensis (Bt) spray can be applied to the leaf surface — the bacteria are harmless to birds, mammals and beneficial insects but lethal to caterpillars that ingest the treated tissue.