Rhubarb is a hardy perennial valued for its tart, edible stalks, but its large leaves often attract garden pests. Although the leaves are not consumed due to high concentrations of oxalic acid, damage to the foliage reduces the plant’s photosynthetic capacity. Significant leaf loss weakens the crown and reduces the size and quality of the harvestable stalks, so protecting the foliage ensures the plant remains vigorous and productive.
Identifying Common Leaf-Eating Pests
The first step in managing leaf damage is accurately identifying the culprits by recognizing their feeding patterns. Slugs and snails are the most common pests, leaving irregular, ragged holes in the leaves. Their presence is confirmed by the silvery slime trails left on the leaf surfaces, as these mollusks primarily feed nocturnally or during damp weather.
Another leaf-notching pest is the Rhubarb Curculio (Lixus concavus), a large, yellow-brown snout beetle measuring about half an inch long. While primarily known for boring into leaf stalks and crowns, the adults also feed on the leaves, creating distinct notches along the edges. When disturbed, these beetles may play dead by dropping quickly from the plant.
Caterpillars, such as Cabbage Worms and Loopers, cause damage by skeletonizing leaves or chewing small, circular holes between the veins. These green larvae are often camouflaged against the foliage and typically feed from the underside of the leaves. Identifying their small, pellet-like droppings, or frass, on or beneath the leaves is a reliable sign of their presence.
Proactive Cultural and Physical Prevention
Maintaining an environment less welcoming to pests is the most effective long-term defense for rhubarb plants. Begin by practicing strict site sanitation, which involves regularly clearing away old leaf matter, weeds, and debris from around the base of the plant. This removes the cool, moist hiding places where slugs, snails, and overwintering curculio beetles shelter.
Proper plant spacing promotes better air circulation and allows sunlight to penetrate the foliage. This reduction in humidity helps dry out the soil surface quickly, making the environment less appealing to moisture-loving mollusks. When watering, apply water directly to the soil in the morning rather than using overhead sprinklers in the evening.
Physical barriers offer immediate protection for vulnerable plants. A ring of abrasive material like diatomaceous earth or crushed eggshells placed around the crown deters slugs and snails by irritating their soft bodies. For large pests like the Rhubarb Curculio, hand-picking and destroying the adults when spotted is a direct control method.
Applying Targeted Treatments
When an active infestation requires direct intervention, several targeted, low-impact treatments are available. For soft-bodied pests like aphids or small caterpillars, a ready-to-use insecticidal soap can be applied directly. Neem oil, an extract from the neem tree, works as both a contact killer and a repellent, disrupting the life cycles and feeding habits of many insects.
Neem oil application should occur in the early morning or late afternoon to prevent leaf burn, as the combination of oil and strong midday sun can damage foliage. To ensure maximum effectiveness, spray both the top and the underside of all leaves until the product begins to drip off. Repeat applications every seven to fourteen days are often necessary.
For controlling slugs and snails, iron phosphate baits offer a safer alternative to older chemical pellets. When ingested, the iron phosphate acts as a stomach poison, causing them to stop feeding immediately. For caterpillar pests, especially Cabbage Worms and Loopers, treatments containing Bacillus thuringiensis (Bt) are highly effective and safe for beneficial insects.
Bt is a naturally occurring bacterium that must be ingested by the caterpillar to be effective, causing the pest to stop feeding within hours and die within a few days. Since sunlight can quickly degrade the bacterium, applying Bt in the late afternoon ensures maximum residual effect during peak feeding time. Ensure the application thoroughly coats the underside of the leaves to be consumed by the larvae.