Rhubarb is a prized perennial, valued for its tart, edible stalks. While the stalks are safe for consumption, the large green leaves contain high concentrations of oxalic acid, which is toxic if ingested in quantity. Monitoring the health of these leaves is important, as damage is often the first indicator of a pest problem. Ragged edges, unexplained holes, or wilting foliage signal that something is actively feeding on the plant. This guide will help identify the culprits and provide effective methods to protect your rhubarb patch.
Primary Pests and Their Specific Damage
Diagnosing the pest requires examining the specific feeding damage on the leaves and stems. One of the most common issues is damage from soft-bodied mollusks, specifically slugs and snails. These pests are primarily nocturnal feeders, creating large, irregular holes in the leaves, often starting near the edges. The unmistakable evidence of their presence is a silvery, slimy trail that glistens on the leaves or surrounding soil.
A more serious threat is the Rhubarb Curculio (Lixus concavus), a large snout beetle measuring one-half to three-quarters of an inch long. The adult curculio is yellowish-brown to brownish-black, often covered with a dusty yellow powder. While they cause minor feeding injury on the leaves, appearing as distinct notches, their most damaging activity is boring into the stalks.
When the beetle attempts to lay eggs in the stalk, the plant exudes a sticky, gummy sap that collects around the wound. Although the eggs typically do not hatch because the plant’s rapid growth crushes them, the resulting damage weakens the stalk and allows decay to set in. If you find small, circular feeding punctures on the stems accompanied by drops of sap, the curculio is the likely offender.
Caterpillars and earwigs also target rhubarb leaves, causing smaller, less uniform holes than slugs. Caterpillars produce holes of varying sizes by chewing directly through the leaf surface. Earwigs, recognizable by their pincer-like appendages, typically chew along the leaf edges, creating smaller, scattered holes. These pests are often found hiding in the folds of the leaves or in debris around the plant crown during the day, making them harder to spot.
Immediate and Organic Control Methods
Once a pest is identified, immediate, non-chemical control methods can stop the feeding damage. Handpicking is a straightforward and effective technique for managing larger pests like the Rhubarb Curculio. The adult beetles are active in the early morning and drop quickly off the plant when disturbed. Placing a bucket of soapy water beneath the leaves before tapping the beetle ensures they fall directly into the solution for destruction.
For slugs and snails, handpicking can be done at night using a flashlight when they are most active, or simple traps can be deployed. A shallow dish filled with beer placed near the rhubarb will attract and drown these mollusks. Physical barriers also offer direct defense against crawling pests, such as creating a perimeter around the plant with diatomaceous earth. This finely ground powder lacerates the soft bodies of the pests, causing them to dehydrate.
Organic sprays are another option for immediate deterrence, targeting general chewing pests like caterpillars and earwigs. Insecticidal soaps work by dissolving the protective outer layer of soft-bodied insects, leading to dehydration. Neem oil, derived from the seeds of the neem tree, functions as a suffocant and contains azadirachtin, which disrupts the insect’s ability to feed and grow.
When applying any spray, ensure the product is properly diluted according to the label to prevent leaf burn. Thoroughly coat the tops and undersides of all leaves, as pests often hide underneath. It is safest to apply these organic treatments in the late evening. This minimizes the risk of leaf scorching in direct sunlight and protects beneficial insects like bees, which are less active at dusk.
Long-Term Environmental Prevention
Preventing future infestations requires modifying the garden environment to make it less hospitable to common rhubarb pests. A significant focus should be on sanitation, especially removing old plant material. The adult Rhubarb Curculio and pests like earwigs overwinter in leaf litter and garden debris near the host plant. Clearing away dried rhubarb leaves and organic matter from the planting area in late fall or early spring reduces sheltered places where pests survive the winter.
Optimizing the plant’s location and ensuring proper air circulation prevent issues with moisture-loving pests. Slugs and snails thrive in constantly wet conditions. Planting rhubarb in an area with good sunlight and spacing the plants appropriately helps the soil and foliage dry quickly after rain or irrigation. Reducing excessive moisture minimizes conditions favorable for these pests to reproduce and feed.
Controlling nearby weeds is an important cultural practice, particularly for mitigating the Rhubarb Curculio. This weevil primarily uses plants like dock, thistle, and sunflower as host plants to lay its eggs, since its larvae cannot survive in the rhubarb stalk itself. Removing these specific weedy hosts from the area in mid-summer, when the curculio larvae are developing within their stalks, significantly reduces the local pest population for the following season.