Geraniums, known botanically as Pelargoniums, are popular flowering plants that bring vibrant color to gardens and containers. A common frustration for gardeners is discovering holes and ragged edges where smooth, green leaves should be. When leaves begin to disappear, it signals that a variety of hungry garden pests have found the geraniums to be a desirable food source. The initial step toward solving this problem is correctly identifying the specific culprit based on the unique damage left behind.
Reading the Signs of Leaf Damage
The pattern of damage on geranium leaves provides the first clue to identifying the pest. Observing the size, shape, and location of missing tissue narrows down the list of suspects. Large, irregularly shaped holes or missing chunks of foliage are typically the work of larger, soft-bodied pests.
The presence of small, neat holes, or leaves that appear skeletonized, suggests a different type of insect is feeding. Skeletonization occurs when an insect consumes the soft tissue between the veins, leaving behind a lacy network of tougher vascular material. If the damage involves missing flower buds or new growing tips, the culprit is likely a pest that targets the most tender parts of the plant.
The Primary Culprit: Geranium Budworms and Caterpillars
The Geranium Budworm, the larva of the Tobacco Budworm moth (Heliothis virescens), is a primary pest of Pelargoniums. These caterpillars grow up to an inch and a half long and vary in color from pale green and yellow to brown and red. The adult moth lays individual eggs on leaves and flower buds, typically becoming more numerous in late summer as generations multiply.
The damage caused by the budworm is distinct because the larva often bores into the developing flower bud before it has a chance to open. This internal feeding results in buds that fail to bloom or flowers that emerge with large, ragged holes. The caterpillars also chew on the leaves, leaving behind noticeable holes and often a trail of tiny, dark-green to brown droppings, known as frass. Finding these small pellets near the damage is a strong indicator of an active caterpillar infestation.
Identifying Slugs, Snails, and Other Chewing Insects
Slugs and snails are common nocturnal feeders that cause ragged damage to geranium leaves. These mollusks leave behind large, unevenly eaten areas and can consume entire sections of the leaf blade. The most certain sign of their presence is the silvery, dried mucus trail they leave across leaves, soil, and nearby surfaces.
Leaf-chewing beetles, such as the Japanese Beetle (Popillia japonica), create a different form of injury. These insects feed between the veins, resulting in the characteristic skeletonized appearance of the leaf. Earwigs are also nocturnal pests that use their pincer-like appendages to leave small, irregular holes in leaves, often seeking shelter in the dark, moist crevices of the plant during the day.
Integrated Pest Management and Treatment
Controlling leaf-chewing pests requires a targeted approach. For budworms and other caterpillars, the biological insecticide Bacillus thuringiensis (Bt) is an effective treatment. Bt is a naturally occurring bacterium that is only toxic when ingested by caterpillars, making it safe for beneficial insects, but it must be applied when larvae are actively feeding.
Manual removal is an effective control method. Regularly inspect your plants, especially at dusk with a flashlight, to hand-pick caterpillars, slugs, and snails and drop them into soapy water. For slugs and snails, commercial baits containing iron phosphate are a pet-safe option, or a shallow container of beer sunk into the soil can serve as a simple trap.
Cultural practices also aid prevention, particularly for slugs and earwigs which thrive in damp conditions. Water plants early in the day so the soil surface can dry before evening, reducing the moist habitat these pests seek out. Remove leaf litter and garden debris from around the base of the geraniums to eliminate daytime hiding spots.