Growing dahlias (Dahlia spp.) rewards the gardener with spectacular, colorful blooms, but they often attract hungry pests. Discovering ragged holes in leaves or distorted flowers can quickly transform the excitement of the growing season into frustration. Identifying the specific culprit is the first step in protecting your plants and ensuring a season full of flowers. This guide teaches you how to read the signs of damage and implement effective solutions.
Diagnosing the Type of Damage
The initial clue to identifying an unseen pest is the type of damage left behind on the plant tissue. Pest damage is generally categorized into three main types: chewing, sucking, and subterranean.
Chewing damage is the most obvious, typically appearing as large, irregular holes in the leaves or petals. This damage ranges from clean cuts to ragged edges and skeletonized foliage where only the veins remain. Chewing pests may also sever plant parts, such as young sprouts cut off at the soil line or buds snipped from the stem.
Sucking damage is more subtle, often mimicking signs of nutrient deficiency or disease. Pests with piercing-sucking mouthparts remove the plant’s cell contents, resulting in stippling or a general yellowing and discoloration of the leaves. If the pest injects a toxin while feeding, new growth or flowers may become severely distorted or stunted.
The most destructive damage is indicated by a sudden wilting or the complete disappearance of a young plant. These symptoms point toward pests that attack the dahlia below the soil line, damaging the roots or the tuber itself.
Visible Culprits Attacking Leaves and Flowers
Many common dahlia pests feed on the above-ground parts of the plant. Slugs and snails are notorious for damaging tender young shoots, creating large, irregular holes in leaves and flowers. These mollusks are primarily nocturnal feeders, so their silvery, slimy trails are often found near the damage.
Caterpillars and certain beetles are also chewing pests. Caterpillars, the larvae of butterflies and moths, chew holes in leaves and flowers; their presence is confirmed by small, dark droppings, known as frass. Japanese beetles, identified by their metallic green bodies and bronze wing covers, skeletonize leaves by feeding only on the tissue between the veins.
Earwigs feed on foliage and flowers, particularly developing buds and petals. They are nocturnal and hide in dark, damp places, leaving small, ragged holes and chewed edges on petals overnight. Aphids, small, pear-shaped insects, cluster on the undersides of leaves and new stems, where they suck sap and cause leaves to curl or deform. They secrete honeydew, a sticky substance that can attract sooty mold.
Subtler Pests and Underground Eaters
Some dahlia pests are difficult to spot due to their size or location. Spider mites are tiny, eight-legged pests that feed on plant cells, creating fine stippling on leaves that eventually turns them bronze or yellow. These mites thrive in hot, dry conditions and are identified by the delicate webbing they spin on the undersides of leaves and between stems.
Thrips are minute, slender insects that feed inside unopened buds and developing flowers, causing the petals to emerge distorted, streaked, or discolored. They also cause silvery patches on leaves where they have sucked out the cell contents. Their damage is often the only sign they are present, as thrips are highly mobile and hide deep within the flower.
Below the soil line, mammals like gophers and voles can destroy an entire dahlia by eating the tuber itself. The telltale sign is a healthy plant suddenly vanishing or wilting completely because its root system has been severed. The dahlia borer, a larva that tunnels into the stem, causes a sudden wilting of the stalk above the entry point. This can be mistaken for a disease, but a small entry hole near the soil line confirms the presence of this hidden pest.
Actionable Prevention and Control Methods
An integrated pest management (IPM) approach combines several strategies to manage pest populations. Cultural practices focus on creating an environment unfavorable to pests. Proper spacing improves air circulation, which reduces humidity and discourages spider mites and fungal issues.
Removing plant debris and weeds around dahlias eliminates hiding places and overwintering sites for pests like earwigs, slugs, and voles. Regular monitoring is important; physically inspecting plants allows you to hand-pick larger pests such as caterpillars and Japanese beetles before they cause significant damage.
Targeted physical barriers and treatments protect vulnerable plants. For slugs and snails, use copper barriers or sparingly apply iron phosphate baits for effective control. For sap-sucking pests like aphids and spider mites, a strong jet of water can dislodge them. Organic options like insecticidal soap or Neem oil disrupt their feeding and life cycle. Applying these sprays in the late evening minimizes harm to beneficial pollinators.