**Dock Sawflies** (*Ametastegia glabrata*) are common pests of plants in the genus *Rumex*, primarily **dock** and **sorrel**. They are recognized by their larvae, which are small, greenish caterpillars that feed on the foliage, often rolling the edges of the leaves to create a protected retreat. They are a common nuisance in gardens and pastures, causing aesthetic damage and sometimes reducing the vigor of host plants, but they are generally easy to control.
Taxonomy and Classification
Dock Sawflies belong to the family Tenthredinidae (common sawflies) in the order Hymenoptera. Unlike true flies (Diptera), sawflies possess two pairs of wings. They undergo complete metamorphosis. Their name comes from the female’s ovipositor (egg-laying apparatus), which is serrated like a saw, used to cut slits into plant tissue to deposit eggs. The larvae are distinguished from butterfly/moth caterpillars by having **more than five pairs of prolegs** on the abdomen (moths/butterflies usually have five or fewer).
Physical Description
Adult Dock Sawflies are small, stout, black, wasp-like insects, about $1/4$ inch long, lacking the narrow “wasp waist” of predatory wasps. They do not sting defensively.
The **larvae** are small, greenish, and slightly wrinkled, resembling a moth or butterfly caterpillar, reaching about $3/4$ inch long when fully grown. A characteristic feature is their behavior of resting with the head curled toward the body. They feed on leaves from the underside, often creating feeding holes or skeletonizing the leaf surface.
Distribution and Habitat
Dock Sawflies are widespread throughout North America and Europe. Their habitat is any area where their host plants—dock (*Rumex* species)—grow abundantly, including pastures, abandoned fields, disturbed sites, and gardens. The larvae are found directly on the leaves, while the adults are active flyers near the host plants.
Behavior and Life Cycle
Dock Sawflies have multiple generations per year. The female deposits eggs individually into slits she cuts in the dock leaf tissue. The larvae hatch and begin feeding on the foliage. When not feeding, the larvae often curl the edges of the leaf, rolling themselves into a protective covering. When mature, the larvae drop to the ground and burrow into the soil or soft wood (like rotting boards) to create a tunnel for pupation.
The most distinctive feature of the pupating larva is its habit of boring into soft material to create a pupation chamber, sometimes surprising homeowners by boring into window sills or decking.
Feeding and Damage
The larval feeding on the leaves of dock and sorrel causes irregular holes and skeletonization, leading to ragged, tattered foliage. While they can be numerous, their feeding typically does not kill the host plant, as dock is a robust weed. However, in sorrel (a culinary herb), their damage can render the leaves unusable, and high populations can significantly defoliate plants.
Management and Prevention
Control is generally straightforward, focusing on hand removal.
- **Hand Picking:** Since they are large and easily visible, hand-picking and destroying larvae is highly effective in small gardens.
- **Biological Control:** Natural enemies, including parasitic wasps and predatory beetles, often keep populations under natural control.
- **Pesticide Use:** Due to the damage being caused by the larvae, simple contact insecticides, or organic options like **insecticidal soap** or **Bt** (though less effective than on true caterpillars), can be used if populations are very high, particularly on cultivated sorrel.
Conservation and Research
Dock Sawflies are minor garden and field pests. Research is minimal, focusing on their specific host range and the efficacy of natural parasites in controlling their populations.