Sawflies are a diverse group of insects, yet they are frequently misidentified. These creatures are often mistaken for true flies, wasps, or, in their immature stage, the caterpillars of moths and butterflies. Sawflies belong to the large insect order Hymenoptera, which also includes ants, bees, and wasps. Over 8,000 species of sawflies have been described, making them a significant component of many ecosystems.
Defining Characteristics and Classification
Sawflies are classified within the suborder Symphyta, distinguishing them from the other Hymenoptera, which belong to the suborder Apocrita. The name Symphyta, derived from Greek meaning “grown together,” refers to their adult morphology. Unlike ants, bees, and wasps, adult sawflies lack the narrow constriction or petiole, often called the “wasp waist,” between the thorax and the abdomen. The abdomen joins the thorax broadly, giving the adult sawfly a stout, fly-like appearance.
Adult sawflies generally possess two pairs of membranous wings, with the hind wings being smaller than the forewings. Tiny hooks, known as hamuli, link the front and hind wings together during flight, allowing them to operate as a single aerodynamic unit. Most species measure between 1 and 2 centimeters in length. Although they may exhibit colors that mimic stinging wasps or bees for defense, adult sawflies are incapable of stinging humans.
The Unique Sawfly Life Cycle
Sawflies undergo complete metamorphosis, progressing through four distinct life stages: egg, larva, pupa, and adult. The female possesses a specialized, serrated ovipositor, which gives the insect its name. This saw-like structure is used to cut a slit into plant tissue, such as leaves or stems, where she deposits her eggs. This egg-laying mechanism can leave small, visible scars on the host plant.
The larval stage is frequently called a “false caterpillar” because of its strong resemblance to true caterpillars. Distinguishing between the two requires counting the fleshy, unjointed legs on the abdomen, known as prolegs. Sawfly larvae typically possess six or more pairs of abdominal prolegs, while true caterpillars have five or fewer pairs. Additionally, sawfly prolegs lack the tiny gripping hooks called crochets that are present on the prolegs of true caterpillars.
Once the larvae have completed their feeding period, which typically lasts four to six weeks, they move into a pre-pupal stage. They then seek a protected location, often in the soil, leaf litter, or sometimes within the plant tissue itself, to spin a cocoon and pupate. Many species overwinter in this stage, with the adult sawflies emerging in the spring or early summer to begin the cycle anew.
Dietary Role and Plant Interactions
Sawfly larvae are predominantly herbivores, feeding on the foliage of plants. The types of plants they consume are highly varied, including many common trees, shrubs, and ornamental plants. Specific groups of sawflies target host plants like pines, roses, willows, oaks, and fruit trees, sometimes causing noticeable damage in cultivated areas. Adult sawflies typically feed on nectar, pollen, or tree sap, or they consume very little food at all.
The feeding habits of the larvae can manifest in several ways on the host plants. Some species skeletonize leaves, consuming the soft tissue while leaving the tougher veins intact. Other types may completely defoliate branches or entire plants, especially since many sawfly larvae feed together in gregarious groups. A different feeding pattern is seen in “slug sawflies,” which chew the leaf tissue between the veins, creating a translucent patch known as “windowpane feeding.”
Heavy sawfly infestations can stress host plants, leading to reduced growth or, in severe cases, the death of susceptible trees, such as conifers that lose too many needles. Despite their reputation as pests in managed landscapes, sawflies are a natural part of the food web. Their larvae and adults serve as a food source for numerous predators, including various species of birds, which often feed them to their chicks during nesting season.