If you see a sawfly, you might think it’s a wasp. Sawflies don’t sting but their appearance in your yard can create problems. Once they lay their eggs, the larvae, which resemble caterpillars, devour large amounts of leaves in a short time, even to the point of total defoliation.
There are many different species of sawflies. What helps with identification is that sawflies are host-specific and feed on one type of plant only. The gooseberry sawfly, for instance, only feeds on members of the Ribes genus, such as currants and gooseberries.
What Sawflies Are
Sawflies are a group of insects named after the saw-like ovipositor, the tube-like organ with which the female sawflies cut holes in plant tissue to deposit their eggs.
The taxonomy of sawflies is complex. The 7,000 sawflies species belong to seven different families, including the true sawflies, the superfamily Tenthredinoidea. The life cycle of sawflies consists of up to six life stages, including egg, larva, pupa, and adult. Some species have one generation per year, others more than one. What is common to all sawfly species is that it is the larvae that cause the damage on landscape plants, and they are usually feeding in groups.
The likelihood of encountering adult sawflies in your yard is relatively small. The short-lived sawfly adults are not buzzing around like other insects; they only make short flights in sunny weather to feed on pollen and nectar, or to lay their eggs.
What Damage Sawflies Can Do to Plants
The damage from sawflies depends on two factors: the number of larvae feeding on the plant, and the size and age of the host. Healthy mature trees and shrubs can survive even severe infestation, but total defoliation can kill young or already stressed plants.
Identification and Common Sawfly Species
Every sawfly species looks different but most share two common traits. Adults can be distinguished from wasps by their broad waist. And while the larvae appear to look like caterpillars, upon a closer look, you’ll see that sawflies have six or more pairs of short and flesh prolegs on every segment of their abdomen. Caterpillars on the other hand only have prolegs in the middle and at the end of their body, and never more than five abdominal prolegs.
Below are a few common sawfly species and the characteristic traits of their larvae. Unless otherwise noted, the larvae feed on the plants they are named after.
Rose sawfly, also called rose slug sawfly: Larvae with a yellow-green body and orange head feed on wild and cultivated roses. They prefer to eat the soft parts of the leaves and bypassing the veins, which gives the leaves a skeletonized appearance.
Hibiscus sawfly: The larvae are green with black heads. They feed on members of the mallow family, starting on the undersides of leaves, and moving to the upper sides but also omitting the veins.
Pine sawfly: The gray-green larvae have an off-white stripe in the center of the back and slightly lighter stripes on either side and a black head. They feed on pine needles from last year’s growth and only eat the surface of the needle. The resulting discoloration of the needles makes them look like straw.
Dogwood sawfly: The larvae are challenging to identify because they change their color and appearance during their development. During the second larval stage, the larvae have a white waxy covering, and in the last development stage, they are yellow and black.
Pear sawfly, also called pear slug: They feed not only on pears but also on other fruit trees such as cherry, as well as ornamentals such as cotoneaster, mountain-ash, and serviceberry. The larvae are dark green to orange and tadpole-shaped. Their unique characteristic is that they cover themselves in their liquid waste, which makes them appear shiny.
Columbine sawfly: The larvae are green with dark heads. After they feed on the perennial, there is not much left from the plant than the stems and flowers.
Mountain ash sawfly: Both European and American mountain ash can be devoured by the larvae, which are greenish with black dots on the sides. The head and the legs are black at first, changing to yellow-orange as they mature.
Scarlet oak sawfly: The larvae are semi-transparent greenish-yellow, flattened towards the front and tapered towards the back. They cover themselves in a slimy substance that makes them stick to leaves better and wards off predators.
White pine sawfly: While the larvae prefer the Eastern white pine, they also feed on red pine, both on this year’s and last year’s needles. They usually defoliate a branch before moving on to the next. The larvae are pale yellow with black heads with four rows of black spots from their head to their tail end.
How to Prevent and Control Sawfly
Sawfly larvae populations are food for predators such as birds, lizards, frogs, ants, predatory wasps, and other insects. Sawflies become a problem when that natural control mechanism is absent.
It is possible that you only notice the damage on your plants after the larvae are done feeding. In that case, there is not much you can do other than to monitor the host plants next year. Check the undersides of leaves or needles for the larvae, and signs of feeding.
If there are just a few, you might be able to remove the larvae manually by knocking them off the plant into a bucket with soapy water.
As a last resort, if the infestation is severe and cannot be managed mechanically because the host plant is too big, you can use chemical controls—insecticidal soaps, horticultural oil, neem oil, or pyrethrin. Whichever method you choose, make sure to apply insecticides only when the larvae are still on the plant. Applying an insecticide after they have left the host plant does not have any effect.
Note that because the larvae are not caterpillars, BT (Bacillus thurningiensis) does not work against sawfly larvae.