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Sawfly larvae look like caterpillars and feed on leaves like caterpillars, but sawflies are wasps. We have several common sawflies in Iowa that are bothersome to gardeners; the pearslug whose larvae feed on the leaves of a variety of fruit trees, the roseslug on roses, the scarlet oak sawfly that feeds on oak, the willow sawfly, and the European pine sawfly.
Sawfly larvae have chewing mouthparts that leave holes in the leaves. Some species of sawfly leave a very characteristic feeding damage called windowpaning. Basically they eat the green part of the leaf leaving a fine layer of clear leaf tissue. Older damage browns and can sometimes look like a disease lesion.
The dogwood sawfly, Macremphytus sp., is an interesting sawfly because the second larval instar (stage in the larval development between molts) is covered in a white waxy covering and the last larval instar is yellow and black. The female sawfly inserts her eggs into the leaf in rows along the leaf veins. The eggs hatch into the caterpillar-like larvae that feed on the leaves.
The dogwood sawfly and sawflies in general can cause considerable defoliation because often they feed in groups. If you catch the infestation when the caterpillars are small they can easily be removed by hand. Insecticides labeled for sawfly control on the plant in question will also work if treatment is applied when larva are young. Unfortunately we often notice the damage after the larvae are done feeding, so it is important to look for the larva before treating.
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Dogwood sawfly eggs lined up along veins.
Closer view of the eggs inserted into the leaf tissue.
Small dogwood sawfly larvae hatched from nearby eggs.
Dogwood sawfly larvae in different larval instars.