The little green worm-like creatures looked like caterpillars, crawled like caterpillars and ate voraciously like caterpillars. They were defoliating my elderberry plant just like caterpillars would. Yet they were not caterpillars.  

So, when is a caterpillar not a caterpillar?  When it is a sawfly larva.

Caterpillars are the name given to the larvae of butterflies and moths, insects in the order Lepidoptera. Sawflies are classified in the order Hymenoptera so their larvae cannot be accurately called caterpillars.

There are other crucial distinctions between the two similar-looking larval insects that makes differentiating between them a proverbial snap if you are willing to get up close and personal with these crawlers.

Both insects have their eyes on you, though one more so than other. Sawfly larvae have a single pair of eyes, one on each side of the head, while caterpillars have up to six ocelli (simple eyes), also called stemmata, on each side of the head, for a total of twelve eyes.  

Next, look to the legs. Most caterpillars have four pairs of abdominal prolegs, while sawfly larvae are leggier, with six or more pairs on their bodies. Tiny hooks called crochets appear on the ends of caterpillar legs, while sawfly larvae lack these clasps.

Another important difference is how they relate to the chemical treatments that humans use to dispose of them. Caterpillars can be killed using certain pesticides such as Bt (Bacillus thuringiensis), while sawfly larvae are immune to many of the oft-used chemical caterpillar deterrents. One helpful website gives this good advice: “Before you apply any pesticide for a caterpillar problem, be sure to count the prolegs and identify your pest correctly.” 

If you aren’t willing to share your plants with sawflies (which can completely defoliate your leafy lovelies), suggestions for eliminating them include squishing, boiling, or removing the larvae manually. Neem oil might also be a solution.

Hymenoptera is the order that bees and wasps belong to, in addition to sawflies, but the adult sawfly doesn’t resemble those relatives, either. Most bees and wasps are known to sting and have thin corseted waists. Adult sawflies are thick in the middle, lacking that waif-like figure for which the others are known, and are stingless.

Sawflies are named for the saw-like appearance of the adult female’s ovipositor, the body part used to deposit eggs. With this sharp appendage, sawflies have no problem cutting into the leaves and stems of plants to place their rafts or pods of up to 90 eggs to develop into those caterpillar-look-alike larvae. 

Since sawflies practice parthenogenesis, the art of reproducing without mating, males tend to be few and far between. Adults only live for a few days to a few weeks, with reproduction their only purpose. Nor are the females very active, since they don’t need to search far and wide for a mate. They rest happily on their plants, which can provide the pollen and nectar needed for food and the substrate for their spawn.

By the time I had learned enough to decide the fate of these mighty munchers, the sawflies had already consumed the leaves of my elderberry bushes. These insects are highly successful, and have been around in some form for over 250 million years. Clearly, my poor plants face an uphill battle surviving a mere few more years from the saw of such a persistent (and easily misidentified) foe.

Suzan Bellincampi is director of the Felix Neck Wildlife Sanctuary in Edgartown, and author of Martha’s Vineyard: A Field Guide to Island Nature and The Nature of Martha’s Vineyard.