Extension News

I Think I Saw a Fly

Note to media editors: Yard and Garden Column for use during the week beginning Nov. 25.


By Donald Lewis
Extension entomologist
Iowa State University

An interesting thing about insects is that out of the tens of thousands of species in Iowa, only a very few are well-known by the general public. Most of these are common and abundant and become familiar by their repeated appearance in our lives. House flies, multicolored Asian lady beetles, monarch butterflies and lightningbugs come to mind. The public also recognizes a few insects that are not common, but they are large and/or interesting enough to have attracted our attention in spite of their limited numbers. The praying mantis and the cecropia moth are examples.

That leaves the vast and unknown majority, some of which are very common and frequently abundant but still no one seems to know their names. I guess life can be unfair that way.

One very large, diverse and relatively important insect group that goes unknown to most people is the sawflies. That is spelled correctly.  ‘Sawfly’ is one word for the same reason that ‘butterfly’ is one word. They are not flies, even though the word ’fly’ appears in their name. ‘House fly,’ on the other hand is two words because that insect actually is a fly. ‘Horse fly’ is also two words.  ‘Fruit fly’ is two words, ‘dragonfly’ is one. If these rules of insect names aren’t making sense to you, just let it go as an entomologist-thing.

Sawflies are wasps. They don’t look like wasps (in the minds of most people). They look like fat-bodied flies without the pinched waist that is characteristic of the better-known wasps. Sawflies have four wings, while all of the true flies have only two. Sawfly wasps cannot sting.

Sawflies got their name from their ovipositor -- the egg-laying apparatus at the end of the female’s abdomen. When the female is ready to lay eggs she uses the ovipositor to saw a slit in a leaf, needle or stem. Eggs are then deposited into the slit.

All ants, bees, wasps and sawflies have a complete life cycle of four stages, egg, larva, pupa and adult. The larva is a worm-like immature stage that eats and grows until it forms a pupa and transforms to the adult stage (the way a caterpillar changes into a butterfly).

For those wasps that most people recognize as wasps, the larvae are carnivores. The paper wasps, hornets, yellowjackets and solitary wasps such as the very large cicada killer all eat some type of meat while they are larvae. In the case of social wasps (hornets, yellowjackets and paper wasps) the food is dead insects brought back to the colony by the workers and fed to the larvae. For solitary wasps the larvae eat insects or spiders provisioned in the nest by the adult female.

Sawfly wasp larvae are plant eaters.  Most resemble caterpillars in general appearance and also in damage. Many sawflies are plant pests that cause noticeable-to-destructive loss of plant foliage. Sawflies are host-plant specific; that is each different species of sawfly feeds on a specific host plant and does not move from one plant type to another. For example, the European pine sawfly larvae are gray-green larvae with shiny black heads that live in clusters and eat pine tree needles in May; they will not feed on other plants. Similarly the dogwood sawfly larvae that eat entire leaves from gray and red osier dogwood plants in late summer will be found only on dogwood shrubs.

Not all sawflies resemble caterpillars. There is a small group of sawfly larvae that resemble slugs. The rose sawfly larvae are velvety, yellow-green in color and up to 1/2 inch long. They have a tapered body and slimy appearance that gives them a vague resemblance to a true slug. Unlike slugs, the sawfly larvae have legs and a small bead-like head at the front end of the body. Neither characteristic is present in true slugs (mollusks).  Other slug-like sawflies are common on oak trees (especially red oaks and pin oaks).

With insects, as in life in general, things are not always what they seem. Things that walk like a duck and quack like a duck usually are a duck.  Insects that look like flies may or may not be!


Contacts :

Donald Lewis, Entomology, (515) 294-1101, drlewis@iastate.edu

Jean McGuire, Continuing Education and Communication Services, (515) 294-7033, jmcguire@iastate.edu

No photos are available for this week's column.