Wednesday, June 27, 2012

Wasp Wednesday: Elm Sawfly

Finally! I have been looking for an adult Elm Sawfly, Cimbex americana, for decades, ever since seeing pinned specimens in college back in the early 1980s. My quest ended this past Monday, June 25, when I happened upon a huge male specimen in the middle of a meadow in Emerald Valley, Colorado.

The Elm Sawfly is a large, robust insect about 20-25 millimeters in body length. They appear even bigger, especially the males with their beefy “thighs” (femora) on the middle and hind legs. The jaws of both genders are strong, and used to strip bark from twigs, sometimes girdling them in their efforts to reach the tasty sap. The clubbed, relatively short antennae are a characteristic of all members of the family Cimbicidae. The overall plump appearance often convinces people that these insects are bees rather than wasps.

The larvae of these wasps feed not on pollen, nectar, or other insects paralyzed and provided by their mothers. Instead, sawfly larvae feed on plant foliage. Despite the name “Elm Sawfly,” Cimbex americana feeds on other trees as well, especially willow, but occasionally on basswood (linden), birch, poplar, alder, and maple. At maturity, the caterpillar-like larva can be two inches (43 millimeters or so) long.

How do you tell a sawfly larva from the caterpillar of a butterfly or moth? True caterpillars have no more than five (5) pairs of prolegs, the “false legs” along the length of the abdomen that look like suction cups. Sawfly larvae have seven (7) pairs of prolegs.

When they are disturbed, Elm Sawfly larvae coil themselves and prepare to release volatile chemicals from glands in the thorax if necessary for their self-defense. They also rest in a coiled position. The rough, pebbly texture, and black midline dorsal stripe help to identify these greenish, yellowish, or whitish larvae. Look for them mostly between June and October, but note that I found the specimen pictured here on May 7 in South Carolina.

The mature larvae crawl to the ground and spin a tough, papery cocoon around themselves amid leaf litter or just below the surface of the soil. There they remain as larvae through the winter, pupating the following spring. There is only one generation per year.

The adults are sexually dimorphic in color. Shown here is a male. Females have the abdomen black with yellow horizontal bands interrupted in the middle. There is some variation in color and pattern from one geographic area to another. This is a very widespread insect, found in the U.S. and Canada from the Atlantic seaboard to the Rocky Mountains, and also the Pacific coast states north to Alaska.

Females of the Elm Sawfly use a saw-like ovipositor to insert eggs in the leaves of the host tree. They do not have a stinger. Both genders simply look intimidating. Males may defend territories. The one I found on Monday I startled from its perch, but after a wide orbit it flew right back to that one shrub, elevated only a little above surrounding vegetation.

The Elm Sawfly does have its enemies, including an egg parasite, Trichogramma minutum, a very tiny type of wasp. A large ichneumon wasp, Opheltes glaucopterus barberi, is an internal parasite of the sawfly larva. The sarcophagid fly Boettcheria cimbicis has been reared from the pupal cocoons of the Elm Sawfly.

See if you can find this species in your own region. Do you find it to be common? Do populations vary from year to year? While this species is rarely a pest (it has been known to defoliate shade elm trees, especially in the northern Midwest), it would pay to know more about it.

Sources: Drooz, Arnold (editor). 1985. Insects of Eastern Forests. Washington, DC: U.S. Department of Agriculture, Miscellaneous Publication No. 1426. 608 pp.
Essig, E. O. 1958. Insects and Mites of Western North America. New York: The Macmillan Company. 1050 pp.
Furniss, R. L. and V. M. Carolin. 1977. Western Forest Insects. Washington, DC: U.S. Department of Agriculture, Miscellaneous Publication No. 1339. 654 pp.
Stein, John D. 1974. Elm Sawfly. Forest Pest Leaflet 142. Washinton, DC: U.S. Forest Service, U.S. Department of Agriculture.

39 comments:

  1. Noticed some Elm Sawfly larvae feeding on Willow leaves this summer. Located in the state of Maryland. Will keep my eyes open for adults and will check for larvae again next summer. The size of the larvae got my attention.

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  2. Thanks for sharing your observations, Mike!

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  3. I saw an elm sawfly larva here in Vermont about 1 month ago; it was crossing a dirt road. My friend and I had a heck of a time id'ing it, as we were looking in a moth/butterfly guide. You can see a photo here: http://northernwoodlands.org/witwit/mellow-yellow-fellow. Thanks for the great info on this critter.

    Meghan

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  4. just found a bunch of these in Reliance, South Dakota. Eating up the America Elm.

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  5. Just found one in Albertville, MN. It was on the ground under a Basswood tree.

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  6. I saw 3 or 4 in a tree in my yard, but judging from the little poop balls all over my patio, there are a bunch of them.

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  7. I have them all over a couple Pecan trees. They first popped up around mid-April here in Central Texas. They don't seem to be doing much damage, but they typically feed and then crawl down the trunk to the soil and then disappear.

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  8. I just found one of these today in my yard , ( Box Elder SD) it took most of my day trying to figure out what this was.. I have never seen one before..

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    1. Congratulations! The only adult one of these I've ever seen is the one pictured above.

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  9. I found one here in Newfoundland, Canada this weekend!!! I have NEVER seen anything like it here before and hope not to see another!

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    1. Ha! They can certainly be intimidating, but maybe I can take your share of future appearances of these wasps.

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    2. I just found one today on my porch in central and. Never seen one before

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  10. I think I may have seen one in my yard. I was scared because I thought it was a huge wasp! I wish I could post pictures because I took a bunch and also a video.

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  11. I found one in its larvae state on my deck today...I would like to know if they are harmful because my kids would like to keep it and watch it change..Thank you

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    1. They are not dangerous in the least. The larva should spin a cocoon shortly if it is large in size and off of its host plant. Simply put the insect into a ventilated container and place it where it will be exposed to a normal cycle of day and night, but out of direct sunlight. Good luck, let me know what happens!

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  12. Larva specimen found on Manitoulin Island in Ontario, Canada. August 23.

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  13. Larva found today 9,000 ft altitude, 2 hrs west of Ft. Collins, Colorado.

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  14. Just found one on my garage trim... Bayside, NS
    Close to Peggy's Cove
    Haven't seen one before.

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  15. My 8yo daughter found one and he has cacooned himself already. She thinks that he needs to have a "winter." How do we take care of this thing? We are in Iowa. We were going to let him go, but then he made the cacoon so now we are in it for the long haul.... Any help would be appreciated! Thanks!

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    1. Hi, Britt: No need to refrigerate, but do put the cocoon somewhere that is exposed to *natural* changes in day and night. It is apparently changes in day length that trigger emergence from the pupa stage. Maybe you have a shelf near a window, in the garage? Natural temperature changes might help, too, or some close approximation.

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  16. I found one of these in my garden a few weeks ago, I`m in the U.K ... Trying to find out what it was, led me to this blog :)

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  17. Saw a female Elm Sawfly today, posted it on iNaturalist. Thanks for the information in your blog!

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    1. Thank you for sharing that!....and you're welcome. :-)

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  18. Single last instar larva of the Elm Sawfly seen yesterday grazing on hazelnut in Athens, GA.

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  19. I saw one today in Romania and I want to raise it !

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    1. Excellent! Your species there in Europe is actually different. It is the "Birch Sawfly," Cimbex femoratus. Good luck rearing it!

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  20. Hello and thank you for this wonderful, informative site. We're in Gainesville, TX and our neighbor's son brought an Elm Sawfly larva to us yesterday asking what it was. We had just found a Polyphemus Moth caterpillar in our own yard a couple days before so had wondered if this white "caterpillar" was going to become a butterfly or moth (although it did also look a lot like a grub worm). Neat to find out it will grow up to be a wasp. It was found under a large Elm (American Elm?) tree. Our Polyphemus Moth caterpillar was found crawling down our Lacebark Elm (what a surprise that was!).

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    1. Thank you for the compliments; and thank you for sharing your personal discoveries!

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  21. Hey I found that as larvae here in Henrico, va. Just today! How cool is that?

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  22. I had a convex Americana on my patio table yesterday. It has greenish stripes on it's back. Holy cow, huge wasp in a fly's body. In 41 years living in Wisconsin, I have never seen this. My husband has lived here 65 years and never seen one of these.

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  23. Saw everyone's post I live in Iowa and have a Chinese elm by my house so I went out and counted the larva that I could see 54.

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  24. My daughter found one in the larval state today -we live in Yellowknife, Canada. I have never seen anything like it before - what a face!! We both thought it was a caterpillar but the internet helped us identify it as an Elm Sawfly larva.

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  25. Im in Wisconsin and I have 3 ten year old willows in my backyard and there are larvae literally everywhere. Probably about half the leaves on the trees are gone. How do I get rid of them?

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    1. Your willows will easily survive if you do nothing. Also, I never give pest control advice here. The whole point of this blog is to encourage better attitudes, like fascination and appreciations, towards arthropods. Thank you for understanding that.

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  26. The Elm Sawfly larva we found in Yellowknife, Canada has turned into a cocoon (it's being kept in a ventilated container). Amazingly, my normally bug leery daughter who picked up bug from a sidewalk wants to see the what it looks like in the Spring- when it's completed its metamorphosis into the winged state.

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    1. That is wonderful to hear! I live for stories and comments like this, so thank you for sharing.

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