Wasp Wednesday: Purslane Sawflies

Not all wasps sting, and not all feed on other insects in their youth. The larvae of sawflies are vegetarians, and many are mistaken for caterpillars. You need not venture far to find sawflies, either. I found the adult female specimen below in my own backyard on the evening of September 18. Through a little detective work I discovered she is a Purslane Sawfly, Schizocerella pilicornis, in the family Argidae.

This species is unique among all argids (worldwide) in that its larvae feed internally on the host plant, mining inside the leaves of purslane. Until recently, it was thought that some larvae of S. pilicornis feed externally. Analysis of mitochondrial DNA has demonstrated that those leaf-nibbling larvae belong to a separate species, S. lineata (Hartsough, et al., 2007).

Adult females can be distinguished by the amount of black markings on the top of the otherwise red (or orange) thorax. Females of S. pilicornis have a solid, or nearly solid, black stripe down the center of the thorax, constricted in the middle. Females of S. lineate have very reduced black markings along the midline of the thorax. S. lineate is also slightly larger, females averaging about 7 millimeters to the 5-6 millimeter length of female S. pilicornis.

Both species are widespread in the New World, ranging from southern Canada to at least Central America. S. pilicornis occurs at least as far south as Argentina. U.S. records for S. pilicornis include Arizona, California, Colorado, Connecticut, Washington, DC, Florida, Indiana, Iowa, Kansas, Maine, Maryland, Michigan, Minnesota, Mississippi, Nebraska, New Hampshire, New Jersey, Ohio, Oregon, Texas, Virginia, and Washington. The species may actually be adventives here, and native to South America. At least, it was not described from North America until 1900.

Life history information may be open to debate since it comes prior to the realization that two species are involved (Gorske, et al., 1977). Females lay one egg per leaf, inserting the ovum into the foliage with the saw-like ovipositor that gives sawflies their common name. Larvae feed for roughly six days, going through five instars (an instar is the interval between molts). Mature larvae tunnel into the soil to a depth of about 3.5 centimeters where they pupate. Adults emerge a week later (during the summer generations; otherwise they overwinter as pupae). Their lifespan as adults is apparently very short, only 24 hours according to one source (Gorske & Hopen, 1976).

There can be up to six or seven generations annually, at least in Illinois, where these insects exert a small degree of biological control against purslane.

Males in this genus are easily identified by the last antennal segment, which is forked. It looks like the insect has four antennae instead of the two it should have.

I have found numerous males, and this one female, “sleeping” out in the open on grasses and other vegetation at sunset. Many wasps (and solitary bees) can be found by looking in fields and meadows for sleeping individuals at dusk, in the early morning, or during inclement weather.

Sources: Gorske, S.F. and H.J. Hopen. 1976. “Purslane sawfly (Schizocerella pilicornis) as a biological control agent of Common Purslane (Portulaca oleracea L.),” a paper presented at the 73rd annual meeting of the American Society for Horticultural Science, August, 1976.
Gorske, S.F., H.J. Hopen, and R. Randell. 1977. “Bionomics of the Purslane Sawfly Schizocerella pilicornis,” Ann. Entomol. Soc. Am. 70(1): 104-106.
Hartsough, Chester D.B., Edward F. Connor, David R. Smith, and Greg S. Spicer. 2007. “Systematics of Two Feeding Morphs of Schizocerella pilicornis (Hymenoptera: Argidae) and Recognition of Two Species,” Ann. Entomol. Soc. Am. 100(3): 375-380.

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.

Wasp Wednesday: Bristly Rose Slug

Wasps are sometimes named after their larvae. This is certainly true of many of the common sawflies in the family Tenthredinidae. Case in point: the “Bristly Rose Slug,” Cladius difformis. True to its epithet, the larvae of this insect are covered in tiny setae (hairs) and feed voraciously on roses. I was lucky enough to document both the adult and immature stages in a friend’s garden in Redondo Beach, California last month.

I spotted the adult female wasp on the morning of May 16 as she basked in the sun and groomed herself on a leaf adjacent to a rose bush. This is not a large insect, only about 6 mm in body length.

A male then suddenly appeared, and just as suddenly they were copulating. Note the sexual dimorphism, especially in the antennae of the male. This branching type of antenna is termed “pectinate” and is not uncommon in some wasps and beetles. The couple soon became unhitched and the male went his merry way.

The female wasp, had I been able to follow her long enough, would have laid her eggs in a rose leaf. She uses her saw-like ovipositor (hence the common name of the family) to cut into the upper surface of the midrib vein and create a pocket where she will deposit a single egg. The process is repeated several times on the same leaf. The ovipositor is a complex arrangement of a “lance” and at least one pair of “lancets,” all normally concealed inside a blade-like sheath when not being deployed. The lancets act in concert to cut through the leaf tissue, braced by the lance.

The tiny larvae that emerge from the egg begin to skeletonize small portions of the leaf. The larvae go through five instars (periods between molts), and their appetites seem to increase with each molt. Eventually they are creating enormous holes in the rose leaves.

I found several larger larvae (12-13 mm, nearly mature) on May 23. Their resemblance to caterpillars is obvious, but they have more pairs of prolegs (“false legs” that run the length of the abdomen) than do larvae of moths and butterflies (which have five or fewer pairs of prolegs). It is easy to see why this insect is considered a garden pest, given the heavy damage it can inflict. It is also known to feed on raspberries and strawberries. The larvae eventually pupate in a papery cocoon.

It is assumed that the Bristly Rose Slug was introduced to North America from northern Europe. Many authorities in fact synonymize Cladius difformis with Cladius pectinicornis of the Old World. David R. Smith, a renowned expert on sawflies here in North America, asserts that the two are distinctly separate species. The Bristly Rose Slug occurs on both coasts of North America, and from Nova Scotia to British Columbia. It seems to range only through the northern third of the United States except for the coastlines.

There are not many options available for control of this species, so handpicking the larvae off of leaves may be your best bet. The biological control “Bt” (Bacillus thuringiensis) is not effective.

There is more information, and more images, available online, of course, including the complete life cycle in images here.

Wasp Wednesday: Tenthredo

Sawflies are about as un-waspish as wasps can get. They don’t have a narrow “wasp waist,” the abdomen being attached broadly to the thorax. The females don’t sting. Their larvae are vegetarians instead of carnivores. They are rather soft-bodied and delicate. They are, however, very abundant at certain times of the year.

The specimen depicted above was identified as a male in the genus Tenthredo, family Tenthredinidae, by Dr. David R. Smith, a world authority on the broader group to which sawflies, horntails, and related primitive wasps belong: the Symphyta. The image was taken near the summit of Mt. Lemmon in the Santa Catalina Mountains of southern Arizona on an aspen leaf, June 12, 2010.

The genus Tenthredo includes approximately 118 species in the U.S. and Canada. The adult wasps are often seen on flowers, but they will eat smaller insects as well as pollen and nectar. Many species mimic stinging wasps like yellowjackets and spider wasps, but the females have no stinger. They retain an egg-laying organ called an ovipositor, modified into a saw-like blade used to insert eggs into plant tissues.

Larvae of sawflies in general are frequently mistaken for caterpillars, and indeed most behave in that manner: they feed on foliage just like a lepidopteran larva. How do you tell them apart then? Caterpillars have only five pairs of prolegs (those fleshy knobs down the length of their bodies that they use like suction cups to grip vegetation) or fewer, while sawfly larvae have seven pairs of prolegs. The larva shown below is typical of sawfly larvae, though it may not be a member of the genus Tenthredo. The image was taken September 6, 2009 at Knightville Dam in western Massachusetts.

Adult sawflies have fairly complex wing venation that gives the wings a reticulated appearance. The common sawflies in the Tenthredinidae family have triangular faces, with slightly bulging eyes, as seen in the image below, also taken at Knightville Dam.

Look for sawflies mostly in the spring, especially at the edges of deciduous forests. Those in the genus Tenthredo actually tend to appear later, in the late summer and early autumn. Watch for larvae in groups feeding on leaves. Watch this space for more on other kinds of sawflies as I encounter them afield.