Aquatic Insects as Indicators of Water Quality

Photo above: Cybister fimbriolatus: Dysticidae (predaceous diving beetle) would have a tolerance score of 5.

Author: Dr. Hope Q. Liu (Biology)

Aquatic insects, also called benthic macroinvertebrates, are ideal bioindicators of water quality. What the heck is a benthic macroinvertebrate? Benthic means “bottom of a body of water” and macroinvertebrate means you can see the insect with your eye and insect has no backbone. Benthic macroinvertebrates are used as bioindicators of water quality because they are sensitive to environmental changes and its presence or lack thereof determines clean water or polluted water. 

How can an aquatic insect like a dragonfly, which lives in the air, help us determine the quality of the water? Aquatic insect adults lay their eggs in the water. The eggs hatch and the immature form lives in the water, sometimes for years, before transforming into winged adults. The composition of the aquatic insects population (aka bioindicators) is used to ascertain water quality and reveal pollution impact. Much like plants are assigned conservation numbers, aquatic insects have a numeric designation, too. This designation is called a Tolerance score and ranges from 0-10 with zero being the least tolerant to pollution.

Peltodytes edentulus: Haliplidae, benthic macroinvertebrate, aquatic insects

Photo above: Peltodytes edetulus: Haliplidae (crawling water beetle) would have a tolerance score of 7.

Photo to the right: This Hydropsychidae (common net spinning caddisfly) nymph would have a tolerance score of 4.

Hydropsychidae , caddisfly

Aquatic insects are a great starting point to get a sense of the water quality.  To assess a body of water using water sampling would require repeated testing visits to the site. Aquatic insects are not highly mobile and reside in the body of water for long periods of time. This means monitoring and testing the water isn’t needed as often.

For example, you are monitoring the water quality of Stream A. You sample the water for aquatic insects in June 2015, June 2016 and June 2017 and find diverse insect populations – stoneflies, caddisflies, beetles, dragonflies – and then you sample 

again in June 2018 and only find beetles and dragonflies. Generally, stoneflies and caddisflies are less tolerant to pollution when compared to beetles and dragonflies, so you deduce that somehow the water was polluted over the past year and wiped out those populations. You determine, based on talking to people along the stream, that the paper mill accidentally polluted the water in November 2017.

Now, let’s imagine you are sampling Stream A using water samples and laboratory tests. You sample in January, March, June and September of 2015-2018. Based on the tests, you determine the water is clean. Because you didn’t sample in November or December of 2017, there were no indicators showing the stream was polluted.

Now that you know WHY aquatic insects are great indicators of water quality, you may be wondering what they look like and how you can identify them. Purdue Extension publishes a Bioindicators of Water Quality Guide that provides detailed instructions for using bioindicators to determine the water quality. Using this excellent guide, the insects and their conservation values (known as a Tolerance Value) can be identified and calculated to estimate the water quality. Remember, there are other factors that impact the ability of aquatic insects to live in water, such as temperature, sediment, etc. Generally, where possible, it’s always best to collect other water data such as pH, temperature, and dissolved oxygen.

Additionally, the University of Wisconsin Extension offers citizen science training for monitoring water quality using benthic macroinvertebrates. For more information on that, check out the Water Action Volunteers (WAV) site. These classes will teach you how to sample bodies of water using a combination of tests, including aquatic insects.

Entomology Today has an article about insects and mites and what they tell us about water quality

Gypsy Moth Treatment

If you received a notice about gypsy moth spraying in your county, you can check to see the target sites with the Department of Agriculture (DATCP) interactive map.

What is the killing agent?

Bacillus thuringiensis serotype kurstaki (Btk) is a group of bacteria which makes it a biological control agent. This bio-control agent is different from those where a non-native insect is brought in to kill off a non-native plant. All bio-control measures need a healthy dose of skepticism applied to them — two non-natives don’t make a native. Since Btk is commonly found in our soils, it does not introduce a foreign entity into our ecosystems.

How does Btk work?

I went in search of how Btk does the “dirty deed.” Btk is not a contact insecticide; the insect must ingest it. It is a stomach poison and will only effect the larval feeding stage (i.e., when it is a caterpillar). Andrea Diss-Torrance, Invasive Forest Insects Program Coordinator for the Wisconsin DNR, tells me that “among moths and butterflies, the effect can vary: about a third of species tested are sensitive, about a third are not [a]ffected at all, and about a third have an intermediate level of sensitivity. Btk is degraded by sunlight and very sensitive caterpillars, such as the Eastern tent caterpillar, are no longer [a]ffected about 11 days after application to foliage”(Andrea Diss-Torrance, personal communication, May 3, 2018).

“When Btk is ingested by a susceptible caterpillar, the highly alkaline environment of the caterpillar’s gut triggers the Btk bacterium to release a crystalline protein called an “endotoxin” that poisons the insect’s digestive system. The endotoxin acts by killing cells and dissolving holes in the lining of the insect’s gut. When a mixture of food, Btk spores, and digestive juices leaks through these holes into the insect’s blood, it causes a general infection that kills the caterpillar. Humans and other mammals have highly acidic environments in their stomachs that destroy Btk before it causes infection” (Ellis 2018).

Two types of Btk mixtures

There are two commercial brands of bio-control mixtures being used against the Gypsy Moth: Foray48® and Gypchek®. The DNR determines which to use based on insects listed in the Natural Heritage Inventory. I have repeatedly expressed my concern with this methodology. Current lists for Lafayette County will be insufficient to ascertain if classified species exist because our county is incredibly undersurveyed for insects (and plants for that matter). I suspect few counties have insect surveys covering the county.

Christopher Foelker, Gypsy Moth Unit Supervisor for DATCP, tells me Gypchek® is used in habitats having known T&E species that are in a vulnerable life stage during the treatment times. (Christopher Foelker, personal communication, May 3, 2018). DATCP considers Gypchek® to be less effective than Btk because it deteriorates quickly and has a much shorter window of efficacy. It is a viral insecticide that is specific to the gypsy moth but it iscostly to produce and there are limited amounts. It is manufactured by raising and infecting gypsy moth caterpillars with a virus (NPV-gypsy moth). These infected caterpillars are ground up and suspended in a liquid solution. This solution is Gypchek® and it is applied to the tree canopy.

Since it is a limited resource, state and federal governments agree to use Gypchek® only where rare species are known and not on every area proposed for Btk treatment. Unless a T&E insect is known, Foray48® is used.

Foelker says all the DATCP treatment plans are reviewed by the US Fish and Wildlife and US Forest Service for any potential effects on T&E species. They present any concerns for areas these species might be impacted.

Who else is affected?

Since I seldom take info from just one source, I continued my sleuthing on this topic. Jay Watson, who works in the Bureau of Natural Heritage Conservation,confirmed my suspicions, “Really, the impacts from Btk on other insects is very poorly understood.  I don’t know of any research that has looked at what impact this might have on insects like bumble bees.” (Jay Watson, personal communication, May 3, 2018). He specifically mentioned bumble bees because of our recently discovered rusty-patched bumble bee on our property.

There are two sides to every issue; this one is no different. Diss-Torrance stated, “the effect, or in this case non-effect, of Btk on a wide range of other creatures is very well known as this bacterially based insecticide has been used extensively in agriculture and forestry since the ‘80’s.”

In general, sunlight and other microbes destroy Btk applied to foliage within three to five days, so Btk does not multiply or accumulate in the environment (Ellis 2018). Yet, in a 1998 study, Btk was added to different types of soil in order to determine how the type of soil affected the persistence and concentration of Btk. The results of the study showed insecticidal activity started to decline after a month in one soil, while in another, toxicity was high after six months. The authors of the study noted that even though Btk is considered non-toxic to non-target species, the accumulation and persistence of the Btk toxins could eventually lead to environmental hazards or the selection of Btk-resistant lepidopterans (Wikipedia 2018).

The EPA has studies demonstrating a small level of toxicity to certain fish, a slight toxicity to honey bees at high level doses, and “practically non toxic” at low level doses. It is slightly toxic to the convergent lady beetle (Hippodamia convergens) (EPA 1998). Caterpillars that become ill or die after ingesting Btk are not considered dangerous to birds or other animals that feed on them (Ellis 2018).

I wasn’t thinking there would be a moral to this story when I began researching it, but I believe that there is. The value of citizen science is priceless and saves lives. Wisconsin DNR’s decision-making about gypsy moth treatments relies solely on the information at the Natural Heritage Inventory. The information behind many decisions that mitigate impacts to our natural community and our T&E community originates from citizen scientists.


Ellis, Jodie A. (04 May 2018). Exotic Insects Education Coordinator Department of Entomology, Purdue University. Retrieved from

Entomological Society of America. (May 2018). Is Bt safe for humans to eat?

Environmental Protection Agency. (1998). EPA 6452 Fact Sheet. Retrieved from

Tobin, Patrick C. and Andrew M. Leibhold. “Gypsy Moth” in Encyclopedia of Biological Invasions. (Los Angeles: University of California, 2011): 298-304.

Wikipedia. (04 May 2018). Retrieved from

By Marci Hess and MJ Hatfield

BugGuide. net is a website worth exploring on the fly! Many of us are interested in the biota living in the ecosystems we work hard to save and protect; learning about them enhances our work and enriches our lives. While I did not set out to work with insects, I suppose it’s serendipitous that during the course of my ecosystem restoration work, insects began to intrigue me. My mom tells me I’ve always liked them –eating them instead of dirt! Once I had cataloged the mammals, birds, and herptiles on our land the next logical step was insects. This curiosity turned into a passion.

I began my insect journey using photography. I’m not unique in this aspect. Digital cameras have revolutionized insect study. We can take great photos with sufficient detail to ascertain an identification, which has taken insect knowledge and interest to a new level.

Equally as revolutionizing is  Troy Bartlett, a computer programmer who loved insect photography created this site. Bartlett calls it “an online community of naturalists who 

enjoy learning about and sharing observations of insects, spiders, and other related creatures.” Its success soon outgrew its first home; it is now housed by Iowa State University. is the pinnacle of citizen science. Referenced and contributed to by anyone, from citizen scientists to academics to professionals, it’s more than just photos. You can find physical details about the insects, their habitats, food sources, identifying characteristics, ranges and references.

For such a colossal resource, it is surprisingly easy to use. You can click on one of the insect silhouettes along the left hand column, and it will take you directly to that order. You also can search for a particular family, genus, or species.


Learn about insect behavior

This site provides photos of unique and unusual behaviors, as well as species not found on other sites. Making a contribution to the betterment of our world motivates me, but I never thought it would be in the world of insects. Yet, armed with only a camera, my photo made history on when a guide page was created for this not-so-common fly (Cryptomeigenia illinoiensis: Tachinidae) found on my Lafayette County property!


Cryptomeigenia illinoiensis, Tachinidae

Because of, Harlan Ratcliff learned of a fascinating mating ritual. The females of the long-tailed dance fly (Rhamphomyia longicauda) inflate their abdomens to trick the males into thinking they are filled with eggs. Why would they want to trick the males? Mating behavior dictates the male offer the female a food item in exchange for mating. Check out the page and learn about their hairy legs.

Right photo: Female Rhamphomyia longicauda after inflating – notice the color change of the abdomen. Both photos by Harlan Ratcliff


Rhamphomyia longicauda, Empididae
Rhamphomyia longicauda, Empididae

Identify Tracks and Sign of Insects

What if you find an interesting marking on a leaf or a gorgeous orange structure? Or better yet, both? has a section (called Tracks and Sign) for these findings. You can post a photo or look up anything from eggs to larvae to leaf mines to droppings. I found this orange object on the backside of this oddly patterned leaf. I posted them on BugGuide and would you believe these are from a fly! It’s a columbine leafminer (Phytomyza aquilegivora). Before BugGuide, I would have marveled at this but I would not have thought I’d find an answer. 

Phytomyza aquilegivora, Agromyzidae
Phytomyza aquilegivora, Agromyzidae

If you like a good mystery or better yet, solving one, BugGuide can help! Ironweed (Vernonia sp.) is a commonly found plant in our area. In December, MJ Hatfield, found spent fly puparia, which encapsulates the pupa, in the seedhead. Based on the host plant and location, the fly was tentatively identified as Neaspilota alba.  Until an adult fly is found, the ID will remain a best guess. If you know of a patch of ironweed, check it out this year, catch a fly and post a photo to and help solve this mystery. 

Neaspilota alba, Tephritidae
Neaspilota alba, Tephritidae

Notice how the seeds form around the puparia.
Photo by MJ Hatfield

Note the spent puparium. Wow! One must be looking closely to find these little nuggets of nature! Photo by MJ Hatfield

There are incredible numbers and incredible diversity in the insect world, and relatively few are identified, described, and named, much less studied for behavior. For years, identifying insects required using dichotomous keys.  Finding those keys was not easy and once found, how up to date were they? Then after struggling through the keys, coming up with an identification, who could confirm it?

Citizen scientists make valuable and significant contributions to science. This is especially so in the insect world. Whether you are a user or a contributor or an editor, BugGuide is an important addition to understanding our ecosystems.

What users say about the site: provides a tremendous service to us entomologists as well as to other biologists, naturalists and interested amateurs, helping make up for the deficiencies which we entomologists know we have. – James Trager

BugGuide is such a great resource for finding names, information, and identification. It has helped me learn the names of insects I have found, but also, it helps me constantly when putting together presentations, providing information like what the insect eats, life cycles, range where found, links to more information, etc.  And I know I do not know how use all the data Bugguide has to offer!!Moni Hayne

BugGuide was, and continues to be, absolutely instrumental in my growth as an entomologist. My formal training is general biology, but BugGuide served to teach me everything I could ever want to know about insect diversity, taxonomy, and distribution. -Darren McNabb

 I started using BugGuide as a technician in a systematic entomology lab run by the State of California’s Dept of Food and Agriculture. It was by far the best and most trustworthy site to assist me in my identification of many, many, MANY small beetles and other critters as, among other duties. – Megan O’Donnell


In 2017, our local librarian wanted some artwork for the library. Heidi and I put our heads together and with her fabulous artistic talent, this exhibit was born!! The following text is from the booklet at the library for self-guided tours. I have added, where the whole moth isn’t obvious in the art, the photos that were the inspiration. We are excited to have 2 other libraries interested in this exhibit. Who knew this would turn into a travelling exhibit. Thank you, Heidi, for coordinating this.


Lepidoptera is an order of insects that includes moths and butterflies. The name is derived from the Greek works “lepido” (scale) and “ptera” (wings), referring to the scales that cover the wings and bodies of adults.

Although butterflies easily capture people’s attention, most Lepidopterans are moths. Of the estimated 11,000 moths found in North America, about 1,300 occur in Wisconsin.

Lepidopteran larvae are called caterpillars and have chewing mouth parts for eating vegetation. Adults have scale-covered wings and in some, their mouth parts form a tubular proboscis for sipping nectar. Some adult moths have no moth parts at all since they don’t eat during their short adult phase. Their goal is to find a mate and lay their eggs.


Heidi Hankley

Heidi has enjoyed a lifelong interest in nature and art.  Childhood summers were spent roaming the hills around the family cottage in Iowa County and countless hours creating things at the art desk she shared with her siblings.  Heidi studied Wildlife Ecology and Environmental Studies at the University of Wisconsin and went on to earn a Master’s Degree from the University of Idaho focusing on Wilderness Management. As a National Park Service ranger and research assistant, she was able to spend some really wonderful years working in several of our nation’s most beautiful parks.  Today Heidi continues to explore and be inspired by the nature found around her home in the rural Blanchardville area.


Marci Hess

Marci began studying insects as part of a larger project designed to understand wildlife living on her 46 acres of habitat. Passionate about providing living spaces for wildlife, Marci began learning about and keeping a list of the birds, mammals, amphibians, and reptiles using the native plants, lichens, fungi, bushes, and trees on her land. She knew good management practices required knowledge of who lived there. Four years into this research and it was time to study insects. None of her formal education prepared her for this adventure! As a lifelong learner, it was remarkable how her world expanded. And simultaneously disappointing at how little is known about 80% of the insects sharing our earth. As her research continues, so has her development of photography skills and website design; both necessary tools as sharing these discoveries are half the fun!

The Exhibits

Halysidota tessellaris, Phyllodesma americana
Forest Moths
Mixed media

A Halysidota tessellaris (Banded Tussock Moth) and a Phyllodesma americana (American Lappet Moth)

From the Latin “tessella” (a little square stone); a tessellated pattern is one laid out in a mosaic of small square blocks and refers to the checkered pattern on the forewing. Tussock moth for the tufts of hair on the caterpillar. These hairs are the caterpillar’s defense against predators, not only while feeding on leaves but they also line their cocoon with these when overwintering. The caterpillars enjoy feeding on a variety of deciduous trees, such as birch, blueberry, elm, grape, oak, hickory, walnut, and hazel. 

Phyllodesma americana – American Lappet Moth

From the Greek “phyllon” (a leaf) and “desma” (a band); refers to the leaf-mimicking shape of the wings, and the pale bands on the forewing and hindwing. It is unusual to find one of the caterpillars by visually searching their host plants, which include various woody shrubs and trees. This is another moth that overwinters as a pupa.

Scientific names are a combination of Latin and Greek. Understanding the roots of these words helps to create a picture in our minds of what those words mean. Each moth has 2 parts to their scientific name – the genus and the species.

Wavy-lined Emerald Moth, Synchlora aerata
Synchlora aerata, Wavy-lined Emerald Moth
Camouflaged Looper
Mixed media, Heidi Hankley

Looper caterpillars are known generally as inchworms.  They feed on a wide variety of plants but commonly are found on asters. The common name of camouflaged looper comes from the caterpillar’s habitat of attaching flower petals and other plant bits to its back. This serves to disguise them from predators. These moths overwinter as caterpillars is in this decorated form. When it’s time to pupate, they leave their covered flower skins to form a cocoon.


Mixed Media Assemblage, Heidi Hankley
Hermaris thysbe – Hummingbird Clearwing Moth

Do moths and other insects sleep? Sorta kinda!  Insects enter a state of torpor. This can be likened to our sleep habits. A state of torpor is when the insects is physically and mentally at rest. Many insects will be active during a portion of a 24-hour period; whether this is daytime activity or nighttime activity depends on the insect. Most moths are nocturnal, meaning they are active during the night hours. As with all insects, this is a general statement as some moths prefer daylight hours for their activity!


Reduction Linocut, Heidi Hankley

Mothra – always on the side of the good and saving the day with her superpowers! Mere mortal moths must employ other means of defense such as camouflaging or startling coloration, or generating foul tasting and smelling chemical compounds.


Mixed Media, Suminiagashi Marbling, Heidi Hankley
Photo element by Marci Hess
Pheosia rimosa – Black-rimmed Prominent

Caterpillars resemble young hornworm caterpillars; for this reason the caterpillars are called False Sphinx. They vary in color and may be yellow, lavender, pink, green, brown or nearly black but regardless of the color, the skin is very shiny. Their desired food is the leaves poplars, aspens, and willows, which they eat very systematically. They eat the leaf edges beginning at the base of the leaf and progressing to the tip. These caterpillars will overwinter as pupa in cocoons.

Camouflaged Mixed Media, Suminiagashi Marbling, Heidi Hankley Photo element by Marci Hess Pheosia rimosa - Black-rimmed Prominent
Camouflaged Mixed Media, Suminiagashi Marbling, Heidi Hankley Photo element by Marci Hess Pheosia rimosa - Black-rimmed Prominent

Cerma cora – Owl-eyed Bird Dropping Moth

There are only 3 species in this genus north of Mexico. This particular species is “Considered rare in most of range and often associated with unusual and/or pristine habitats.” There are few documented occurrences and other than Wisconsin, probably no state has more than five recently verified occurrences. We are very lucky to have found this moth in our county and in our township!  The pupa overwinters in dead wood. Its vulnerability highlights the importance of leaving fallen dead trees and limbs in your woods.


Cerma cora

Harrisimemna trisignata – Harris’s Three Spot

This uncommon moth has a most unusual caterpillar that feeds on various woody species. Aside from the beautiful brocade appearance of the moth wings, it’s claim to fame is the unusual appearance of its caterpillar. This black and white caterpillar can look like a fresh bird dropping or a spider! When winter arrives, the caterpillar bores into the wood to survive the cold months.


Harrisimemna trisignata - Harris’s Three Spot

Paonias myops — Small-eyed Sphinx

There are only 3 species in this genus north of Mexico. The caterpillars are bright green, which disguises them as they feed on the leaves of cherry, hawthorn and serviceberry. Once they have feasted on a leaf, they cut it off at the base. Birds have become quite savvy about noticing damaged leaves and honing in on a dinner for themselves. This practice mitigates that. The adult uses the eyespots on their hindwings as their defense, flashing them as predators near. This style of defense if called a startle defense; visualize yourself as a bird, flying toward a delicious morsel, and as you approach, you suddenly see LARGE eyes staring back at you. Yikes!


Paonias myops -- Small-eyed Sphinx
Mixed Media, Heidi Hankley<br />
Grammia virgo - Virgin Tiger Moth

Mixed Media, Heidi Hankley
Grammia virgo – Virgin Tiger Moth 

Grammia virgo - Virgin Tiger Moth

Grammia virgo – Virgin Tiger Moth
This is the moth that inspired the artwork.

Once Heidi had the exhibit on display, we held an event at the library. Heidi offered suminagashi marbling as an activity. Marci had her photography equipment on display and a few unique beetle specimens for folks to see insects close up. Here’s some fun photos of the evening.

Insect Overwintering

As winter nears, I look forward to my “long winter’s nap” where I catch up on reading, writing, various research projects, and processing photography. It’s generally a bit slower pace of life and I ready for it as the days grow shorter and chillier.

For many insects their “long winter’s nap” is called diapause, a period of suspended development. This is the most common overwintering mode (Leather et al. 1993). This period is marked with a preparatory time followed with a time when the insect does not feed. The end of diapause is not an immediate happening. It is an extended period of time coupled with particular environmental events, one being the return of a certain photoperiod. In the course of an insect life, it typically will enter diapause as it progresses from egg to adult. The life form — egg, larva, pupa, or adult – the insect overwinters in depends on the particular species.

What cues do the insects use to determine the onset of winter?

How do they tolerate the cold northern winters?

While diapause is genetically controlled there are environmental factors stimulating the beginning and ending of this time period. Three important environmental cues are photoperiod, temperature, and nutritional needs. Photoperiod is the more reliable cue and is defined as the insects’ response to day-length; the corresponding change in temperature interacts with this. Insects also get cues from their food plants; the senescence (aging and deterioration) of their nutritional sources signals impending harsh conditions. Diapause is also synchronized so emergence occurs when chances of finding a mate are high. Isn’t nature incredible?

Insects have different strategies of cold hardiness that allow them to survive at low temperatures. While some can tolerate freezing, most insects in our climate use the biochemical strategy of supercooling or freeze avoidance and many of them overwinter in an immobile state as larvae or pupae (Leather et al. 1992). Supercooling is when water cools below the freezing point yet does not freeze. This process is initiated in the fall. The insect stops feeding and clears the digestive system, this removes water and naturally increases soluables. Next an increase in polyols (an alcohol) and sugars occur (glycerol is the most common sugar) which act like antifreeze and are collectively known as cryoprotectants. Overwintering larvae can supercool to temps ranging from -68F to -86F! 

Goldenrod gall fly larva, Eurosta solidaginis, Canada goldenrod, Solidago canadensis, gall
Dendroides canadensis, larva
Dendroides canadensis

The Goldenrod gall fly (Eurosta solidaginis) produces 3 cryoprotectants with polyols having the highest concentration (Baust and Lee 1981). These flies spend 11 months of their life in this gall as a larva.


The fire-colored beetle (Dendroides canadensis) is a beetle that has the ability to switch from freeze tolerant to intolerant, depending on the weather conditions of that particular year. While the red bark beetle (Cucujus clavipes) is also known to use both strategies, Dendroides canadensis was the first insect discovered to have this ability (Horwath and Duman 1984).

What stage in their life cycle are they?

Whether an insect overwinters as an egg, nymph or larva, or pupa is important to their survival strategy. They must control their development and reproduction in order to optimize their survival throughout the cold winters.

The recurring theme of ecological specificity applies to insect overwintering as well. The stage used by the insect is dependent upon its taxon and life cycle. It is unfortunate we don’t know more about the overwintering strategies of many of our known insects. I suppose this is expected when as only 20% of insects are described and an even lower percentage of those have full known biologies.

Eggs are the most cold hardy but they are not impervious to the rigors of winter’s cold. Praying mantis (Mantis religiosa) overwinters as eggs yet in a 6-year study their mortality ranged from 15-86% depending on the temperature fluctuations (Salt and James 1947).

Moths and butterflies tend to overwinter in a particular stage depending on the family taxa. While one can find overwintering forms in eggs, larvae, pupae, and adults, in moths 55% are pupae and in butterflies 56% are pupae (Leather et al. 1993).

Overwintering adults can be found alone or in aggregations with other insects. Think of those pesky Boxelder bugs and Asian beetles that swarm our homes as the thermometer dips! Others you’ll find overwintering as adults are Snow fleas (Hypogastrura nivicola)  and Winter crane flies (Trichocera sp) . On a sunny, warm winter day, these can be found on top of the snow. 

Common Green Darner, Anax junius

Most aquatic insects overwinter as larvae or nymphs under the ice. They continue feeding and growing until spring comes and they are ready to emerge. Common Green Darner (Anax junius)  takes 2 summers at the larval stage before emerging as an adult. These larvae are aquatic.


And, there’s always the exceptions! Some insects require 2 or more years to reach maturity. One example is the Osmoderma beetles; they spend three years in the larval stage in rotting wood before pupating in fall and emerging the following year. A sidebar interesting fact is there are only 3 beetles of this genus living in the U.S. (Galloway, MLBS website)


Osmoderma eremicola
Osmoderma eremicola

The forked fungus beetle (Bolitotherus cornutus) can overwinter as an adult or a larva depending on whether they “hatched” in spring or fall. The larva overwinter in their host fungus but we don’t know where the adults will overwinter. Perhaps in the dead or dying tree where the fungus is growing? Maybe in the soil near the roots of the tree? Maybe in the fungus? 

Forked fungus beetle, Bolitotherus cornutus, Ganoderma applanatum
Forked fungus beetle, Bolitotherus cornutus

The Virginia Ctenucha (Ctenucha virginica) moth overwinters as a caterpillar (larva) in the leaf litter of grasses, sedges, and iris. It has 2 generations in a year so one can find the caterpillar in the dark stage for the cooler months and in the light stage for the warmer months. BugGuide page has great photos of the differing caterpillar colors. Check out your prairie and see if you can find this one!


Ctenucha virginica, Erebidae
Ctenucha virginica, Erebidae

Not only do insects have to change their chemistry (supercooling) and know the most strategic life cycle stage in order to survive, they also need to add coloration to their winter survival “to do” list. Darker colors not only hide them from predators, it absorbs heat. Most of the insects overwintering as eggs are black. Geometridae moths pupae start out green and darken as they overwinter in the soil. The Eastern tent caterpillar moth (Malacosoma americana) lay their eggs in masses on tree limbs. They are white when laid and then turn dark and blend into the tree limb.


Eastern tent caterpillar moth, Malacosoma americana

Where do they overwinter?

While we know very little about most insect overwinter habits, we can generally say some of the places they overwinter include leaf litter, under dead plants or grasses, crevices of tree bark, stems of plants, tree stumps, galls, old rodent burrows, and under just about anything. Bumblebees like to choose overwintering spots under another object; it could be under a rock, a fallen tree, a deck, or a porch.

Soil and snow are good insulators. Many moths and butterflies overwinter as pupae at the soil surface. Moths such as Spotted cutworm (Xestia c-nigrum) overwintering at the soil surface have much lower lethal temperatures than insects such as the Japanese beetle (Popillia japonica)  whose larvae can burrow 4-8” into the soil.

These insects using the soil surface, leaf litter, and under dead plants have the added benefit from the insulating effects of snow during the winter but this places them at greater risk of mortality with early spring management practices.

How do they choose an overwintering site?

Topography plays a key role in many aspects of ecology. It is often the explanation to the variations found in the general tenets and paradigms of ecological restoration because it can create micro-local situations. What might appear to be similar habitat can result in quite different weather patterns. These micro-climates affect how insects exploit overwintering habitats.

Like all aspects of life strategies and survival, there are costs associated with overwintering. The immobility predisposes them to unpredictable events and predation. Dessication and precise timing of emergence pose threats as well. Overwintering has its threats and the mortality rate depends on the severity of the weather. The Codling moth (Cydia pomonella)  has an 80% mortality rate in a typical winter.

How do insects know when it’s time to emerge after overwintering?

Photoperiods and temperature are again the most important cues. Synchronizing this is important. Being out of harmony with the environment and the growth of their food plants or hosts (if one is a parasitoid) can be disastrous.

There are so many unknowns when learning about insects. This lack of knowledge can be an opportunity. Whether you are one who gets out into nature during the cold, snowy, winter months or you prefer to stay warm inside, you can contribute. While snowshoeing, look up and around and see if you can spot an egg mass or a gall housing a larva. Insects are more cryptic and harder to find in their overwintering mode. Take a picture and post it on TPE Facebook page being sure to note the plant and the date the photo is taken. If you prefer to do inside research, pick out an insect or two and dig through the literature and note your findings. However you choose to do the research, you can be certain that your contributions are important!



Baust, J.G. and R.E. Lee. 1981. Divergent mechanisms of frost hardiness in two populations of gall fly, Eurosta solidaginis. Journal of Insect Physiology 27: 485-490.

Galloway, Hazel. “Hermit Flower Beetle.” Mountain Lake Biological Station, University of Virginia. Accessed 04 Oct 2016.

Howath, K.L. and J.G. Duman. 1984. Yearly variations in the overwintering mechanisms of the cold-hardy beetle Dendroides canadensis. Physiological Zoology 57: 40-45.

Leather, S.R., K.F.A. Walters, and J.S. Bale. 1993. The Ecology of Insect Overwintering. New York: Cambridge University Press.

Salt, R.W. and H.G. James. 1947. Low temperature as a factor in the mortality of eggs of Mantis religiosa. Canadian Entomologist 79: 33-37.

Insects and Milkweed

Authors: Marci Hess and MJ Hatfield

Insects and milkweed plants have been a topic of much conversation lately. It is wonderful that monarchs and their corresponding host plants, milkweeds, are getting the attention and grant money they deserve. Yet there are a number of insects that utilize milkweeds and depend on them for various reason; many of these are less well known by the general public. By planting, encouraging and appreciating milkweeds, folks will be helping these insects, too.

Do you know how many types of milkweed are in your area? Can you name them? [Test yourself first then check the list at the bottom of this posting for milkweeds in The Driftless Area.]

There are some unique characteristics of milkweeds making it an intriguing plant. Milkweeds contain cardiac glycosides which are toxic to humans and mammals. Insects can sequester these in their systems, making them unpalatable to their predators. This chemical is usually in a low enough dose it does not kill the bird or mammal but it will make them vomit. Often it only takes once for these predators to learn not to do that again! Defensive systems are pretty remarkable. Do you think the plant or the insect evolved this defense first?

Another characteristic and plant defense is the white, milky sap that exudes when a leaf or stem is cut. Longhorn milkweed beetles are able to feed on milkweeds and dogbanes despite the latex secretion oozing from the leaves when they are cut. The beetles sever the midvein of the leaf, disconnecting its flow to the rest of the leaf. Once this is done, the beetles can freely nibble on the leaf tips without fear of having their mandibles glued shut with this sticky protective substance (Eisner, 2003:284).

Cerambycidae, milkweed, red milkweed beetle, Tetraopes tetrophthalmus

Leaf nibbled by Tetraopes tetrophthalmus (Red milkweed beetle)

“The cerambycid genus Tetraopes is the most diverse of the new world milkweed herbivores and the species are generally host specific, being restricted to single, different species of Asclepias, more often than most other milkweed insects” (Farrell 2001). Tetropes produce one generation annually. Eggs are laid at the base of the stem or cut into the stem. Either way, the larvae migrate to the roots, boring into the plant stem if the eggs weren’t laid there.  The adults feed on the leaves, flower buds, or blossoms.

Along with the Tetraopes tetrophthalmus, some of the other Tetraopes species you might find are, Tetraopes annulatus and Tetraopes femoratus.

Cerambycidae, milkweed, Tetraopes annulatus

Tetraopes annulatus


Cerambycidae, milkwee, Tetraopes femoratus

Tetraopes femoratus

Another beetle whose larvae participate in the vein cutting prior to feeding is the Swamp Milkweed Leaf Beetle (Labidomera clivicollis). Common names can be misleading as this beetle is not host-specific to swamp milkweed.

Chyrsomelidae, milkweed, Labidomera clivicollis, Swamp Milkweed Leaf Beetle

Labidomera clivicollis (Swamp Milkweed Leaf Beetle) preparing to feed

The Milkweed Stem Weevil (Rhyssomatus lineaticollis) feeds on the stems of common milkweed (Asclepias syriaca) and oviposits there as well. An interesting comment I read on BugGuide is the scar length the female cuts for ovipositing is an “accurate predictor of the number of eggs laid by the adult female.” Can you imagine being able to measure the slit and count the eggs laid by a 6mm insect?

Curculionidae, milkweed, Milkweed Stem Weevil, Rhyssomatus lineaticollis

Milkweed Stem Weevil, Rhyssomatus lineaticollis

Milkweed leaf-miner fly (Liriomyza asclepiadis) larvae feed on the foliage of milkweed. As the name implies, they “mine” between the outer layers of a leaf, leaving colorless mines that often turn brown. Leaf-miners are interesting because one can usually tell who the insect is by the characteristics of the mine and the type of plant being mined; even the frass pattern is unique enough to offer an ID for leaf-mining insects. Charley Eiseman and Noah Charney have a great book, Tracks and Sign of Insects, which show pictures of various leaf-mining activity. More photos of the larvae and the blotchy leaf-mining pattern of the Liriomyza asclepiadis can be found on BugGuide.  

Diptera, agromyzidae, Liriomyza asclepiadis, milkweed

Liriomyza asclepiadis larvae

Diptera, agromyzidae, Liriomyza asclepiadis, milkweed

Liriomyza asclepiadis adult

 Some of the more commonly found insects are the milkweed bugs in the Lygaeidae family. The small milkweed bug (Lygaeus kalmii) and the large milkweed bug (Oncopeltus fasciatus). The following pictures show two stages of the nymph growth and highlight their clustering habit.

Hemiptera, lygaeidae, Large milkweed bug, Oncopeltus fasciatus, milkweed

Large milkweed bug, Oncopeltus fasciatus, sucking on an Asclepias tuberosa stem

Hemiptera, lygaeidae, milkweed, lygaeus kalmii

Lygaeus kalmi

Hemiptera, lygaeidae, Large milkweed bug, Oncopeltus fasciatus, milkweed

Large milkweed bug nymphs, Oncopeltus fasciatus, inside and outside of the milkweed pod.

Hemiptera, lygaeidae, Large milkweed bug, Oncopeltus fasciatus, milkweed

Large milkweed bug nymphs clustering, Oncopeltus fasciatus

In the Hemiptera order there is an aphid that uses milkweeds, the oleander aphid or milkweed aphid (Aphis nerii).

Milkweed aphid, Aphis nerii, hemiptera, aphididae, milkweed

Milkweed aphid, Aphis nerii

Seems no matter what plant we pick, there’s a moth or two that use it! The moths sequester the secondary metabolite (compounds not directly related to primary functions) of the milkweeds making them unpalatable to bats. Dogbanes have this same chemical; the Euchaetes egle (Milkweed Tussock Moth) is munching a dogbane leaf in the photo. The bright colors of the caterpillars warn predators of their bad taste, but the adults warn with clicking sounds (Simmons and Conner 1996). Tussock moths are not to be handled without protection; they have urticating hairs between the soft ones which can irritate your skin. Cycnia collaris is a tiger moth who feeds on milkweeds; its caterpillar is a brilliant orange while the adult is white with a bright yellow edge.

Cycnia collaris, lepidoptera, erebidae, milkweed

Cycnia collaris caterpillar

Euchaetes egle,(Milkweed tussock moth, lepidoptera, erebidae, dogbane, apocynum

Euchaetes egle (Milkweed tussock moth) caterpillar munching on a dogbane (Apocynum sp) leaf.

Euchaetes egle,(Milkweed tussock moth, lepidoptera, erebidae, milkweed

Euchaetes egle caterpillars skeletonizing a milkweed. These are earlier instars (developmental stage) than the single one of the dogbane leaf.

Pollinators of milkweeds are diverse ranging from bees, wasps, flies, ants, to beetles. To ensure they are pollinated sufficiently, milkweeds have a mechanism in their flowers allowing them to capture and trap an insect for a period of time (Jolivet, 1998:189). This trapping is caused by a sticky substance and results in the pollinia sac attaching to the insect. The larger insects can carry this sac to the next plant, completing pollination. Small insects can suffocate if they cannot get free from this. Many of these smaller insects are careful and only dip their tongues into the blossoms like the small sweat bee, an Augochlora species shown below. Have you found an insect caught inside a milkweed blossom?

Geometrid, milkweed, lepidoptera

A geometrid moth stuck on a milkweed blossom

Augochlora, hymenoptera, halictidae, milkweed

Augochlora species

We did not include the monarch (Danaus plexipllus) in this article because most of you are familiar with them. The Xerces Society has many good resources about this butterfly and its relationship to milkweeds; one in particular is an article in their 2011 newsletter, Wings. A PDF can be accessed via this link:

We’re hoping this kindles some excitement and you’ll enjoy exploring milkweeds in more detail.


Eisner, Thomas. 2003. For Love of Insects. Cambridge: The Belknap Press of Harvard University Press.

Farrell, B.D., 2001. Evolutionary Assembly of the Milkweed Fauna: Cytochrome Oxidase I and the Age of Tetraopes Beetles. Molecular Phylogenetics and Evolution 18(3): 467–478.

Jolivet, Pierre. 1998. Interrelationship Between Insects and Plants. Washington DC: CRC Press.

Simmons RB, and WE Conner, (1996). Acoustic cues in defense and courtship of Euchaetes egle Drury and E. bolteri Stretch. Journal of Insect Behavior 9: 909–919.


Asclepias viridiflora, A. hirtella, A. lanuginosa, A. tuberosa, A. ovalifolia, A. incarnata, A. syriaca, A. purpurascens, A. amplexicaulis, A. exaltata, A. verticillata, A. sullivantii, A. meadii, A.speciosa, and A. quadrifolia.