“What’s Crawling in the Lab?” is a blog featuring short stories, pictures, and highlights from the UW-Madison Insect Diagnostic Lab. Topics range from the insects most commonly diagnosed in the lab to emerging arthropod pests and unique and bizarre cases from the lab.
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When most folks think of “cutworms” they probably envision big fat caterpillars munching on plants in their yards and gardens. Sure enough, we have many species from the cutworm & armyworm family (Family Noctuidae) that can be notable crop pests, such as black cutworm, bronzed cutworm, variegated cutworm, true armyworm, fall armyworm, and others. Readers should keep in mind that the Family Noctuidae is surprisingly diverse with over 2,500 species known from the US and Canada. Many of these species play important roles in the food web and can be as important as bees for pollination of certain crops!
While cutworms can be common during the summer months into the fall, we usually don’t expect to see them this time of the year. Surprisingly, there’s one cold-hardy species that has been common in Wisconsin recently—the “winter cutworm” (Noctua pronuba). This species gets its name due to the fact that the caterpillars are cold-tolerant and can be active when temperatures dip. While they won’t be out-and-about during a polar vortex, I’ve had many recent reports at the UW Insect Diagnostic Lab of winter cutworm caterpillars wandering on the snow when temperatures have been in the 20’s. Overall, there are very few caterpillars that venture onto the snow, period.
Similar to other cutworms, the winter cutworms are plump, earth-toned caterpillars—they are brownish with pairs of black dashes bordered by white running down the sides of their backside, which makes them easy to identify. (Technically, a greenish form of the caterpillars also occurs). In addition, if you see any caterpillar that even looks “cutworm-like” out on the snow, it’s almost certainly this species. They pass through the the winter as nearly-mature caterpillars before pupating in the spring. The adult moths are active during the warmer months and display an amazing array of different color forms—ranging from very light beige to grey or brownish—although the hindwings (typically tucked under the body at rest) are a diagnostic yellow color. Thus, adults are commonly known as “large yellow underwing” moths.
The winter cutworm can be found from coast-to-coast in much of the US and Canada, although this wasn’t always the case. This species is common in Europe and wasn’t known from North America until 1979 when it was first spotted in Nova Scotia. While the caterpillars do feed on a wide range of plants, they’re rarely a notable pest. Outbreaks and reports of damage aren’t common, although there was a notable outbreak in Michigan around 2007-2008. In most cases, these caterpillars are simply a curiosity as they wander across the snow on mild winter days—sometimes by the thousands. In addition, winter cutworms can occasionally sneak into structures as well, which can be another surprise to see caterpillars actiely wandering in garages or barns when the temperature is in the 40’s.
Each insect species has a “story” to tell—where they came from, what they do, and what their presence means for humans. Knowing this story can make a big difference when it comes to our own actions. For example, imagine finding an insect near your bed. If you recognized the insect as a harmless ground beetle that accidentally snuck in from outdoors, you might shrug your shoulders and carry it outside in a Tupperware. But, if the insect were a blood-feeding bed bug, it could mean an expensive bill to have your home professionally inspected and treated by a pest control company. Knowledge is power!
In my mind, the first step towards getting this knowledge is through proper identification. I say this with a certain amount of endearment, as I am an insect diagnostician after all. However, once you’ve properly identified an insect, it’s easy to look up its “story” (biology, behavior, etc.) in books, websites, or even the scientific literature. This process can allow you to more efficiently identify the root of a problem (if there is one) and figure out a solution.
A good example of this concept would be flies in and around homes. There are easily dozens of different types of flies that can be encountered in typical yards and homes in the Midwest and each has a slightly different story to tell. Some can breed in piles of grass clippings or seaweed (e.g., stable flies), some parasitize earthworms but like to sneak indoors (e.g., cluster flies), and others can hitchhike on plants. A common and widespread group of flies is the Family Calliphoridae—the blow flies.
Luckily, blow flies can be identified easily in many cases due to their metallic green or bluish coloration. Some members of this group are even known as “green bottle flies” or “blue bottle flies” due to their distinctive colors. Overall, the blow flies are mid-sized flies and are often slightly larger than house flies (~1/4 – 3/8” long). The larvae are your stereotypical pale and slender “maggots”, although they can be definitively distinguished from similar-looking fly larvae under the microscope.
When blow flies are found indoors, it’s often easy to understand the situation and remedy the problem. One of the biggest factors in the “story” of blow flies is dead stuff. Out in nature, blow flies play an incredibly important role in helping break down dead animal matter. These flies are often the very first insects to show up at dead animals—sometimes within seconds. After laying eggs, the resulting larvae do a lot of work feeding on and breaking down animal tissues. Being poikilothermic (“cold-blooded”), they also develop at a predictable rate depending on the temperature, so these flies are even used to estimate lengths of time in forensic investigations.
There’s always a chance that a blow fly spotted indoors might have accidentally snuck in from outdoors, but in many cases it’s an indication of something dead. Blow flies are able to detect the faint odors of death from long distances, so a single dead mouse, bat, or other creature in a home can attract these flies. Rather than spraying an insecticide, the best remedy is to find the root of the issue and clean it up. In many cases, it’s just a matter of checking mouse traps and discarding the mouse that had recently met its demise.
Another situation that I often see indoors is a scenario that has simply proceeded further than the last one. If an adult female blow fly is able to locate a dead mouse (or other animal) and lay eggs unnoticed, the resulting larvae can consume most of the remains and complete their development. If this happens, the first thing noticeable is often a bunch of maggots wandering the nearby area (the maggots often wander away from their food source when ready to pupate). When they pupate, small brownish “pods” (puparia) may be noticed and upon reaching maturity, dozens of adult blow flies can seemingly appear out of nowhere. In such a case, it’s generally an indicator that a dead mouse or other animal had been present for some time (often a few weeks). Unless there are additional resources for them, these flies usually perish quickly indoors. This scenario frequently comes into play in second homes or cabins that are unoccupied for longer periods of time or if the dead animal is location in a tucked away or inaccessible spot (such as in a wall or ceiling void).
If you’re like me, you’ve probably been out appreciating our recent fall weather. Likewise, boxelder bugs (Boisea trivitatta) have also been enjoying the warmth. I’ve seen an uptick in reports of boxelder bugs at the UW Insect Diagnostic Lab this year compared to most other years in the last decade and weather patterns in the Midwest have played an important role.
Whether you recognize them or not, there’s a good chance you’ve bumped into boxelder bugs before. These black and red insects can be common throughout the warmer months and can be especially abundant in late summer and early fall. Adults are approximately ½ inch long and have a criss-cross pattern on their backside created by their wings. Juveniles (nymphs) are smaller with much more red on their bodies. As the nymphs mature, their developing black wing pads become noticeable. True to their name, boxelder bugs are commonly associated with boxelder trees (Acer negundo). Botanically speaking, boxelders are technically a type of maple, and some other maples can also be a host for these insects as well as ash trees and a few others.
From a plant-health perspective, boxelder bugs cause little damage to plants and are of little concern. These insects get the most attention when they’re spotted on the sides of homes and other structures. They’re often particularly fond of the southern and western sides of homes where the warm afternoon sun hits. Boxelder bugs are often spotted on buildings in fall as they search for sheltered overwintering spots. If they can squeeze in through a gap or crack, they can easily hunker down in a wall void, or similar spot for the winter. Sometimes, boxelder bugs can make it to a location where they become active indoors during the winter months, much to the chagrin of the humans living in the home. While these insects can be perceived as a nuisance, they’re really quite harmless to people, pets, and homes (although they could stain light-colored fabrics if crushed). Like other “fall invading” insects (e.g., cluster flies, multicolored Asian lady beetles, brown marmorated stink bugs, and western conifer seed bugs), a helpful approach is to seal up potential entry points on the exterior of your home before these insects make their way inside.
Looking at long-term patterns, boxelder bug populations have generally been low in many parts of Wisconsin over the last decade. A reason for this is moisture. When we have rainy years, entomopathogens (insect-infecting pathogens) can keep boxelder bug populations low. In contrast, boxelder bugs tend to thrive under drier and warmer conditions. There’s even been research on a closely-related species (the western boxelder bug, Boisea rubrolineata) in the western US suggesting that those insects deliberately sunbathe and secrete a chemical to help inhibit the common entomopathogenic fungus Beauveria bassiana. Here in Wisconsin 2021 was surprisingly dry in many areas which likely helped boost boxelder bug numbers. Likewise, some parts of Wisconsin also saw dry conditions continue into 2022, which may have helped them further, leading to an increase in boxelder bug populations and reports this year.
With the arrival of fall, we’re starting to see pumpkins, spooky yard decorations, and pumpkin spiced everything (*shudder*). Not to be left out of the festivities, one insect can be quite noticeable this time of the year—the large milkweed bug (Oncopeltus fasciatus).
If you’re wondering, we do also have a small milkweed bugs (Lygaeus kalmii), although it’s not usually as common as the ubiquitous, larger cousin. Both juveniles (nymphs) and adults of the large milkweed bug are fittingly adorned for fall. Adults reach lengths of nearly 3/4 inch long and are mostly orange with black patches on the wings and body. Going through “simple metamorphosis” (technically, they’re paurometabolous), the juveniles or nymphs look similar in body shape to adults (although smaller) and are also orange with some black. In more mature nymphs, the developing blackish wing pads are quite noticeable. When you see a group of nymphs and adults hanging out on milkweed in the fall, it’s easy to recognize that they’re all the same species.
Large milkweed bugs specialize on milkweed plants and are one of the many creatures that can be found in and around milkweed patches. When they feed, large milkweed bugs use their sucking-type mouthparts to sip fluids from plants; they’re especially fond of the developing seed pods and are often spotted on pods. While home gardeners hoping to rear monarchs might be concerned about competition, these insects generally cause little harm to plants and are more of a curiosity than a pest.
Large milkweed bugs have a very seasonal pattern in the upper Midwest. They can’t survive the winter at more northern latitudes, so they must migrate northwards each year. Here in Wisconsin, you’d be hard pressed to find large milkweed bugs in spring, but by July they’ve often arrived in low numbers. Large milkweed bugs can become quite common by late summer and early fall as it typically takes 40+ days for a new generation of adults to appear from eggs laid in our area.
In much of Wisconsin, we’re just starting to see foliage change to reds, oranges, and yellows, but as you’re out enjoying the fall weather, keep an eye out for these festive-looking insects around milkweed patches.
If you’ve spotted small, crunchy beetles in your home this summer you aren’t alone. Broad-nosed weevils (Curculionidae: Entiminae) have been a surprise this summer at the UW Insect Diagnostic Lab. While I see cases of these insects every year, things have been a bit more intense this summer with a flood of reports from around Wisconsin.
What’s a weevil? Out of the 100+ different families of beetles, the weevils (Family Curculionidae) are extremely diverse with over 50,000 species in this group alone. In terms of their appearance, most weevils might remind you of Gonzo from the Muppets with their very pronounced “snouts”. A great example of this in Wisconsin is the genus Curculio (the “nut and acorn weevils”). Some species in this group can have a snout (technically “rostrum”) as long as the rest of their body.
On the other hand, some members of the weevil family lack the pronounced snout . One such group, the subfamily Entiminae, is commonly referred to as the broad-nosed weevils. Our common broad-nosed weevils tend to be small (around ⅛ – ¼ inch-long) and have pear-shaped bodies with very hard, crunchy exoskeletons; they also have “elbowed” antennae similar to ants. The color of the broad-nosed weevils can vary by species, but many are blackish or grayish.
When it comes to broad-nosed weevils, we have over 100 species in the Midwest alone. In general, these are “outdoor” species associated with plants. The larvae tend to feed on the roots of plants while the adults often chew small notches out of the edges of foliage. Interestingly, a few species in this group have the habit of sneaking indoors during the summer months. Once inside, these insects are completely harmless but can be a minor nuisance as they seem to mindlessly wander on walls or floors.
Conditions in Wisconsin over the last year and a half must have been just right for some of these species, since I’ve had a flood of requests to help identify broad-nosed weevils in homes and other structures during the summer of 2022. Often when I see “weevil” cases it’ll be a handful of weevils indoors, but this year I’ve also seen plenty of reports of large numbers of weevils (hundreds or thousands!). The top three species I’ve been seeing in Wisconsin have been the strawberry root weevil (Otiorhynchus ovatus), the imported longhorn weevil (Calomycterus setarius), and the black vine weevil (Otiorhynchus sulcatus). There are a number of other species in this group that can invade structures as well. Due to the similar appearance of these beetles, it’s often necessary to get a sample under the microscope to help confirm the exact species. If you come across broad-nosed weevils in your home and want to know the exact type, feel free to send in a sample to the UW Insect Diagnostic Lab: insectlab.russell.wisc.edu/samples.
While these weevils can be a bit of a nuisance, there usually isn’t much of a need to spray (especially indoors). In the grand scheme of things, good physical exclusion (i.e., sealing things up better with caulk, expanding insulation foam, better weatherstripping, etc.) can go a long way to help prevent broad-nosed weevils from getting indoors in the first place. For the weevils that do make it indoors, insecticide really aren’t necessary and sweeping or vacuuming up these slow-moving pests is the best course of action.
If you’ve noticed all the lawn signs for mosquito treatments, you may be wondering if mosquito yard sprays harm other insects. If so, you’re not alone. As an Extension entomologist, this is a common question that I get from the public every year. To get to the bottom of this question, it helps to understand the different types of mosquito spraying that’s done.
One approach (Ultra Low Volume or ULV) is sometimes used by municipalities or abatement districts to control mosquitoes. These ULV applications generally involve using specialized equipment mounted on trucks or aircraft to apply extremely tiny droplets which kill adult mosquitoes by direct contact as the droplets float in the air. Such applications use very small volumes of insecticides either undiluted or with minimal dilution and are often applied after dark when mosquitoes are most active. The microscopic droplets from ULV treatments disperse relatively quickly and have little residual activity—think of them like a “one time strike” to knock down mosquito numbers.
The vast majority of research on mosquito sprays and non-target organisms has looked at these ULV-type treatments. Some good news is that these studies suggest that impacts to non-target insects are relatively small and short-lived. It turns out that the ULV treatments are most effective on insects with very small body mass, so insects larger than mosquitoes tend to be spared. A good summary of the impacts of ULV treatments on non-target organisms can be found in a 2012 review paper by J.A.S. Bonds in Medical and Veterinary Entomology.
Case closed, right?—Not quite. Here in Wisconsin, we don’t really use ULV treatments a whole lot for mosquitoes. The common yard treatments are what we’d call “perimeter”, “barrier”, or “residual” treatments. Such treatments are applied via a backpack sprayer to create a coating or “barrier” on treated surfaces which affects mosquitoes that land on it. These treatments involve applying a residual insecticide (usually from the pyrethroid group) to vegetation in yards and around structures. The pyrethroid products are broad-spectrum and often last for a few weeks or longer depending on the formulation. These same ingredients (and sometimes the exact same products) can also be used to control a wide range of yard, garden, and structural pests (e.g., Japanese beetles, garden pests, household ants, etc.).
While the pyrethroids are very common and widely used for a range of purposes, there’s a knowledge gap when it comes to the impacts of mosquito “barrier” treatments on other insects. While this knowledge gap exists, a few studies raise concerns. One study by Dr. Karen Oberhauser and colleagues found that monarch caterpillars could be harmed or killed even 3 weeks after spraying. A more recent (2022) study by Qualls et al. found that honey bees were harmed 28 days after “barrier” treatments were applied. Thus, if a yard is being sprayed for mosquitoes monthly during the warm season, there are reasons for concern. More research is needed to help understand the effects of these “barrier” yard treatments on insects that often land on vegetation in yards, such as moths, butterflies, fireflies and other beetles, true flies, bees, wasps and other pollinators.
References:
Heading into the growing season, spongy moth (Lymantria dispar, formerly known as the “gypsy moth”) was poised to have a big year in Wisconsin. That prediction has held up and I’ve seen an influx of reports of spongy moth caterpillars and damage at the UW Insect Diagnostic Lab (IDL) this spring. In some areas, these caterpillars are causing conspicuous damage, which has also led to a flurry of questions from the public on what to do about them. Let’s take a look at how this season has shaped up, how the next few weeks could turn out, and what can be done.
After a cool start to spring, we saw some unseasonably warm temperatures during the second week of May, which jump-started a lot of insect activity. I saw a distinct increase in diagnostic requests at the IDL around this time as well as my first reports of spongy moth caterpillars.
Initial sightings of small caterpillars mostly involved larvae dangling from trees and structures from silken threads—a dispersal mechanism down as “ballooning”. In other cases, thousands of tiny, dark caterpillars stood out against light-colored siding of homes. At first, these tiny caterpillars couldn’t cause much damage—with their small size, they simply don’t eat much. It isn’t until caterpillars are larger and more mature that they really start to chow down and damage increases dramatically. It’s estimated that 80-90% of the damage caused by these caterpillars is from the final two larval substages (instars). Reports of notable damage started to pop up a few weeks later in early June.
Based on the reports coming in to the UW Insect Diagnostic Lab, the heaviest spongy moth activity in 2022 spans from southeastern Wisconsin (Lake Geneva area) west through Rock, Green, and Dane Counties, and north to Sauk, Juneau and Monroe Counties. Overall, Dane and Walworth Counties stand out for the number of spongy moth sightings and reports of damage that I’ve received.
The end of caterpillar activity is in sight—but we’re not there yet. I’m still getting reports of spongy moth caterpillars and likely will for a few more weeks. In many cases, the caterpillars being spotted are now pretty large (1¾ – 2 inches), meaning that they’re feeding voraciously and causing lots of damage to plants. If there’s a silver lining, it’s that these large caterpillars should also be pupating in the near future—putting an end to their damage for the season. However, I’ve been receiving reports of mixed caterpillar sizes, with some caterpillars only measuring ¾ – 1 inch long. These smaller “stragglers” will continue to feed and cause damage into July, meaning we’re not entirely out of the woods yet.
Another variable that could be at play this year is a beneficial fungus known as Entomophaga maimaiga. This disease can specifically infect and kill spongy moth caterpillars and can play an important role in regulating their populations over time. Last year’s drought likely helped set the stage for 2022 by suppressing this beneficial fungus. This spring we’ve had pretty regular precipitation in many parts of the Midwest, which could help put a dent in spongy moth populations if this pathogen kicks in.
This has been one of the commonest questions I’ve been getting recently and have seen plenty of posts on social media sites like Facebook and Nextdoor asking this same question. Management of spongy moth really depends on the life stage of the insect. The UW-Madison Division of Extension Spongy Moth website has an excellent month-by-month discussion of management approaches.
For small numbers of yard trees, the burlap band method can be a way to remove larger caterpillars from the equation. However, it’s important to understand that this method can be time and labor intensive as you need to check bands daily and brush caterpillars into a container of soapy water to maximize effectiveness. [Note: don’t touch the caterpillars bare-handed, it hurts!]. For large trees, there’s not much else that an individual homeowner can do other than discussing chemical treatment options with an arborist. Many of the online posts I’ve seen have had an element of panic, but it’s also important to keep in mind that trees that are in otherwise good health can generally tolerate defoliation and will push out another batch of leaves later this year. I start to worry more about plant health when trees are defoliated repeatedly, as that can lead to secondary issues over time.
I’ve also seen a number of questions about aerial sprays for spongy moth. This year, the Wisconsin Department of Agriculture, Trade and Consumer Protection (DATCP) has been coordinating aerial spraying in the western parts of the state to slow the overall spread of this invasive species. The treatment used in early-season aerial sprays (Bacillus thuringiensis kurstaki) is most effective against small caterpillars. Later spraying will focus on disrupting the ability of adult moths to successfully find a mate. In theory, members of the public could band together to coordinate aerial spraying in their local area, but the planning process for this can take months. By the time folks were posting on social media expressing a desire for aerial treatments in their neighborhood, that option was no longer feasible.
One key thing to pay attention to later this summer will be the egg masses laid by adult female spongy moths. Each egg mass can contain upwards of 1,000 eggs, so surveying for egg masses can give insight into what the spongy moth situation could be like in 2023. Those egg masses will also remain in place for roughly nine months until they hatch next spring, which gives lots of time for a search-and-destroy scavenger hunt in your yard.
For additional information on managing spongy moths, check out the updated UW-Madison Division of Extension factsheet on this insect and the Extension spongy moth website with month-by-month recommendations.
If you’re seeing insect damage on your roses this spring, you aren’t alone and there are a number of early-season insect that can catch our attention. For example, a number of caterpillars can occasionally be spotted on roses in spring, such as the rose plume moth (Cnaemidophorus rhododactyla). There’s also the non-native rose leafhopper (Edwardsiana rosae) which can cause faint speckling on leaves. Later in the growing season, rose chafers (Macrodactylus subspinosus) and the notorious Japanese beetle (Popillia japonica) can be a concern.
Perhaps the most noticeable damage to roses in spring is caused by the larvae of sawflies. These insects cause two types of damage: “windowpane” damage when they only feed partway through foliage and leave the upper leaf surface intact or small holes or notches when larger larvae chew entirely through the leaves. In the Midwest, there are three different species of sawflies that can commonly be encountered on roses in spring: the roseslug sawfly (Endelomyia aethiops), the curled rose sawfly (Allantus cinctus), and the bristly roseslug sawfly (Claudius diffiformis). To the naked eye, the larvae (up to ~ ½ inch long, pale greenish, and caterpillar-like) and the adults (~ ⅓ inch long, dark-colored, and wasp-like) all look similar. However, determining the exact species under magnification can be helpful to understand the potential impacts on your roses.
The roseslug sawfly (Endelomyia aethiops) is the commonest of the three sawflies that I see samples of at the UW Insect Diagnostic Lab. The larvae of this species have a smooth texture and brownish head capsule; their bodies are translucent and they usually have a pale greenish color due to ingested green leaf material. Some good news about the roseslug sawfly is that this species only has a single generation in spring. Their damage tends to be mostly just a minor cosmetic issue. As the plants really take off in late spring, they tend to “shrug off” this damage and it’s quickly covered up by new growth.
The curled rose sawfly (Allantus cinctus) goes through two generations early in the growing season. Similar to the roseslug sawfly, their feeding damage tends to be minor and plants are usually able to chug along just fine. The larvae of the curled rose sawfly also are pale green with a brownish-orange head capsule. They do have tiny spots on their bodies, but these are only visible under high magnification. This species tends to feed while curled up and they also chew notches on the edges of leaves which can help distinguish this species from the other two sawflies.
The third sawfly commonly seen on roses also tends to be the most problematic—the bristly roseslug sawfly (Claudius diffiformis). Unlike the first two sawflies, this one can continue to reproduce throughout the growing season. Because they go through many generations per year, their damage can accumulate over time and tends to be more notable. Like the other two sawflies, the larvae of this species are also pale green with a brownish head capsule, but they are covered with fine, hair-like bristles when viewed under magnification. Very few sawflies have a hairy or bristly appearance like this, which helps distinguish this species on roses.
If you do bump into these sawflies on your roses in spring, they’re pretty easy to deal with. Options for managing them include:
The United States Department of Agriculture (USDA) declared April to be Invasive Plant Pest and Disease Awareness Month. To support this effort, the University of Wisconsin Insect Diagnostic Lab recently launched a new Wisconsin invasive insect mapping page to help track invasive insects in the state.
If you’ve followed this blog for a while, you’ll notice that quite a few of my posts focus on invasive insects. Why? In part, it’s because these non-native insects tend to be new or emerging issues and a key role of the UW Insect Diagnostic Lab is to help identify and track new and trending insects in the state. In addition, these invasive insects can sometimes cause significant damage or capture our attention for other reasons. In a typical year, I see 2-3 new non-native insects show up in Wisconsin, which really adds up over time. For every species that has arrived here, many more are making progress towards the state (e.g., spotted lanternfly). In other cases, non-native insects show up completely out of the blue.
Because of the impacts mentioned above, it’s helpful to track invasive species so we can better understand where they may be having impacts, and also to get the word out about new detections and allow folks to take appropriate action. To help in this regard, the IDL’s new invasive insect mapping page hosts a series of maps showing the known county-level distributions of a select list of invasive species.
These particular species have been included due to their relatively recent arrival in Wisconsin, and the ability to track them on a county-by-county basis. Keep in mind that many other non-native insects can be found in the state, but some of these have been around a long time, are now widespread, and tracking on a county-by-county basis is no longer feasible or helpful (e.g., Japanese beetle, European paper wasps, German yellowjackets, European earwigs, and many more).
The maps on this page will be updated when new detections occur, and additional species maps will be added over time. If you believe that you’ve observed one of the listed insects in a county where it has not been documented or a new invasive insect species, please collect evidence (physical specimens and/or digital images) and contact me to work on officially confirming the detection. An example entry from the map page can be found below:
The spongy moth, Lymantria dispar has recently been in the news because of its new name. If you haven’t heard of the “spongy moth” before, it’s probably because you learned of this insect as the “gypsy moth”. It’s the same exact creature, just with a new common name.
Why the change? The word “gypsy” in this insect’s name was originally a reference to persons of Romani descent—“the popular name of the gypsy was no doubt suggested by the brown, tanned kind of color of the male” [Forbush & Fernald, 1896]. In 2021, the Entomological Society of America’s Better Common Names Project started to review the common names used to communicate about insects. Common names that include derogatory or inappropriate terms are being assessed. After a lengthy review process, the term “spongy moth” was ultimately decided upon to describe Lymantria dispar—and fittingly so. The beige egg masses of this insect have a soft, spongy consistency. In French-speaking parts of its range, this species has long been known as La Spongieuse for this very reason. Thus, you’ll be hearing more about the “spongy moth” over time as the term “gypsy moth” is phased out from educational/government websites and other resources.
In addition to the name change, the spongy moth should be on our radar for other reasons. Despite being in Wisconsin for decades, this pest can still be a serious defoliator of hardwood trees, both in yards and forested areas. From the period of 2014 – 2020, spongy moths haven’t been much of an issue. An important reason for this is a beneficial fungus known as Entomophaga maimaiga. This fungus was introduced from Japan and it is strongly associated with the spongy moth. Although it took some time to make an impact in the US, this fungus is now viewed as an important “check” on spongy moth populations. Spring rains encourage this fungus, which can cause high mortality amongst spongy moth caterpillars. However, in many parts of Wisconsin we saw an unusually dry year in 2021 which likely curbed the impacts of this fungus. As a result, I saw an increase in cases and reports of spongy moth caterpillars and their damage, adults, and egg masses at the UW Insect Diagnostic Lab last year and Wisconsin also saw a subtle uptick in defoliation of forested areas.
As illustrated in the chart above, spongy moth populations can be very dynamic and can explode under the right conditions—leading to extensive defoliation. The egg-laying strategy of this species plays an important role in this dynamic. Adult female spongy moths deposit egg masses that can easily contain upwards of 1,000 eggs. In late summer and fall of 2021, I saw plenty of reports where trees contained dozens of egg masses, which could turn into tens of thousands of hungry caterpillars this spring.
Luckily, there’s still a bit of time to take advantage of this knowledge as the young caterpillars typically don’t become active until late April or early May. In the meantime, removal or destruction of the egg masses could help reduce local populations. While often found on trees, the egg masses can also be located on just about any surface in a yard—stacked boards, sides of structures, piles of firewood, and even on vehicles. Don’t delay if you noticed spongy moth activity in your area last year, since it won’t be long before the caterpillars are out and active this spring.
Reference: Forbush, E. H. and C.H. Fernald. 1896. The gypsy moth. Porthetria dispar A report of the work of destroying the insect in the commonwealth of Massachusetts, together with an account of its history and habits both in Massachusetts and Europe. Boston, Wright & Potter. 495pp.
