Note: As of late 2023, elm seed bug (ESB) has not yet been found in Wisconsin, but could show up in our area in the near future.
The elm seed bug (Arocatus melanocephalus| Hemiptera: Lygaeidae) is an invasive insect species native to parts of Europe. It was first detected in North America in 2012 in western Idaho and is now established in western parts of the continental US and Canada. In the eastern US, we haven’t seen much of this insect yet. It was technically spotted in the general Detroit area in Michigan in 2015 and reports continue to pop up in southeastern Michigan. This last summer, I helped confirm the presence of the elm seed bug (and the Asiatic garden beetle!) in the Twin Cities area for the Minnesota Department of Agriculture. Given the proximity to the Wisconsin state line, the elm seed bug could make an appearance in our state in the not-too-distant future.
Overall, elm seed bugs have habits similar to boxelder bugs and birch catkin bugs in that they feed on trees outdoors before invading structures later in the year. True to its common name, the elm seed bug is associated with elm trees and is especially fond of feeding on the seeds. They have little overall impact on the health of the trees. Later in the year, adult elm seed bugs seek out sheltered overwintering spots. In natural settings, they would typically overwinter beneath loose bark of trees or similar locations. However, they can readily invade homes and other structures in large numbers. Indoors, elm seed bugs are harmless to humans, but can be a general nuisance. Like brown marmorated stink bugs, elm seed bugs can also produce an unpleasant odor when crushed. Because ESBs are a nuisance invader like boxelder bugs and multicolored Asian lady beetles, management is going to be very similar and should focus mostly preventing these insects from getting indoors, e.g., physical exclusion and exterior crack and crevice treatments.
Infographic describing the main features of the elm seed bug (Arocatus melanocephalus). Infographic created by PJ Liesch, UW Insect Diagnostic Lab. Click for larger version.
Since we have not yet seen elm seed bugs in Wisconsin, having this insect on our radar and being able to recognize it are the most important things at the moment. Elm seed bugs are similar in body shape to boxelder bugs, but are slightly smaller at roughly ¼ – ⅓ inch long. The adults are a dark brownish-black color with rusty-colored patches behind the head. There is a blackish, triangular structure (scutellum) on the middle of the back which sits within a rusty-colored, square-shaped patch. The edge of the abdomen is marked with a series of small, pale spots and the underside of the abdomen is a rusty, reddish color as well.
Wisconsin residents believing that they’ve found elm seed bugs are encouraged to collect a sample and contact me at the UW Insect Diagnostic Lab to definitively confirm the identity of the specimens: insectlab.russell.wisc.edu.
As one of the coordinators of Wisconsin Insect Fest, I’m thrilled to announce that Insect Fest will be back this year and is coming to the Upham Woods Outdoor Learning Center (Wisconsin Dells) this August. The event will be held from 2–10 PM on Saturday, August 19th, 2023 and is free and open to the public. A family-style cookout will be provided to participants (please RSVP if interested). Insect enthusiasts of all ages are welcome to attend!
Wisconsin Insect Fest is a celebration of insects and attendees will have the opportunity to learn about the different types of insects found in Wisconsin and the many important roles they play in the environment. Insect Fest will feature a variety of indoor and outdoor activities suitable for insect enthusiasts of all ages, including educational presentations, hands-on activities, guided insect hikes, live insects, and an informal BioBlitz activity. This is also a great opportunity to meet entomologists and learn about what they do.
The Wisconsin Insect Fest is a family-friendly event, and we encourage everyone to attend and learn about the fascinating world of insects. We hope to see you there!
Spongy moth (Lymantriadispar) season has officially begun here in Wisconsin. Earlier this week (May 3rd), the Wisconsin Department of Agriculture, Trade and Consumer Protection (DATCP) Pest Survey Program reported caterpillars emerging from an egg mass in far southwestern Wisconsin. Based on forecasting models from the USA Plant Phenology Network, we should expect to see caterpillar emergence begin across much of southern Wisconsin over the next week. As things continue to warm up, the pattern will push further north in the state.
Forecasting map from USA National Phenology Network for predicting hatching of Spongy Moth eggs. Source: USA-NPN (www.usanpn.org)
We may be facing a challenging year from this invasive insect. Dry conditions suppressed a beneficial fungal disease (Entomophagamaimaiga) the last two years and allowed spongy moth populations to build up. Along these lines, DATCP reported a 102% increase in male spongy moths caught in trapping surveys last year. Likewise, the Wisconsin Department of Natural Resources (DNR) Forest Health Team reported a significant uptick in defoliation last year—from 294 acres defoliated in 2021 to over 85,000 acres defoliated in 2022.
At this point in the season, it’s hard to know how much damage will ultimately occur on yard and forest trees in 2023. However, you can get an estimate of potential defoliation in your own yard by counting egg masses. Each overwintered spongy moth egg mass contains upwards of 1,000 eggs, so a yard with dozens of egg masses could soon face tens of thousands of hungry caterpillars in the near future.
Dozens of spongy moth egg masses on a tree trunk. Photo credit: Karla Salp, Washington State Dept. Agriculture via Bugwood.
Over the next few weeks, we can expect spongy moth caterpillar activity increase. These small caterpillars will cause a trivial amount of damage at first but will gradually become larger, hungrier, and more damaging over time. Keep an eye out for activity in your yard. If you are inundated with caterpillars, consider using sticky barrier bands and burlap barrier bands to trap them as described on the UW-Madison Extension Spongy Moth website.
Looking at the bigger picture, Mother Nature could hold a trump card for our spongy moth situation. If we end up having a rainy spring, damp conditions could encourage the fungal disease Entomophaga maimaiga to kick in and crash spongy moth populations (this halted outbreaks in SE Wisconsin in 2004 and 2010) . However, if we experience another dry season, it could allow spongy moth populations to build further—stay tuned and hope for rain!
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.
Winter cutworm caterpillar wandering on the snow. Photo submitted to UW Insect Diagnostic Lab in late 2022.
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.
Larger yellow underwing moth showing the distinctive yellow color of the hindwings. Photo credit: Olaf Leillinger via Wikipedia CC.
The winter cutwormcan 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.
Winter cutworm caterpillar found in a garage last December. Photo submitted to UW Insect Diagnostic Lab.
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 cluster fly (Pollenia sp.). These flies are common parasites of earthworms, but frequently enter structures in late summer and can be a nuisance. Preventing their entry into homes is the single most important management step. Photo credit: Katja Schulz, via Wikipedia.
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.
An adult blow fly. The metallic green coloration is a helpful identifying feature. Finding these indoors often indicates a dead mouse. Photo credit: Brian Gratwicke via Wikipedia.
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.
Adult boxelder bug (Boisea trivitatta) on the side of a building. Photo credit: PJ Liesch, UW-Entomology.
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 (Acernegundo).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.
Boxelder bug nymphs (juveniles) on the UW-Madison campus. Photo credit: PJ Liesch, UW Entomology.
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.
Boxelder bugs congregating on the side of a home. Photo submitted to UW Insect Diagnostic Lab.
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.
Group of large milkweed bugs (Oncopeltus fasciatus) on milkweed at UW-Madison; both adults and nymphs are present. Photo credit: PJ Liesch, UW Insect Diagnostic Lab.
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.
The pecan weevil (Curculio caryae). The pronounced rostrum or “snout” is a classic feature of many members of the weevil family. Photo credit: Jennifer C. Girón, Museum of Texas Tech University. Bugwood.org.
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.
The imported longhorned weevil (Calomycterus setarius)—a type of “broad-nosed weevil” that occasionally sneaks indoors. Photo credit: Natasha Wright, Braman Termite & Pest Elimination, Bugwood.org
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.
During the summer months, some broad-nosed weevils can sneak indoors—occasionally in large numbers. In this case, hundreds of strawberry root weevils (Otiorhynchus ovatus) were wandering on the side of a home. Photo submitted to UW Insect Diagnostic Lab.
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 (Calomycterussetarius), 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.
Many mosquitoes like to rest on vegetation during the day, so some mosquito sprays specifically target these resting sites. Photo credit: Public Domain Image.
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.
Fireflies can be very common on vegetation in a yard during the summer months. Scientists view pesticides as a concern for fireflies globally. Photo credit: RachelEllen via FLickr CC.
References:
Bonds, J.A.S. 2012. Ultra-low-volume space sprays in mosquito control: a critical review. Medical and Veterinary Entomology. 26: 121-130.
Oberhauser, K.S., Brinda, S.J., Weaver, S., Moon, R.D., Manweiler, S.A., and N. Read. 2006. Growth and Survival of Monarch Butterflies (Lepidoptera: Danaidae) After Exposure to Permethrin Barrier Treatments. Environ. Entomol. 35(6): 1626-1634.
Qualls, W.A., Moser, B.A., Periera, R.M., Xue, R-D, and P.G. Koehler. 2022. Impacts Of Barrier Insecticide Mixtures On Mosquito, Aedes Aegypti And Non-Target Honey Bee, Apis Mellifera. Journal of the Florida Mosquito Control Association 69: 34-42.
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.
What’s happened so far (as of mid-June)?
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.
Unseasonably warm temperatures (90+˚F) for Madison, WI in early/mid May kicked off a lot of insect activity. Source: Weather Underground.
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.
What will the next few weeks be like?
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.
A large spongy moth caterpillar showing the distinctive color pattern (click for larger view). The larger, more mature caterpillars cause the bulk of the feeding damage. Photo credit: PJ Liesch, UW Insect Diagnostic Lab
Another variable that could be at play this year is a beneficial fungus known asEntomophaga 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.
What can be done about spongy moth?
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.
Burlap band around an oak tree. Large spongy moth caterpillars feed mostly at night, so burlap bands offer a convenient daytime hiding spot. Caterpillars can then be brushed into a container of soapy water. Photo credit: Bill McNee, WI-DNR.
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.
