Category Archives: Phenology

Spring’s Coming…and so are the Insects

With daylight saving time beginning over the weekend and warmer temperatures knocking at our door, spring is finally crawling our way.  Last winter is one we won’t soon forget—the season started out mild before temperatures plummeted with January’s polar vortex.  During the coldest stretch, our coping strategy might have involved layers of blankets and reruns on Netflix, but what about the bugs? Questions regarding the winter impacts on insects have been some of the commonest at the UW Insect Diagnostic Lab this year.  There will undoubtedly be some impacts of this year’s polar vortex, although many insect species are well-equipped to deal with the cold.  Before we know it, overwintering insects will become active again in the Midwest and many species will simply shrug off the polar vortex as if it hadn’t happened.  For insects that didn’t fare as well in the cold, high reproductive capacities will likely allow their numbers to bounce back relatively quickly.

Thus, 2019 isn’t going to be insect-free by any means and intuitively this makes sense.  We know that every year insects make it through the winter months and become active as temperatures creep up in spring.  Looking at an evolutionary time scale, this year’s cold snap wasn’t the first time that the species in our area have encountered frigid temperatures before, and many creatures are adapted to survive surprisingly cold conditions.   We might have chosen to block it out of memory, but the Midwest experienced a very similar situation a mere five years ago.  Weather patterns in January of 2014 saw temperatures dip to -20˚F and colder in some spots of the Midwest.  The following summer, we still had plenty of insect activity in the region.

Thermometer from a cold and crisp Wisconsin morning. Photo credit: PJ Liesch, UW Insect Diagnostic Lab

Since we don’t see insects bundling up with tiny mittens and scarves, how do they make it through the winter?  It turns out that insects and other arthropods have a number of strategies to help them survive.  For starters, insects typically have a particular life stage (e.g., egg or pupa) that is more tolerant of adverse environmental conditions, such as freezing or desiccation.  Passing through the winter as a more resilient life stage is a good starting point.

Some of the other strategies are surprisingly similar to humans.  Just like snowbirds heading to warmer states for the winter, certain insects like monarch butterflies and green darner dragonflies migrate southward to avoid the coldest temperatures.  Our official state insect (the honey bee) doesn’t migrate, and instead chooses to remain active.  Honey bee colonies shiver together as an insect version of central heating to keep the inside of their hive a constant temperature.  Other insects simply seek shelter and overwinter in protected locations to avoid the worst of the cold.  Insects like the multicolored Asian lady beetle, boxelder bugs, and the invasive brown marmorated stink bug are fond of sneaking into man-made structures to spend the winter.  If insulation and central heating make homes warm enough for us, it’s plenty warm to prevent insects from freezing.  In more natural settings, such insects might end up sheltering in rock piles or beneath the loose bark of a dead tree.  Those locations might not be as toasty as a house, but they can still provide adequate respite from the cold—meaning that insects using this strategy should have been well protected from this year’s cold spell.  Similarly, many insects and other arthropods spend the winter below ground or on the surface of the ground amongst a layer of insulating leaf litter.  In addition, many parts of Wisconsin had a solid covering of snow by the time the polar vortex arrived, so creatures such as ticks had a thick layer of insulation from the coldest of the cold.

Another strategy utilized by insects is the production of natural antifreeze compounds (specific alcohols or proteins) which serve as cryoprotectants to help prevent freezing within their bodies.  We know that a cup of water will turn to ice at 32˚F, but dissolve salts or other substances in that same water and it will require colder temperatures to freeze it.  Insects producing high concentrations of these cryoprotectants can remains unfrozen at surprisingly low temperatures, similar to a bottle of high-proof spirits kept in a freezer.  Taking it even further, the common black and brown woolly bear caterpillars seem to embrace the cold and actually allow ice to gradually form within their bodies.  This may sound like a fatal mistake, but by regulating the formation of ice crystals on their own terms, woolly bear caterpillars are able to control where ice formation occurs and limit it to specific areas of their bodies to prevent damage.  If the same caterpillars were unprepared and froze rapidly, their cells might burst like a can of soda put into a freezer.

The ubiquitous woolly bear caterpillar (Pyrrharctia isabella) is well adapted to winter conditions. Photo credit: Dave Govoni via Flickr.

And then the ash borer
The insect I’ve gotten the most questions about lately has been the emerald ash borer.  While not native to our area, this invasive pest comes from similar latitudes of eastern Asia and the cold-hardy larvae are fortified with cryoprotectants as they spend the winter beneath the bark of ash trees.  These natural antifreeze compounds have their limitations though, and just like sidewalk salt failing to melt ice on a really cold day, the cryoprotectants only work down to certain temperatures before freezing (and death) occurs.  For emerald ash borer, the point at which freezing spontaneously begins to occur (the supercooling point) is when temperatures dip into the range of -13˚F to -23˚F.  This year’s polar vortex did see temperatures fall into and below that range, which would have killed plenty of emerald ash borer larvae, although the insulating effects of the tree bark likely provided some buffering.

The pale end of a surviving emerald ash borer larva sticking out from its tunnel. When larvae are killed by freezing, they typically become discolored. This sample came from the Milwaukee area in early March, 2019. Photo credit: PJ Liesch, UW Insect Diagnostic Lab

Emerald ash borer populations will almost certainly take a hit from this year’s polar vortex, but it’s not going to be a knockout blow.  Give it some time and the reproductive capacity of this invasive species will allow populations to rebound.  The news reports of cold-induced EAB mortality in early February might have been encouraging, but scientific models from the US Forest Service suggest that to really knock down EAB in the long run, we’d have to experience arctic blasts on a regular basis—news that many Midwesterners aren’t likely to receive warmly.


Further Reading: For a great read on how wildlife survive the winter, check out Bernd Heinrich’s Winter World

Signs of Autumn: Orbweavers

Without looking at a calendar, certain things tell you autumn is approaching—pumpkin spice encroaches upon your food and beverage options, weekends are filled with football, the leaves are turning various hues, and brightly-colored orbweaver spiders adorn the landscape.

A beautifully patterned shamrock orbweaver (Araneus trifolium) on the side of a Northwoods cabin. Photo Credit: PJ Liesch, UW Entomology.

Like the overwhelming majority of spiders, the orbweavers (Family Araneidae) of autumn are harmless to humans.  There are a dozen or more common species in the Great Lakes Region and these can be good sized as far as spiders are concerned—easily over 1” long when you include their legs.  Our commonest species are from the genus Araneus and include the cross orbweaver, shamrock orbweaver, and the marbled orbweaver.  They can be quite common in yards, gardens, on plants, and on your back patio.  Other common species in the genus Argiope (the “garden” spiders) are even larger, spanning over 2” with outstretched legs.  In addition to their large size, flashy “fall” colors and patterns conspicuously adorn these spiders—yellows, oranges, reds, stripes, polka-dots, and more.

Despite the large size, orbweaver spiders are harmless. Photo credit: PJ Liesch, UW Entomology.

Their life cycle is another reason why many orbweavers can be so noticeable in autumn.  Our common species overwinter in the egg sac and the young spiderlings usually go unnoticed as they grow and develop the following spring and summer.  By the time they’ve reached maturity in late summer, it’s mating season and the adults have a month or two to go about their business. During that time, they’re easiest to spot sitting in their large circular webs, which were an inspiration for the children’s classic Charlotte’s Web.

Further Reading:
Unfortunately, most folks never take the time to learn about these beautiful and fascinating creatures.  If you ask someone their thoughts of spiders, feelings of fear, disgust, repulsion, and anxiety might come to mind.  In society as a whole, there seems to be a feeling that spiders are something to be loathed or feared, which really shouldn’t be the case. It doesn’t help when the internet has an abundance of myths and preposterous stories about spiders [here’s a good source to debunk some of those myths].  In the grand scheme of things, you’re more likely to be injured by a pet dog than you are to be harmed by a spider.  If anything, spiders should be considered beneficial as they eat an astonishing mass of insects every year.

If you’d like to learn more about spiders, one of my favorite books for the Midwest is Spiders of the Northwoods by Larry Weber.  There are also some great spider blogs out there; my favorites include: SpiderBytes by Catherine Scott and Arthropod Ecology by Chris Buddle. To this day, two of my all-time favorite spider posts are from Chris Buddle’s blog and have the self-explanatory titles of “Spiders do not bite” and “Update: spiders STILL don’t bite”.

Just like Clockwork

We’re all familiar with phenology—that regular progression of plant and animal life through the seasons—to a certain extent.  We might not stop to think about it in detail, but we recognize the crabapples blooming in spring, the fireflies lighting up the nighttime sky in June and July, and the southward flying geese and rutting deer in fall.  When you think of the 25,000+ insects in the Great Lakes Region, there’s a rich diversity of seasonal patterns to pick up on.  Some insect patterns, like cicadas, katydids, and tree crickets calling during the summer months, are hard to miss—although it can be challenging to decipher exactly who’s making that racket (Hint: here’s your translator).  Others are much harder to pick up on unless you’ve been briefed on the subtle clues.  For example, take the tiny foreign grain beetle (Ahasverus advena) which conspicuously pops up in unexpected places in August, September, and October.

To the naked eye, these tiny (1/16 inch long) brownish insects can be a bit tricky to see and it’s hard to tell if they’re beetles, ants, or something else.  Even to budding entomology students pushing the boundaries of what they can interpret under the microscope, foreign grain beetles and relatives might be jokingly referred to as “little brown nothings” and passed over for easier-to-identify specimens.

Foreign Grain Beetles next to a US nickel. Photo credit: PJ Liesch, UW-Entomology.

Around the UW Insect Diagnostic Lab, foreign grain beetles are one of my favorite samples when they arrive in late summer and early fall as they give me the faintest sensation of what it must feel like to be Sherlock Holmes.  Picture a client coming in with a Ziploc bag of tiny brown insects.  After a cursory glance and before the specimens even make it under the microscope, I ask, “are you in a new home by any chance?”  The standard reply is often along the lines of, “Well, yes—but how did you know?”  A quick check under the microscope and the specimen’s identity is confirmed.  It’s elementary, my dear Watson.

Up close view of the Foreign Grain Beetle (Ahasverus advena). Actual size: ~2mm long. Photo credit: PJ Liesch, UW-Entomology.

How is there such a reliable association with an unexpected source: newly constructed homes, where intuition wouldn’t have you expecting insects?  The secret to this seasonal pattern lies in understanding the biology of the foreign grain beetle and its relatives.  These insects love to feed on fungal spores—often in musty stored grains on farms.  It turns out that during the construction of a new home, residual dampness in construction lumber, plaster, sawdust, and other materials can lead to the growth of a trivial amount of mold.  Like vultures to carrion, these beetles fly in looking for a fungal smorgasbord.  Eggs are laid and entire life cycles can be carried out in the wall void of a new home after the drywall, insulation, and siding are put up.

Fast-forward to late summer and just like clockwork the proud new homeowners suddenly have hundreds of tiny brown beetles crawling out through nooks and crannies, causing immediate dismay.  While this can be alarming, these insects are harmless to people, pets, and the home, and are simply a temporary nuisance.  As the construction materials lose that lingering moisture, conditions become unfavorable for the beetles and activity drops off over time.  Pesticides often aren’t needed as the beetles already face an inevitable demise.  Vacuuming or sweeping them up and running a dehumidifier are often the remedy in fall until the dryness of winter puts a final end to the beetle activity.