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A Wandering Horde of…Millipedes

It’s a dark, overcast night as the horde emerges from the nearby woods. There’s no real coordination, but thousands of them—perhaps tens or even hundreds of thousands—seem to wander aimlessly through the yard.  Some approach the darkened farmhouse and a few even manage to make it inside…

If this were and episode of The Walking Dead, the protagonists would be in a tough spot, but we’re not talking about zombies in this case.  Instead, the topic is millipedes, which have been surprisingly abundant this summer in parts of the Upper Midwest.

Greenhouse millipede.
Greenhouse Millipede (Oxidus gracilis). Photo Credit: Joseph Berger, Bugwood.org

Most everyone is familiar with millipedes.  They technically aren’t insects, but they are related as demonstrated by their segmented legs and “crunchy” exoskeleton (both are types of arthropods).  These multi-segmented, worm-like creatures can be common in damp areas and are perhaps most recognizable by their slow walk and their habit of curling into a spiral when disturbed.

Unlike the zombies portrayed in on TV, millipedes are really quite harmless.  Some millipede species have been documented as minor crop pests, but in the grand scheme of things, I mostly think of millipedes as being beneficial detritivores.  Millipedes feed on decaying plant materials and they return nutrients to the soil.  Their feeding also breaks down plant materials into smaller pieces, allowing microbes to more easily assist in the decomposition process.  Millipedes can be especially common in damp locations with abundant plant materials: compost piles, rich soil with high organic content, mulch beds, wooded or prairie areas, CRP land, lawns with a heavy thatch layer, and similar.

Millipede curled up in a spiral
A millipede curled up in a classic defensive posture to protect its legs. Photo credit: Joseph O’Brien, USDA Forest Service, Bugwood.org

While mostly beneficial, millipedes can occur in very high numbers under the right conditions and can be a nuisance when they seem to suddenly appear in yards and homes.  Hopkin and Read’s The Biology of Millipedes (1992) describes situations where massive millipede hordes have covered acre after acre of farmland or stopped trains, quite literally, in their tracks.  The Midwest does see large masses of millipedes on occasion and it was a particularly busy year at the UW Insect Diagnostic Lab for calls about these creatures.

The reasons behind millipede mass migrations aren’t fully understood, but moisture is often noted as a common factor.  Other potential reasons range from general weather patterns to habitat disruption, competition, and reproduction.  When millipedes do move about, many species shun the sun and prefer to move at night or during very overcast days.  When they encounter a building, millipedes can sneak inside, although this is really accidental—it’s too dry for them to survive indoors and they typically die within a day or two.

Millipedes on a home's foundation
Thousands of millipedes along a resident’s home. From a case submitted to the UW Insect Diagnostic Lab this summer.

Millipedes can be frustrating when mass migrations occur as there’s not much that can be done to completely stop them.  It’s not uncommon to have cases where hundreds or thousands of of millipedes crawl onto the foundation or siding of a home every night.  If they mostly stay outside, that’s one thing, but this summer I’ve had multiple cases where large numbers of millipedes (hundreds) had snuck under a building’s siding and then rained down through ceiling light fixtures.  This sounds like something out of a sci-fi film, but if you were trying to sell your home it could be a real-life nightmare scenario.  In such cases, there simply isn’t any way to make the millipedes magically disappear.  Insecticides may be tempting but only help to a certain extent because more millipedes can simply show up the next day.

If you’re staring down a millipede horde, one of the most important approaches is physical exclusion.  Inspecting the exterior of a home and physically sealing up cracks, crevices, and other potential entrance points with caulk, expanding foam, or new weather stripping can be a chemical free, long-term solution to at least keep millipedes outdoors.  Because millipedes prefer damp areas with decaying plant material, keeping landscape pants, fallen leaves, and thick layers of mulch away from the foundation of a home could also help reduce hiding areas for millipedes.

Luckily, millipede mass migrations eventually run their course and quiet down on their own.   This year, I saw a spike in millipede reports starting in mid-June and running into early August before subsiding.

Wisconsin’s Top Insect Trends of 2015 (#5 – #1)

In this post, we’re continuing to count-down 2015’s top insect trends in the state.  This is the final post in a three part series.  Part I (2015’s diagnostic lab statistics) can be found here and Part II (Top Insect Trends Numbers 10-6) can be found here.

5) Pollinators
With all the headlines about bees, it’s not surprising to see pollinators in the top insect stories again in 2015.  Similar to other years in the recent past, honeybees and other pollinators have been facing declines.  Unfortunately, Wisconsin saw some of the highest honeybee losses in the country, with over 60% colony loss during the 2014-2015 period.  Some good news over the past year has been the development and release of pollinator protection plans.  A federal pollinator protection plan was released in May with the goals of reducing honeybee losses, increasing the population of Monarch butterflies, and increasing pollinator habitat.  In addition, a Wisconsin pollinator protection plan was announced in 2015, and was just released in January of 2016.   Due to the recent declines and their importance to agriculture in the state and nation, pollinators will continue to be in the spotlight in the future.

A ground nesting bee (Colletes sp.) near the stump of the former President's Oak on the UW-Madison campus. Photo Credit: PJ Liesch
A ground nesting bee near the stump of the former President’s Oak on the UW-Madison campus. Photo Credit: PJ Liesch

4) Spring caterpillars
An unexpected insect trend in the spring of 2015 was the surprising abundance of a number of caterpillar species feeding on plants in the landscape.  Caterpillar species, such as the humped green fruitworm, speckled green fruitworm, eastern tent caterpillar, forest tent caterpillar, gypsy moth caterpillar, and the euonymous caterpillar are typically present to some extent, although their numbers have been low the past few years.  For a number of potential reasons, these species had a great spring and during a period in May and June, caterpillars made up roughly 30% of the cases coming in to the diagnostic lab.  Weather patterns (i.e., rainy weather) and natural predators/parasites/diseases can have significant impacts on caterpillar populations each year, so it’ll be interesting to see if we’re faced with a plethora of caterpillars again in 2016.  Additional details of this story were featured in a blog post last June.

3) Viburnum Leaf Beetle
In terms of a new emerging pest with the potential to impact a commonly planted landscape shrub, Viburnum Leaf Beetle is near the top of the list.  As of late 2014, we only knew of a single infested viburnum bush in northern Milwaukee County, which raised the question: is the infestation small enough to contain and/or eradicate?  Some ground truthing this past spring identified many new infestations in SE Wisconsin, in many cases miles from the original site.  At the moment, the viburnum leaf beetle seems to be centered around the four county area where Milwaukee, Waukesha, Washington, and Ozaukee counties meet.  While this insect only feeds on viburnums (and related plants like Arrowwood), the damage can be significant.  It may be some time before this pest spreads elsewhere in the state, but if you have viburnum plants in your yard in SE Wisconsin, be weary!  Additional details of this case were featured in a post last June.

VLB Damage
“Skeletonizing” feeding damage from adult viburnum leaf beetles. Photo courtesy of Paul Weston, Cornell University, Bugwood.org

2) Brown Marmorated Stink Bug
Populations of the invasive Brown Marmorated Stink Bug increased dramatically in 2015 and this insect takes the overall #2 spot in this list (up from #8 last year).  This invasive species was first spotted in the state in 2010, and each year a handful of lone adults have been found in Wisconsin.  In the fall of 2015, we had more sightings of BMSB (30+), than in the past 5 years combined! (Spoiler: this trend has continued into early 2016)  At this point, the “hot spots” in the state are: Dane County, the greater Milwaukee area, and the Fox River Valley.  In addition to being an indoor nuisance pest, BMSB can also feed on and damage a wide variety of plants in home gardens, agricultural fields and orchards.  In other places in the country, the first reports of plant damage have typically been noted ~3-5 years after the initial detection of this species.  With that said, 2016 could be the year that BMSB really takes off and starts wreaking havoc for gardeners and agricultural growers alike.  Additional details of this case can be found in this post from last October.

1) Magnolia Scale
While scale insects have already been mentioned in the “sucking insects” section (#9 on the list), one species in particular, the Magnolia scale (Neolecanium cornuparvum), seemed to stand out amongst all other insects in 2015.  This species is often present in low numbers in the state, but the conditions must have been perfect for their populations to explode last year.  During the months of June and July, reports of Magnolia scale were coming in on almost a daily basis.  Being one of the unusual scale insects, Magnolia scale adults look more like a fungus than an insect (note the whitish blobs in the image below).  Not only did this bizarre looking species pummel Magnolia shrubs and trees in many parts of the state, but the honeydew produced by these insects rained down below, attracting ants and yellowjackets and leading to the growth of unsightly black sooty mold.  A number of predators, parasites, and diseases typically keep Magnolia scale in check, but with the extremely high infestations noted last year, it’s likely that we’ll continue to see some Magnolia scale activity into 2016.  If you experienced magnolia scales first hand, there’s a helpful factsheet available here.

Magnolia twig coated with whitish, fuzzy magnolia scale adults.