Tag Archives: Insect Pests

Fall Armyworms: A Late Summer Surprise in Wisconsin

It’s been hard to miss the recent news headlines about fall armyworms “FAW” (Spodoptera frugiperda). States east of the Rockies have seen historical outbreaks of this insect in 2021, including a bit of fall armyworm activity here in Wisconsin. In some cases, the caterpillars have decimated entire crop fields or home lawns overnight before marching onwards in search of “greener pastures”.

We usually don’t see much of the fall armyworm in Wisconsin and it’s primarily a pest of warmer areas, such as the gulf coast states.  The FAW is native to tropical and subtropical parts of the western hemisphere and the larvae (caterpillars) can feed on dozens of different types of plants—ranging from field crops to fruits and vegetables and even turfgrass. They can be particularly important pests to crops such as corn, grains, and alfalfa.

Fall armyworm caterpillar
Fall armyworm caterpillar. Photo credit: Frank Peairs, Colorado State University, Bugwood.org.

The fall armyworm can’t survive the winters in the US, other than the southernmost areas (e.g., southern Texas and Florida). However, in spring and summer the adult moths migrate northwards and lay eggs. Over the course of many generations and subsequent northward migration, fall armyworms can make it to the upper Midwest and even parts of southern Canada. Historically, fall armyworm has rarely been a notable pest in Wisconsin or the upper Midwest—it simply arrives too late or in too small of numbers to be a concern. To a certain extent, every year is a roll of the dice, but the odds are usually in our favor in Wisconsin and other northern states.

Fall armyworm adult moth
Fall armyworm adult moth. Photo credit: Lyle Buss, University of Florida, Bugwood.org.

This year has been different though, with large numbers being spotted northwards and reports of significant damage coming in from nearby states such as Illinois, Indiana, Iowa, and Ohio. Many other states ranging from Kansas to the mid-Atlantic region have also been impacted in the later parts of summer. While there have been scattered reports of fall armyworm damage to field crops in southern parts of Wisconsin, the lateness of this pest’s arrival and our declining temperatures have likely spared us from the widespread damage seen in other states.

Under hot conditions (e.g. temps in the 90’s), the life cycle of the fall armyworm—from eggs to adult moths—can take only a few weeks. However, fall armyworms are “cold blooded” creatures and cooler temperatures slow down their growth and development. Depending on how chilly it is, their life cycle can be “stretched out” to take 60 days or longer—leaving them much more vulnerable to predation, parasitism, or exposure to frosts.

Eggs of the fall armyworm
Fall armyworm eggs from a residential yard. Photo submitted to UW Insect Diagnostic Lab in September, 2021.

One study* found that fall armyworm eggs didn’t hatch at all if temperatures were cool enough (though not particularly chilly by Wisconsin standards). That particular study simulated daytime/nighttime temperatures of 21˚C (70˚F) and 8˚C (46˚F)—temperatures that are “in the ballpark” for many parts of Wisconsin by mid-September and are often considered downright “pleasant” by Wisconsinites.  Eggs held at warmer temperatures in the experiment hatched just fine.

For eggs that did hatch this year in Wisconsin, cool temperatures also could have helped us out by slowing down their development. As they grow, fall armyworms pass through six sub-stages (instars). The early instar caterpillars are so small, they simply can’t eat much and cause little damage. It’s not until FAW caterpillars become more mature fifth and sixth instars that they really start to chow down and cause significant damage to plants. Thus, falling temps could help prevent the fall armyworm caterpillars from making it to the destructive late instar stages and could also leave them more exposed to a variety of threats.

Chart showing quantity eaten by fall armyworm larval instars.
Graphic representation of the amount eaten by fall armyworm caterpillars in an early USDA experiment. Early instar caterpillars eat little compared to late instars. Cool temperatures limiting their development could help prevent damage by the FAW. Credit: USDA Technical Bulletin No. 34

The fall armyworm outbreak of 2021 could very well be a “once every few decades” type of event, and our northern location likely helped us avoid the significant problems seen in other states. However, if changing climate gives the fall armyworm a “head start” by overwintering farther north, it’s possible that we could see more of this pest in Wisconsin in the future.


*Barfield, Mitchell, and Poe. 1987. A Temperature-Dependent Model for Fall Armyworm Development.  Annals of the Entomological Society of America. 71(1): 70-74.

The Summer of Springtails

With the unusually dry weather we’ve had in Wisconsin this year, I would not have predicted that springtails (Collembola), would have been one of the commonest samples at the UW Insect Diagnostic Lab this summer. In general, springtails tend to thrive under damp conditions. These tiny (<1/10 inch long), insect-like creatures are commonly associated with leaf litter, rich soil, compost, and mulch or plant beds outdoors where they feed on fungi and decaying plant materials.

Photograph of a springtail on a log.
A tiny springtail on a decaying log outdoors—a common place to spot these creatures. Photo credit: Melissa McMasters via Wikipedia.

Springtails are ubiquitous and can be found around the globe under a wide variety of conditions. In many parts of North America, you can even bump into large numbers of dark-colored springtails (snow fleas) bouncing around on the snow in winter.  Springtails are a fairly diverse group with over 15 Families in North America, so there’s a lot of variability in terms of the color and overall appearance.

Indoors, it’s usually too dry for springtails to survive or reproduce unless damp conditions exist. Occasionally, they can be found indoors in association with overwatered houseplants, new construction (due to residual moisture in construction materials), plumbing leaks, and other moisture issues. In many cases, when springtails are spotted indoors, they originated outside and simply snuck in but perish shortly thereafter due to desiccation.

Chart shwoing drought conditions in Wisconsin in June of 2021.
Drought conditions in Wisconsin—June of 2021. Much of the state experienced drier than usual conditions this year, with some parts experiencing extreme drought conditions. Photo credit: US Drought Monitor Program: https://droughtmonitor.unl.edu.

With the lack of precipitation and this year, I haven’t gotten the impression that springtails are necessarily thriving outdoors. However, the hot and dry conditions are likely forcing springtails to sneak into structures or other spots that may be slightly damper, darker, and cooler. When they do make it inside, springtails are often spotted near moisture sources—kitchen or bathroom sinks, showers or bathtub drains, and basement floor drains. Some good news is that springtails are completely harmless to humans, pets and homes, and in most cases won’t survive long.

Hundreds of springtails along a home's foundations.
Hundreds of springtails along a home’s foundation. Photo submitted for a recent case at the diagnostic lab.

The following tips can be helpful when springtails are a problem indoors:

  • Monitor both indoors and outdoors—Visually inspect for areas with lots of springtails and see if potential entrance points into structures are nearby. Indoors, glue board traps from the hardware store or garden center can be helpful tools for monitoring and management.
  • Manage vegetation near the foundation of homes and other structures—Plants near a foundation can hold in moisture and create good habitat for springtails, spiders, millipedes, and insects that can wander indoors. Having a gap of 1-2 feet or more between a structure and plants increases airflow and can decrease humidity.
  • Avoid excessive mulch—A thick layer of mulch can hold in moisture and may create good habitat for springtails and other arthropods.
  • Physical exclusion—Sealing up potential entrance points into a structure can help with many pests, including springtails. Caulk and expanding insulation foam can be helpful in this regard. Inspect and replace weather stripping to make sure that windows and doors seal properly. Pay particular attention to high priority areas such as along the foundation, around window and door frames, and basement window wells.
  • Keep indoor humidity low—Running a dehumidifier and/or air conditioning can help decrease moisture and make it harder for springtails to survive. Repair or correct any drainage, plumbing, or moisture issues that may be increasing humidity levels within a structure. Allow houseplants to dry out between waterings; soil that is kept damp can be a potential hangout for springtails indoors.
  • Insecticides—Since springtails sneaking in from outside don’t survive long, spraying indoors is generally not warranted or helpful. If springtails are highly problematic, treating cracks and crevices on the exterior of a structure may decrease the number making it inside but won’t eliminate them outright.

Fungus Gnats: Tiny Flies Around Your Houseplants

With winter’s arrival, the caseload at the UW Insect Diagnostic Lab is low, although ID requests continue to trickle in. Recently, dark-winged fungus gnats (Family Sciaridae) are one of the commonest insects I’ve seen at the lab. These tiny (1/16 – 1/8 inch long), dark-colored flies are almost always associated with houseplants.

Adult fungus gnat showing the distinctive, curved “Y” fork in the wings.  Photo Credit: B. Schoenmakers, via Wikipedia.

Despite their small size, adult fungus gnats can readily be identified by their wing venation. Using a bit of magnification, the translucent wings of these insects possess an oblong, rounded “Y” towards the tips of the wings. While the delicate adults gnats may catch our attention, the larvae are at the root of the problem. The slender, worm-like larvae possess a dark head capsule and live in moist environments.

The connection with houseplants has to do with the feeding habits of the larvae. Fungus gnat larvae generally aren’t plant pests, but scavenge on fungal spores. If the soil of a houseplant is kept too moist, it can create ideal conditions for them. An abundance of fallen decaying plant materials (leaves, etc.) on the soil surface can also contribute. Some of the commonest sources of fungus gnats (and other houseplant pests) are “outdoor” plants which were brought indoors in the fall.

Adult fungus gnats may be a nuisance, but are harmless and short-lived. They will often be observed near potted houseplants (where females can lay eggs) or at nearby windows. When it comes to fungus gnat problems, the following approaches can be helpful:

Sticky card traps can be a useful non-chemical approach for monitoring fungus gnat populations over time and capturing adults. Photo credit: PJ Liesch, UW Entomology.

1) Dry out the Soil: Fungus gnat larvae thrive in damp soil or potting mix. Cutting back on waterings is often the single most important step in dealing with fungus gnats and the mere presence of these gnats is often an indicator of overwatering. Allowing the soil to dry out between waterings decreases survival of the larvae. For succulents and other popular plants that are tolerant of dry conditions, cutting back on waterings can often correct a fungus gnat issue over time without any additional steps.

2) Sticky Card Traps: These traps look like bright yellow index cards, but are covered with a sticky adhesive. When purchased at local hardware stores, garden centers, or online, they usually include small stakes to help place these cards into pots. The adult fungus gnats are attracted to the color of the traps and can become stuck in the adhesive. Used alone, these traps will not eliminate fungus gnats. However, these traps are a non-chemical way to capture adults and can help when used in combination with other approaches. If you have significant fungus gnat numbers, swapping out sticky cards on a regular basis can help monitor for trends over time.

3) Soil Monitoring: If you have lots of houseplants, determining which plant(s) is/are harboring the larvae can be a challenge. One helpful approach is to place slices of potato on the soil surface of potted plants. If larvae are present in a given pot, some may come to the surface to feed on the readily-available starches of the potato. Checking the potato slices for the presence of the dark-headed larvae can help determine where to focus your attention.

4) Cultural Practices: If fungus gnats are really bad, it can sometimes make sense to discard a problematic plant to prevent it from serving as a continual source of fungus gnats. Alternatively, a favorite plant could be washed to bare roots and re-potted in fresh potting mix. When obtaining new plants, it can be helpful to isolate and monitor new plants for fungus gnat or other insect activity before placement amongst other houseplants.

5) Soil Treatments: With severe or persistent infestations of fungus gnats or in cases where watering can’t be reduced, treating the soil is an option to directly target the larvae. The commonest option is to use a product labelled for use on houseplants containing Bacillus thuringiensis israelensis (Bti). These products are based on a naturally-occurring bacteria that is toxic to fly larvae. Such products can be applied as a liquid drench (e.g., Gnatrol) or as granules (e.g., Mosquito Bits) to the soil surface and watered in. Such products won’t eliminate fungus gnats overnight, but can be effective over the course of several weeks.

6) Sprays for Adult Fungus Gnats: Spraying for adult fungus gnats with is not generally effective or recommended.  Targeting the adults will only provide temporary relief and it is much more effective in the long-run to target the larvae at the heart of the problem.