Category Archives: Indoor Flies

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.

2018’s Top Trends from the Diagnostic Lab (Part 1)

Each year the University of Wisconsin’s Insect Diagnostic Lab receives thousands of arthropod samples and reports from around the state and region, providing a unique perspective into insect and arthropod trends in Wisconsin and beyond.  This post is the first half of a series counting down the top arthropod trends in our area last year.  The second part will be posted in early February and can be found here.


10) Dagger and Tussock Moths:
A few species of fuzzy caterpillars were surprisingly abundant last year and there’s a good chance you might have bumped into these in your own neighborhood.  Two similar-looking yellowish species, the American dagger moth and the white-marked tussock moth, were extremely common around Wisconsin and were two of the most widely reported caterpillars last summer. Another tussock moth associated with milkweed was also surprisingly common in 2018. With many Wisconsinites growing milkweed to attract monarch butterflies, the black and orange caterpillars of the milkweed tussock moth were also noted in abundance around the state last year.

Fuzzy black and orange Caterpillar of the milkweed tussock moth (Euchaetes egle)
Caterpillar of the milkweed tussock moth (Euchaetes egle). Photo Credit: Katja Schulz via Wikipedia

9) Fungus Gnats:
Pick any spot on a Wisconsin map and 2018 was most likely a soggy year. Understandably, rain encourages insects and other creatures that thrive under damp conditions. Last year’s rains created great conditions for fungus gnats, which became quite abundant by late summer. While fungus gnats are harmless to people and pets, they can be an annoyance if present in high numbers. Fungus gnats thrive in damp organic materials, meaning that rich soil, compost piles, and decaying plants can produce masses of these tiny, dark-colored flies. The larvae of these insects can also be common in the soil of houseplants.  As Wisconsinites brought their favorite potted plants indoors in autumn to avoid approaching frosts, reports of indoor fungus gnats were common.

Small dark coloured gnats captured on a yellow sticky card trap
Tiny (2mm long) fungus gnat adults captured on a sticky card trap near indoor plants in fall of 2018. Photo Credit: PJ Liesch, UW Insect Diagnostic Lab

8) Purple Carrot Seed Moth:
With several new, non-native insects showing up in Wisconsin every year, the impacts of each species can vary significantly. Some exotics, like the emerald ash borer make massive waves, while others cause merely a ripple. The impacts of one of our newest invasive insects, the purple carrot seed moth (Depressaria depressana), are not yet fully known. This European species was spotted in Wisconsin for the first time last summer and the tiny caterpillars love to feed on the flowers (umbels) of plants from the carrot family. Below-ground plant structures (e.g., the taproots of carrots) aren’t impacted, but notable damage to herbs like dill, fennel, and coriander can occur. As a result, this pest may be a concern for seed producers, commercial herb growers, or home gardeners with a fondness for dill and related herbs. The purple carrot seed moth has been reported in 8 Wisconsin counties thus far [Brown, Columbia, Dodge, Kewaunee, Milwaukee, Racine, Sheboygan and Washington Counties], so new county-level reports are encouraged at the UW Insect Diagnostic Lab.

Tiny (<1/4" long) spotted caterpillar of the purple carrot seed moth on dill.
Caterpillar of the purple carrot seed moth. Photo Credit: PJ Liesch, UW Insect Diagnostic Lab

7) Odorous House Ants
Imagine the stereotypical black ants zeroing in on sugary foods at a picnic and you’d have a fitting profile of the odorous house ant (Tapinoma sessile). Of the 100+ ant species in the Midwest, the odorous house ant stood out in spring and early summer last year with its sheer abundance. The UW Insect Diagnostic Lab was flooded with calls about these sugar-loving ants during 2018’s rainy spring, especially when these ants wandered indoors. The spring rains may have forced the ants from waterlogged colonies to seek out higher-and-drier locations, making odorous house ants the most commonly reported ant at the diagnostic lab last spring.

SMall black ant—an odorous house ant worker
Odorous House Ant (Tapinoma sessile) worker. Photo Credit: JJ Harrison via Wikipedia

6) Stink Bugs:
While the Midwest is home to over 50 species of stink bugs, one particular species—the invasive brown marmorated stink bug—stands out to give the rest a particularly bad reputation. If you live in a part of the state with the brown marmorated stink bug, you may have already encountered this species. With its habit of sneaking indoors in the fall, this insect replaced boxelder bugs in some areas as the top home-invading nuisance pest of 2018. This Asian species has made the diagnostic lab’s Top 10 list for several years now and unfortunately doesn’t show any signs of slowing down. In 2018 alone, BMSB was detected in 8 new Wisconsin counties, which hints at potential damage to fruit and other crops in those areas in the coming years.

Adult brown marmorated stink bug
Adult brown marmorated stink bug. Photo Credit: PJ Liesch, UW Insect Diagnostic Lab

To see the rest of Wisconsin’s top arthropod trends of 2018, check out part 2, available here in early February.