Fungus Gnats and Fruit Flies – How to Tell the Difference

fungus gnats

Tips from an ecologist

Fungus Gnats: A Common Pest

Houseplants harbor any number of animals on the plant itself or in the soil.  I received an unpleasant surprise by way of a philodendron on a north-facing windowsill outside my office.  After many years, there came an outpouring of small black flies that mostly hung around the pot but also explored the conference table nearby.  I had to move the plant down the hall to spare my colleagues the too-frequent approaches of these tiny explorers.

These flies belonged to one of the many species of fungus gnats, probably a dark-winged fungus gnat (family Sciaridae), and after a while, their numbers subsided.  Several months later, in the new location, they appeared again, but the people now in charge of watering the pot solved the problem by simply forgetting to water it.  The plant died and that was that.

The Life of the Fungus Gnat

The larval gnats are quite small and, as the name implies, feed on the fungi that inevitably grow in the pots of houseplants.  Yes, there are many kinds of fungi in normal, healthy soil, and you should welcome them.  Root fungi (mycorrhizae) can help plants absorb water and nutrients.  Others decompose the organic matter in the soil, releasing inorganic nutrients that your plants need.  Some consume bacteria and other fungi (it’s a tough life among the decomposers), while a few attack tiny soil animals.  And there are still others doing things we don’t understand.  Fungi tend to grow in thread-like strands of cells called hyphae, and the gnat larvae are the right size to feed on fungal hyphae.  If a female gnat lays eggs in a pot of soil, the larvae all hatch from their eggs at about the same time, feed on the fungi at essentially the same rate, and mature nearly simultaneously, leading to an emergence of adults that appears to come out of nowhere.  Then it’s breed, cycle, repeat.

While there are a few species of fungus gnats that attack and damage plants, these are unlikely to be the ones emerging from a typical pot. 

The gnats can be annoying, as were the ones in my plant. Any normal person would prefer not to have them flying around our homes, faces and food.  According to Wikipedia (the font of all knowledge) they can be controlled by adding a biological control agent, the nematode Steinernema feltiae.  These roundworms parasitize the immature stages of many kinds of flies in the soil, including the larvae and pupae of fungus gnats.  I’ve never tried to apply nematodes for this problem, but there are several vendors willing to sell them to you for a range of prices.  You might want to check the information about these worms from Dr. Anthony Shelton at Cornell University’s College of Agriculture and Life Sciences, where entomologists have been studying insect control for many decades.

Be sure to get the right kind of nematode.  Some will attack plants, pets, or even people.  But nematode parasites tend to be host specific, and S. feltiae attacks only flies. Plant supply stores will provide the correct variety.

What About Fruit Flies?

Sometimes we see some small flies in our houses that look like miniature houseflies, but are tan instead of black.  These are fruit flies, or more properly, pomace flies or small fruit flies, members of the family Drosophilidae (entomologists reserve the name fruit flies for the family Tephritidae, which includes some major plant pests like the Mediterranean fruit fly).  The fruit flies that show up in our houses are not emerging from your plant pots, but are instead investigating any fruit (especially overripe fruit, including tomatoes) as a possible place to lay their eggs.  Their larvae love fruit pulp, and will mature in about two weeks.

Pomace flies are not harming your plants, and will land on leaves and stems only incidentally.  They might take nectar from flowers, but they won’t stay too long.  Small flies tend to be annoying, but they are not coming from your pots.  They are either strays hoping to find something good, or they have already found what they want in your kitchen.  Leaving fruit in the open will attract fruit flies. Storing it in airtight containers can thwart fruit flies.

One last thing: fruit flies do like alcohol and will seek out a glass with any amount of alcoholic beverage in it.  The flies drown quickly, but you might not want to have even dead flies as a garnish.

Owen Sholes is a retired professor of ecology and author of the book “Stopping By Woods: Robert Frost as New England Naturalist”.

Stay-at-Home Houseplant Guide

In the name of practicing social distancing, we are all spending more time at home to help stop the spread of the coronavirus. If you’re like me and trying to do literally anything to keep busy, why not take the opportunity to spend even more time with your houseplants? When you’ve binged all you can binge and baked all the bread you can stomach, check out the list below. At the very least, you can make sure your indoor jungle is thriving while outside is off-limits.

Inventory and Inspection

Just how many plants do you have in your house, anyway? Every time I check there seem to be a few more than I remember. Take a pen and paper, or use your favourite notes app on your phone, and make a list of your plants. Now is a great time to look up all of the names and check out the individual care instructions. When you’re making your list, it’s also a good idea to take a look at the leaves, stems and soil. Check out often neglected spots like under the leaves, the spots where leaves meet stems and soil surfaces. If you notice any pests, webbing or strange spots try and resolve that issue as soon as possible.

Move Plants for Spring

It’s warming up in North America, and summer is around the corner. If you have air conditioning that will be in use soon, consider moving plants away from air vents that can shock them. If you’ve got hot water radiators in front of windows that you’re turning off, consider reclaiming that prime plant real-estate as well. If during your plant inspections you noticed signs of etiolation (stretching due to lack of sunlight), find a sunnier spot. Likewise, consider moving plants out of direct sun that might burn as the days get longer. Resist the urge to move all the furniture and redecorate the entire house. Or go for it. You have time.

Tidy Up and Dust

While dirt around the roots of your plants is good, dirty leaves are not. Houseplants with larger, broader leaves can tend to be dust magnets over time. There are leaf shine products that are available, but they can do more harm than good. Grab a soft towel or an old t-shirt and gently wipe down leaves. If you have delicate plants or succulents use an old soft toothbrush. Dust buildup can get in the way of your plant absorbing the sun it needs. Take this opportunity to remove dead leaves and wipe down pots and drip trays as well, keeping an eye out for pests or loose change. We’ll all need it once this thing is over.

Repot or Pot-up

Did you notice roots poking out of the bottom of your pots? Long roots on your cuttings sitting in water? If you happen to have soil and pots handy, take the time to move your plant into a bigger pot. If you noticed your aloe pot overflowing with pups, it can be a fun afternoon spent splitting them into their own pots.

If you only have fresh soil or amendments available, you can still help your plants out. Trim up to half of the existing roots, add fresh nutrients or soil and place it back in the previous pot. In a pinch, old take-out containers or disposable drink cups both make great temporary pots. Remember to add drainage holes!

Propagate and Prune

Spring is a great time to take cuttings of your plants to propagate them and make more. When tidying up maybe you noticed your pothos vines have long bare sections. Prune it back and take some cuttings to keep it nice and bushy. Cut back other plants that are growing a little wild, much like cutting your own hair – use your own judgement. Save the plant cuttings where possible and plant them. If anyone asks, yes you do need more plants and at least these are free.

Tame Those Unruly Vines

A happy pothos can grow very long and unruly. The aptly named monstera can also very quickly become spread out. A fun project that’s not too hard is to fashion a stake out of scrap wood for a growing support. It is easier to do this while repotting but not too hard to add to an existing pot. Add the stake/trellis and use twist ties or garden twine to hold the vines. If you haven’t got any stakes handy or you’re a vampire and staunchly against the idea, fishing wire or twine can make great supports for a pothos or philodendron to grow across a wall or above a window.

Make More Houseplants

Avocado pits? Grow them. I know everyone is doing extra groceries these days which means – more things to grow! Seeds from fruit, vegetable scraps and even extra herbs can be started in water. Growing from kitchen scraps is a fun and FREE project. I like to keep the ends of my green onions in my kitchen window so I can watch them while I stare outside every five minutes, trying to remember what a patio beer tastes like.

Do you have any more ideas for fun activities to do with your houseplants at home? We would love to hear them, sound off in the comments below and please stay safe during this pandemic. We can get through this by working together.

Bringing Home a New Houseplant

You’ve gotten a new houseplant – Great! Then you get it home and start to wonder “what the heck am I supposed to do with this thing?”. I’m going to go over a few things you can do to ease your (and your plants’) anxiety a little bit. Personally I pick up new houseplants all over the place. From hardware stores, or plant swaps and a number of them came as gifts (read: abandoned by owner). No matter where you got your plant from or what sort of plant it is, here’s a good general plan on what to do when you bring a new plant into your home.

Inspect and Isolate

While new plants are always exciting to bring home there’s always the question of what’s coming along with it. I’ve made the mistake of bringing in a very cute plant only to find days later it was infested – what a nightmare! You can imagine that bringing in bugs or diseases and putting them right beside your existing plants is a recipe for disaster.

An ounce of prevention is always better than the cure, especially when the cure could mean seriously disturbing your other already established plants. Take the time to carefully look at the leaves, where the leaves meet the stems, where the stems meet the soil and the soil itself. Keep an eye out for tiny bugs, webbing, or any signs of rotting. Pests like spider mites or mealybugs are common but a real nuisance once they spread, so if you have a magnifying glass or a zoom on your camera get a real good look.

In transit leaves can get bumped and bruised and that can cause topical damage, this is usually permanent but not risky. It’s good to look for mushy or dark spots on stems and leaves, to make sure nothing is rotting. It’s also not a bad idea to get a good sniff, you might look a little crazy but it’s an easy way to tell if something is rotting in the pot. Remove and all rotting plant material immediately as it will spread.

It’s a good idea to place your new plant somewhere a little further away from your collection for a few days while you keep an eye on it to make sure no stowaways pop up in the short term.

Identify and Research

If you’ve identified any issues during your initial inspection now is also a good time to find out how serious the problem is and what remedies there are to try. If you bought your plant at a nursery often they offer a refund or return if there are major issues.

It’s a good idea to find out what type of plant you have, and once you know, use that to look up what sort of care it requires. Knowledge is power. It’s easy to assume all plants want as much sun and water as possible but that isn’t always the case. Take special note of the type of sun your houseplant prefers, how wet or dry the soil should be, and how often you need to fertilize. I also like to familiarize myself with growth habits and problems that can happen so I can be proactive in spotting them You don’t need to be an expert, but a general idea on what your houseplant needs to thrive will go a long way.

Pick a Spot and a Pot

Now that you know what type of houseplant you have and what conditions it prefers, pick out a spot for your plant. Keep in mind the direction of the sun, how far away from the window and whether there are blinds or curtains filtering the light. The idea here is that you want to find an ideal spot and not move your plant around too much. Consider that once your plant is established and settled, moving it again will mean it has to readjust.

Now is also a great time to consider the soil conditions that your plant needs. Plants that prefer to dry out can benefit from being placed in clay pots that will help wick moisture away. Avoid clay pots for plants that prefer to stay moist or you’ll find yourself watering more often – these can do better in glazed or plastic pots. In all cases make sure your plant pot has drainage or you will definitely see issues in the future.

While plenty of plants show up well potted, some could use a better soil either for the plant or for you and how often you’ll actually remember to water it.

Have Patience

Once you’re sure your new plant is pest-free, and settled into its new home, the best thing is just a little patience. A lot of people get frustrated that their new plant isn’t doing anything at all and start trying all sorts of things to get it to grow, which does more harm than good.

The journey from where it was to your home could have been very tough on your houseplant. It’s in a new place with new light, different humidity and different water. Give some time for it to adjust fully and it’ll start growing soon enough.

Water according to the needs of the plant and keep an eye out for any signs the plant is struggling (browning, rotting, etc.). In some cases plants will drop all their leaves when moved just to grow them all back to fit their new environment better, but hopefully you’ll be aware of that if you’ve done your research.

Resist the urge to disturb the roots or move the plant to new locations constantly which will just continue to stress it out. Have patience and before you know, it will be thanking you with new leaves.