Thursday, January 14, 2021

What's This Bug? The Fungus Gnat.

A close-up of a Fungus Gnat
These itty bitty insects are fungus gnats, although they go by many names, including “soil maggots”, “soil gnats”, and, (very commonly) “fruit flies”. People often mistake fungus gnats for fruit flies, but if you look closely you will see that they look more like tiny mosquitoes than any kind of fly. Although they look like mosquitoes, they don’t behave like them. They can’t bite and are utterly harmless to humans. However, they do have a nasty habit of flying around people’s faces, so it’s pretty easy to inhale or swallow one. Unpleasant but not necessarily dangerous.

Houseplants in pots on a windowsill looking our onto a wintry scene.
Many people say that fungus gnats are worse in winter, but they may have been around all year long and you just didn’t notice them. Conditions in the home during winter are perfect for fungus gnats to thrive and people are inside more with their plants, so if you have a problem you will notice it during the cold weather months. And then there is the bringing-inside factor: when the temperature plummets, people scramble to bring in their plants that have been sitting on the porch or patio. It's very common to have hitchhikers on recently-outdoor plants, so always check them carefully before re-homing them.

Tiny black insects, fungus gnats, all over a green leaf.
Fungus gnat adults are harmless to plants – it is their offspring that cause the trouble. The adults lay their eggs in soil and within 3 days the larvae emerge and they are mighty hungry. Their foods of choice are fungi, decaying organic matter, and plant roots and they begin feeding as soon as they hatch. This feeding continues for about 10 days, at which time the larvae will pupate. A short 4 days later, the adults emerge from the pupae ready to reproduce. This short life cycle means that you could potentially have a continuous population of fungus gnats in your plants. When you take into consideration that these insects will thrive all year round in your climate-controlled home, it pays to be on guard and prepared to put down any potential infestations. 

Rows of potted plants on light-colored shelves. It's a greenhouse in Dublin, Ireland. Photo by Soff Garanavano Puw on Unsplash.
Fungus gnats can be a big problem in greenhouses. The conditions (moist & warm) that are optimal for
propagation are also optimal for fungus gnat development. Their larvae will not only damage roots and cause wilting or plant death, their root-eating behavior can open wounds that allow soil-dwelling pathogens in. One small opening can lead to massive damage to crops or plant collections. For more on what to watch out for and ways to control fungus gnats in these situations, check out 
this short but informative article.

 Because the larvae do their feasting underground, the signs of their activity are often hidden until the damage has been done and the plants are visibly suffering. Your best tactic is to be pro-active and take the following steps:

An arm holding a yellow watering can and watering a tall green plant in a green container on a wooden table.
Watch for flying adults - Fungus gnats are not great fliers, so you won’t see swarming like other insects do. What you may see is their sudden appearance as you water or turn lights on. The water will disrupt their hop-flying around the soil and, like other insects, light will draw them to it. Bear in mind that fungus gnats are very small, only 1/8”-1/10” long, and have clear wings and long antenna. If the insects flying around your plants don’t have these characteristics, then you have another insect pest problem. Here’s some more information on how to identify fungus gnats.

Watch your watering – Proper watering is not only important for overall plant health, it is an invaluable tool to control soil-dwelling larvae. Simply put, fungus gnats like living in soggy potting soil. Remember that plants require less water in winter, so your regular watering schedule should be cut back in cold months. Make sure that your plants are draining thoroughly and give the top couple of inches of soil (where the larvae like to dwell) a chance to dry out between waterings. 

A yellow sticky trap with insects on it. It sits in a container of basil.

Use sticky traps – Whether you have 15 or 15,000 plants, sprinkling some sticky traps around your growing area is a great way to find out what insects you have in there. And, with a little math, you should also be able to get at least a rough idea of how many there are. We carry a wide variety of sticky traps; you should be able to get whatever size, shape, and color you want here.

If you are a visual learner, we have a video about fungus gnats that you may enjoy. It’s a cartoon and (I think) rather fun.

An image of tiny gnats flying beside a plant.

If you think you have (or know you have) a fungus gnat problem, don’t despair - there are a lot of ways to get rid of them. Here at ARBICO, we have many solutions, including Beneficial Nematodes non-toxic sprays and soil drenches like Earth Juice® goGnats™, and beneficial insects that prey on fungus gnats. Interestingly, although fungus gnats are not related to mosquitoes, many of the products used to control mosquitoes work for them as well. If you would like more information on ways to control fungus gnats, go to our ARBICO Organics fungus gnats page here.

Submitted by Pam



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