Fungus gnats can be a real pain to the indoor gardener. They particularly favour permanently moist conditions so can be the bane of growers using dripper systems. Here we discuss how to get rid of them using natural methods.
When you garden indoors, it's good to know who your true friends are. Outdoors, nature keeps a complex web of predators and prey in balance. The bugs that feed of your plants are, themselves, food for other bugs.
Indoors it's a different story. If left unchecked, pests can breed unchallenged by their natural predators and quickly take over your garden.
So, it stands to reason that, beyond simple preventative measures, one of the best ways of controlling garden pests indoors is to introduce their natural enemy. These "insect predators" target the pests that are munching on your plants without doing any damage to your plants themselves!
Fungus gnats are very common root pests. If you have a microscope or a keen eye then you'll be able to spot them lurking in growing media (particularly organic substrates such as potting soils and coco coir) in the form of tiny larvae. However it's far more likely that the first signs you'll spot are harmless looking small 1/10 - 1/5 inch (3-5 mm) black flies. The flies themselves don't do any damage. It's their egg laying that causes issues for indoor gardeners.
Wet growing media, especially where a layer of algae has formed (rockwool growers listen up!) are particularly attractive to Sciarid flies. Once they've hooked up successfully with a male, the female flies can lay anywhere between 50 and 200 eggs which are ready to hatch in as little as 48 - 72 hours. Once hatched the larvae, which look like tiny maggots, develop through four life stages or 'instars' over a period of two to three weeks. At the end of this time they normally reach around 5mm in length and are translucent white color with a distinctive black head. You can often see their guts throughout the length of the larvae with their previously eaten food moving through.
So what's the big deal with these larvae? Well, in addition to feeding on decaying organic material, algae and fungus the little blighters also eat living plant material, mostly roots but also stem tissue. As a result of this root tissue damage they open the plant up to a variety of disease such a Pythium, Phytophthora and Fusarium. Plants under attack from fungus gnats will often slow down in growth, leaves may discolor, nutrient and water uptake will be affected and in severe cases the plant will wilt and if left untreated it may die.
After two-three weeks of feeding the larvae will enter a pupation stage for three-four days until they emerge as an adult fly. Adult fungus gnat flies can live and lay eggs for between one and three weeks. At temperatures above 77°F (25°C) the complete life cycle from egg to larvae to adult fly takes three to four weeks.
Introducing Your New Friend: Hypoaspis miles.
He's a tiny mite, less than 1/25 inch (1mm) in size and resides in the top few inches of the growing media preying on a variety of soil organisms, but Hypoaspis Miles is especially useful for the control of fungus gnat larvae (aka Sciarid fly larvae).
Hypoaspis are very useful predators indeed. They feed on fungus gnat larvae, springtails, thrips pupae and other small harmful soil insects. When introduced into a growing area that isn't already plagued by fungus gnats, they have a significant impact in reducing and eventually eradicating them. They don't work effectively when pest number are high, mainly because they can only eat a limited amount in any given timeframe!
Hypoaspis miles mostly live in the upper surface of growing media. Under a microscope you'll see that they are light brown in color with a darker V-shaped marking on their backs. (This dark V is due to the slightly darker dorsal shield on top of a slightly lighter colored body.) In compacted growing media that may be only the first half inch but in looser more open growing media they may roam down as deep as two inches below the surface or more. Hypoaspis prefer growing media rich in organic matter such as peat or coco coir but they will also happily establish populations in rockwool, perlite and expanded clay balls too.
Females lay oval eggs in the growing media at the rate of two to three per day. The eggs hatch in approximately six days (at 68°F (20°C) but this figure is dependent on temperature) into six legged white-beige larvae. After two days they enter the protonymphal stage followed by the deutonymphal stage which actively feed for around ten days before becoming adult. The total development time from egg to adult takes between 17-18 days (again at 68°F (20°C)) when they have plenty of prey to eat. Hypoaspis miles adults can live for between 4- 5 months, and usually have an even population ratio of one female to every one male.
As mentioned earlier, temperature plays a huge role in the development time for Hypoaspis, at 59°F (15°C) the development time extend to 34 days and at temperature lower than 50°F (10°C) development almost stops. The mites will recover when the temperature rises, as long temperatures no lower than 10°C are sustained for long periods. Temperature below 46°F (8°C) and above 90°F (32°C) are detrimental to their development. At 75°F (24°C), the Hypoaspis development cycle shortens to around 12 days showing just how important temperature is to their reproduction and growth.
The really good news is: Hypoaspis are absolutely ferocious eaters of fungus gnat larvae! An astonishing eight first instar larvae can be consumed by a single adult Hypoaspis in a single day! Contrary to popular belief, Hypoaspis will not eat fungus gnat eat eggs or pupae. They can attack and kill up to one fourth instar larvae per day, but as fungus gnat larvae reach maturity they can get up to seven times larger than Hypoaspis adults making it possible for them to be attacked but not completely consumed. Some uncommonly large fungus gnat larvae may be attacked but not be killed by Hypoaspis due to their size. However, the most import life stage for our friends to target is the young developing fungus gnat larvae. When these are removed fewer and fewer make it to the pupae stage which equates to a huge reduction in adult flies. This effectively throws a huge spanner in the cogs of the fungus gnat breeding machine, totally disrupting their life cycle, and as the Hypoaspis population grows the fungus gnats get wiped out.
One key attribute that makes Hypoaspis such a good biological control is their ability to last for up to 70 days without food! In the absence of fungus gnat larvae Hypoaspis can sustain themselves on nematodes, soil microorganisms, algae or in some cases decomposing plant debris, but they do not feed on live plants. This handy generalist feeding approach prevents the population from crashing after eating their way through lots of fungus gnat larvae. Once up to a healthy population size they literally form an army of mites scavenging through the growing media surface constantly in search of prey. This makes Hypoaspis an excellent preventative measure as well as a key tool in the fight against existing fungus gnat populations.
Hypoasis are also know to feed on thrips pupae, which often fall from plants and into the growing media before turning into adults. Hypoaspis are not efficient as a standalone solution to thrips control, but they are effective when used in combination with Amblyseius cucumeris (above ground thrips predator) or spray programs.
While they are mostly found below ground during the day, many entomologists have observed Hypoaspis miles climbing up stems and onto foliage low down on plants during humid nights. Here they have been observed feeding on mealybugs, thrips larvae and other organisms.
Practical Tips: Purchasing and Release
When purchasing Hypoaspis miles from your grow store, garden center or specialist supplier they most often come in a carrier material of peat and vermiculite. They are bred on this carrier material in controlled laboratory conditions using the tiny storage mite (Tyrophagus putrescentiae) as a food source. This is then packaged and sold in cardboard tubes ranging in size, the most widely used are either 17 ounce (0.5 liters) tubes with 10,000 Hypoaspis or 34 ounce (1 liter) containing 25,000. For larger growers some suppliers offer larger buckets containing a gallon or more with over 100,000 predatory mites in. The peat and vermiculite mix will contain all stages of Hypoaspis including adults, eggs and immature mites.
Releasing the mites into your garden is easy. Simply sprinkle the mixture onto the surface of the growing media. Application rates vary depending on the level of infestation. Most biological control companies recommend using 30 mites per square foot of ground area as a preventative measure and up to 120 per square foot for treating existing infestations. If you're growing in pots, around one level teaspoon at the base of each plant will be enough as a preventative and one heaped teaspoon for existing fungus gnat problems. A 34 ounce container of Hypoaspis miles retails for around $40.00 (USD) and can treat between 100-200 three gallon pots.
If fungus gnats are a constant problem in your indoor garden, you should introduce Hypoaspis predators one to two weeks after planting into the final pot/system. Top up two to three weeks later with a second application to help build up the Hypoaspis population.
Before distributing the mites into your garden roll the tube back a forth a few times and leave it on its side for ten minutes. This will create a more uniform spread of Hypoaspis mites within the peat-vermiculite mix before dispersal. If stored upright for too long all the mites will try to make their way to the surface of the mix which will lead to the first few applications getting all the goodies.
Hypoaspis are sensitive to some chemical foliar pesticides as run-off can drip onto the surface of growing media. Pyrethrum products and more importantly their synthetic cousin's permethrin and bifenthrin will have longer term harmful consequences on the growth of Hypoaspis populations and should be avoided. Be sure to check the compatibility of your chosen pesticide with your predators before application.
If you are struggling to control fungus gnats in your garden and you need a quick fix help the Hypoaspis predators get on top of the situation try using a bacterial larvicide containingBacillus thuringiensis israelensis (Bti). This bacteria kills the active larvae but not Hypoaspis. Another compatible bio-control is the beneficial nematodesSteinernema feltia. These natural microscopic parasites will infect the fungus gnat larvae quickly and effectively without harming the Hypoaspis mites.
Using sticky yellow traps around the canopy and at the base of the plants is also great idea. Not only will these trap the adults and prevent them from laying eggs, but they can be used as monitoring cards to keep an eye on fungus gnat numbers. Simply look at the cards, make a note of the amount of flies trapped and replace the traps each week. A steady decline in fungus gnat numbers should be observed after releasing the predators.
Hypoaspis miles eat:
- Preferentially; fungus gnat larvae and thrips pupae
- Secondarily; other tiny soil organisms algae and plant debris
- Control fungus gnat population keeping the pest to a minimum and over time eradicate them
- Aid in the control of thrips species that pupate in the growing media
They should be used:
- When fungus gnat numbers are low
- When growing media temperatures are above 10°C
They should not be used:
- As a sole treatment for a fungus gnat infestation, they will not be able to cope with the amount of pests.
- As a sole treatment for thrips.
- In conjunction with pesticides before checking compatibility.