You’ve all heard the old adage, “Your enemy’s enemy is your friend.” Well, it’s never truer than in the world of indoor gardening! Savvy growers have been using “predators” for years. A predator is just another name for a beneficial insect – an insect that feeds on the pests you hate! Usually Mother Nature keeps everything in check. Indoors, you have to take the role of Mother Nature, otherwise a rogue pest can breed uncontrollably and quickly overwhelm your garden. With this in mind, we thought we’d put together a regular feature on predator insects. After all, it’s good to know who your friends are!

Hypoaspis Miles vs. Fungus Gnats

Seeing the first signs of fungus gnats? Allow us to introduce you to a good friend of ours: Hypoaspis miles. These guys are truly fantastic predatory insects all gardeners need to know about. This tiny mite is less than 1mm in size and resides in the top few inches of the growing media preying on a variety of soil organisms, but is especially useful for the control of fungus gnat larvae (aka Sciarid fly larvae).

What are Fungus Gnats?

The most common root pests that lurk in your growing media are without doubt the larvae of fungus gnats. These pesky little insects will usually appear in your garden as harmless looking small 3-5mm black flies. True, the black files themselves won’t actually do any direct damage to your plants, it’s their habitual egg laying that’s the problem. The flies are attracted to wet growing media, particularly organic substrates like potting soils and coco coir or anything with algae growing on it. After mating the female flies can lay anywhere between 50 and 200 eggs which hatch in two to three days. 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 period 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.

The larvae move through the upper surface of the growing media feeding on decaying organic material, algae and fungus but more importantly they 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.

Hypoaspis miles?

Hypoaspis are predators that 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’t eat everything!

Hypoaspis miles is light brown in color with a darker V-shaped making on its back, this dark V is due to the slightly darker dorsal shield on top of a slightly lighter colored body. They mostly live in the upper surface of growing media. In compacted growing media that may be only the first half inch but in looser more open growing media that may roam down two inches below the surface or more. Hypoaspis prefer growing media high in organic matter such as peat or coco coir but they will happily establish populations in rockwool, perlite and expanded clay pebbles 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.

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.

Hypoaspis are ferocious eaters of fungus gnat larvae! Hooray! An astonishing eight first instar larvae can be consumed by adult Hypoaspis each 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 to target is the young developing fungus gnat larvae. When these are removed fewer and fewer make it to the pupae stage which mean 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. Read it and weep fungus gnats!

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.

Geeky Fact
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.

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.

Left to right, 17 ounce tube, open tube showing peat/vermiculite mix, one low infestation dose

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.

Top Tip!
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 containing Bacillus thuringiensis israelensis (Bti). This bacteria kills the active larvae but not Hypoaspis. Another compatible bio-control is the beneficial nematodes Steinernema 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.

In a nut shell

Hypoaspis miles eat:

1) Preferentially; fungus gnat larvae and thrips pupae

2) Secondarily; other tiny soil organisms algae and plant debris

They will:

1) Control fungus gnat population keeping the pest to a minimum and over time eradicate them

2) Aid in the control of thrips species that pupate in the growing media

Should be used:

1) When fungus gnat numbers are low

2) When growing media temperatures are above 10°C

Should not be used:

1) As a sole treatment for a fungus gnat infestation, they will not be able to cope with the amount of pests.

2) As a sole treatment for thrips.

3) In conjunction with pesticides before checking compatibility.