Oh, Rats, Fungus Knats!

If you have houseplants, then you will at one time or another have an infestation of fungus knats, especially if you put them out for the summer. These little flying knats look like small black flies that can be bothersome flying around the face and eyes of people. The common greenhouse knat roams freely among interior growing plants and can become very annoying if left untreated. Fortunately, getting rid of them is fairly easy but will take a bit of effort over a few weeks and can be done without the use of insecticides.

Fungus knat adults will burrow down moist soil and eat developing fungus that grows among the roots of the plant. They usually do not harm the plant in any way. They will fly around plants and land on the soils that are on the dry side to lay their eggs. When the eggs hatch, the larvae may nibble on the tips of plant roots as well as fungus in the soil but will not do much harm unless the soil is swarming with them. When the plant is watered and the soil gets wet, the larvae swarm to the surface until the water abates, then they will resume munching, borrowing back into the soil. Once they pupate into adults, they will fly off and continue the cycle. The life span of one of these little flies is only a few weeks, but if there is a large infestation, then the constant hatching can seem like they live forever.

Many times there is a lull in the cycle and they seem to disappear only to come back with a vengeance. This is due to the incubating of the eggs. It can take two to three months for eggs to incubate and hatch. If, however, there is a large infestation, eggs may be hatching all the time and it seems that they never die.

When plants come in from being outside all summer, they probably have eggs of fungus knats in their soil as well as adults that may be harboring in the warmer soil as the temperatures fall. Once inside, they resume their life normally, becoming bothersome as they fly around looking for food sources. They may seem to disappear within weeks of coming in and one may think that the problem is solved. Unfortunately, once the eggs hatch, and the larvae pupate into adults, they appear as if out of nowhere in late winter to early spring. On many occasions I have houseplant owners calling me in March wondering where these little flies came from.

So what can one do when these little beasties are flying around the house, buzzing eyes and faces and being just annoying? The solution is rather easy. If you notice the larvae swarming to the top of the soil after watering your plants, you can leech the soil to drown them and wash them and any other unhatched eggs right out of the soil. Leeching is done by submerging the plant, pot and all, into tepid water until it is ½ to 1 inch above the rim of the pot. Allow the plant to sit in water for two seconds and then quickly pull the pot out of the water and let it drain. Most of the eggs and larva will flow out with the water, giving your soil a good rinsing. If the plant is one that likes to have a moist soil most of the time, then one more leeching will not harm the plant. If it is a succulent in mid winter, I would only recommend one leeching so the plant will not get too much water.

This usually takes away any eggs and larvae that may be lurking in your plant's soils. To get rid of the adults, other methods of eradication are necessary to completely get rid of the infestation. One method is to place sticky, yellow cards around the plants. Hanging them from branches near the soil or attaching them to sticks placed around the pot into the soil will attract the adults when they fly from the plants. They are known as White Fly traps and are sold in most garden centers. The traps consist of bright yellow cards coated with a sticky glue. Most flying insects are attracted to this color yellow and when they fly onto the paper, they get stuck and die. When the card is full, simply remove it a dispose of it in the trash. The glue is not harmful to humans so if the kids get stuck, just wash the areas with soap and water and no harm done. Continue using the traps until you no longer see flies. I usually recommend leaving them in place for a few months just to make sure.

The other method is by coating the soil of each plant with one inch of either course sand or Diatomaceous Earth. You need sand in course texture in order to make it difficult for the insects to craw through it to get to the soil. This is more of a deterrent rather than a way to kill them. Diatomaceous Earth is a fine power consisting of crushed shells mined from the ocean floor. The power is extremely course under magnification. It cuts into the soft bodies of the knats, killing them as they crawl through. It is a very fine powdery dust and must be handled with caution as it can harm lung tissue if breathed in. However, it tends to be a bit pricey but it is a great organic way to rid infestations of many soft-bodied insects, such as ants, that may infest your home. This is extremely effective in ending the cycle of this insect and can remain on the soil without any harm to the plants.

Both these methods are extremely successful ways to rid your plant of a fungus knat problem. However, if you do not put your plants outside for the summer, the chances of infecting them with insects is much less than if you put them out each year. The reason for this is because when the plants are outside, they are in the most perfect environment they can have. These plants are tropical, developing over eons in humid, high light climates that have constant temperature ranges, just like most summers here in the US and most temperate climates. When these conditions change rapidly, as when the plants are brought inside, the plant reacts to this change by decreasing its food production due to the decrease in light levels. Because this weakens the plants, they become mores susceptible to an infestation, not having the strength to fight the insects.

By checking your plants once a week, you can stop any insect invasion before it becomes and infestation. By keeping your plants inside all year and keeping the environmental conditions as close to the same as possible, your plants will remain stable, grow slower and remain healthier throughout their lives. They will age as gracefully as you do and remain a member of the family for many years