What Fun, Gus.

I have been bouncing e-mails back and forth with a friend of mine about his Kaffir Lime. It is strictly an indoor tree due to the weather conditions he has, apparently it is always windy and rips the leaves off of the Citrus trees. He has experienced an array of different problems, Spider Mites, Mealies, pruning issues, but the latest e-mail had to do with a new problem. He wrote: "I am also noticing what appears to be some tiny flies...almost like fruit flies that are hanging around the soil level.
I love answering all kinds of questions, but it is nice to get an easy one occasionally. I sent a message back letting him know he has Fungus gnats (Bradysia species).
If you could see one under a microscope, they look like this:



These things are small, mosquito-like insects, often found in homes and offices, usually in the vicinity of houseplants. Adults are 1/8 inch long, delicate, black flies with long legs and antennae. There is a distinct “Y-shaped” pattern on the forewings. A great way to monitor and see them for yourself is to use yellow sticky cards. The adults are attracted to yellow and will be captured on them. This may be helpful in mass trapping adult females, which will also help in reducing the number of larvae in the next generation.
Fungus gnats are prevalent during most of the year, but they develop significant populations in Winter and Spring or when the weather has been cloudy and overcast for a number of days. In a home setting they can appear at pretty much anytime.
Fungus gnats are typically harmless to healthy plants and people. The adults are insects that do not bite, but can inflict extensive damage to seedlings,cuttings and young plants. The larvae also feed on the developing callus of directly stuck cuttings, delaying rooting. Fungus gnat larvae usually are located in the top 2 to 3 inches of the growing medium, depending on moisture level. The larvae are wormlike and translucent, with a black head.



They can often be found under decaying plant material on the soil surface. Their presence can be readily detected by placing a small piece of potato or carrot on the surface for a couple days and then observe it with the aid of a hand lens. Larvae also produce thin "webs" on the soil surface which can become obvious when droplets of moisture collect on them. Spraying a fine mist of water on the soil surface can aid in this.
The fungus gnat's life cycle from egg to adult may be completed in as little as three to four weeks depending on temperature. Eggs are laid in cracks and crevices in the media surface and mature in four to six days. During their seven to ten day life span, adult females may lay up to 200 eggs.
Control of fungus gnats can be difficult but not impossible. Proper water management is crucial. The most important strategy is to allow the growing medium to dry between waterings, especially the top 1 to 2 inches. The theory is, the dryness of the growing medium will decrease survival of any eggs laid and/or larvae that hatch from the eggs.
Insecticides may be necessary if fungus gnat problems persist several weeks after watering practices have been adjusted. The most effective treatments are those that are persistent; killing the adults for up to three days. A number of pyrethroid based insecticides, with extended persistence, are available for use on houseplants. Please make sure you read the label and it has listed the pest and the plant that you are using it on, this is the law!
The use of short persisting contact insecticides such as those containing soaps, oils, and neem oil, do not provide sufficient long term control of fungus gnat adults and require repeat applications at short intervals (usually every couple of days) to work.
I mentioned earlier about the use of the yellow sticky cards helping to reduce female populations. And for something else to remember, the larvae in the growing medium will not be directly affected by any insecticides applied to kill adults.
Unless you have a really bad infestation, these creatures will not cause enough noticeable harm. Just watch your watering, allow the medium to dry out. Some things to watch out for are; plants with succulent stems, such as geraniums, sedum, coleus and poinsettias, these are especially prone to injury and can suffer serious losses. As the young feeder roots and stems are damaged, the affected plants wilt. Leaves may turn yellow and drop.
Hopefully you will not ever have a major infestation, Though, I am pretty sure at some time or another you will have some of these guys come for a visit. If the soil is moving like an earthquake or the plant starts to fly around, you may want to consider losing that particular plant.
Happy Growing!
Darren