Gnats - Nuisance Pests

Adult fungus gnats are primarily a nuisance pest, although some damage can be done to plants due to larval feeding on root hairs. This type of damage can be serious in greenhouses and nurseries.

Fungus gnats thrive under moist conditions, particularly where there is an abundance of decaying vegetation, algae, and fungi. They infest the soil and container media, specifically those rich in organic matter. Fungus gnats do not bite people or animals and are not known to carry human pathogens. They have a beneficial role as decomposers helping to convert dead vegetation into nutrients for plant growth.

The adults are dark in color and similar in appearance to mosquitoes. They are about 1/16 to 1/8-inch long, although they can be larger. The gnats are relatively weak fliers and are usually not found flying around indoors.

Females lay tiny eggs in moist organic debris or potting soil. The immature, legless larvae have a white to clear body with a shiny-black head and when fully grown are about 1/4 inch long. The larvae live in the soil and feed on organic mulch, leaf mold, grass clippings, compost, root hairs, and fungi.

Control

Cultural and physical control includes reducing moisture as much as possible, eliminating any plumbing or irrigation system leaks and providing good drainage. Moist grass clippings, other organic mulches and composts are favorite breeding spots. The use of partially composted matter in potting mix will encourage the infestation of gnats.

Insecticides are used in commercial plant production but are seldom warranted to control fungus gnats around the home.  Bacillus thuringiensis subspecies israelensis (Bti) applied to the soil or potting soil is a safe product for the control of fungus gnats. Repeated applications are needed for long-term control. Note: This particular Bt product is different from the one used to control caterpillars. Bt labeled for caterpillars is not effective against fly larvae. Insect growth regulators (kinoprene, cyromazine) applied to the potting mix can also be effective.

Pyrethrins or a pyrethroid (bifenthrin, permethrin) are an option to provide temporary, fast-acting control. Pyrethrins have low toxicity to people and pets. When spraying houseplants, it is best to move the plants outdoors for treatment and wait a day or so before bringing them back inside.

Biological Control - Predators, such as some beetles, mites and nematodes will help to control fly larvae outdoors.   

pH Can Make A Difference

Why are certain plants in the garden growing poorly while others flourish?  
Obvious answers maybe insect pests, fungal diseases and abiotic ailments such as soil deficiencies. The pH of the soil, however, is an often overlooked problem. pH measures the acidity/alkalinity of the soil. Seven is neutral; below 7 is acidic and above is alkaline.

To address this, the University of Hawaii Cooperative Extension has a publication “The pH Preference of Plants.”  The pamphlet includes a chart with many crops including fruit trees, vegetables and ornamentals, and their suggested pH growth range. All plants prefer a specific pH range, usually around neutral; some require slightly acidic while others favor slightly alkaline.

The pH of soil is important and can be a reason for poor growth. Take note which plants are doing well and which are not. Compare those plants to the list in the publication. If all or most of the poorly growing plants prefer a soil pH which is not indicative of your soil, then pH may be a problem. Get a soil analysis. Often taken through a local university cooperative extension, the analysis will be  helpful for a variety of reasons.  

As an example, breadfruit, mac nuts, parsley, lilikoi (passionfruit) and gardenias prefer an acidic soil and will grow well down to a pH of 5 and even 4.5.  These plants may be flourishing in the garden,  On the other hand, plants like bananas, mock orange, broccoli, leeks and lettuce all like a pH much higher, between 6 and 7. If these plants are growing poorly, then your soil may be too acidic.  

For additional information on pH and a useful chart, search this blog for the articles, Problems With Acidic Soils: Lime Can Work Wonders, and Unsuspected Causes of Plant Damage.