By NICK SAKOVICH
I would like to know how I can control the fungus gnats in my vegetable greenhouse. Many hours were spent on the Internet looking for possible answers, but I am finding that what you find on the Internet may not be reliable because a lot of the reports were done several years back. New and better biological and chemical controls are constantly being introduced.
Your help will be greatly appreciated Right now, fungus gnats are my biggest problem. -- Sincerely, O.
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 do 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.
The Internet offers a plethora of information on a vast range of subjects: some accurate, some not. When searching the internet for horticulture related inquiries, begin with the University of Hawaii CTAHR website. Here are three of their websites: http://www.ctahr.hawaii.edu/site/, http://www2.ctahr.hawaii.edu/oc/infoctr.asp and http://www.agroforestry.net/.
Other websites to search include the University of California www.ucanr.org and http://www.ipm.ucdavis.edu/ and the University of Florida http://www.ifas.ufl.edu/. And last, but not least, my website at www.gardenguyhawaii.com features science based information as discussed in this column.
In general, when searching the internet, garden websites are interesting and can yield some good information. But keep in mind, much of the information is opinions and personal experiences of the writers, not necessarily factual or replicable.
Class
Announcement
I will be teaching a gardening class from 9 a.m.-12:30 p.m. Saturday, Feb. 4, at the University of Hawaii at Hilo campus. The class is called "Common Pests of the Garden & How to Control Them." We will look at many of the widespread diseases and insect pests that attack garden plants. Call 974-7664 to register; or go online at http://hilo.hawaii.edu/academics/ccecs/fitness/. There is a fee.
Hilo resident Nick Sakovich is a professor emeritus of the University of California. He has worked in the field of agriculture for 30 years and is a member of University of Hawaii Master Gardeners. Email your questions to Sakovich at askthegardenguy@earthlink.net.
