Indoor Insect Control Over the Winter

J W Keeth sends out the newsletter for the Cactus and Succulent Society of Tulsa. The newsletter is called "Stuck Up News".

Keeth has allowed me to use photos and an article from the December 2007 issue.

Photos: Keeth's Chaos Cactus Nursery in Sand Springs, near Tulsa OK where the greenhouse is stuffed with succulents and cacti awaiting the first sale of the spring. The yellow flowering succulent is glottiphyllum pygmaeum.

Read the informative article from "Stuck Up News" below.

Fungus Gnats by Thomas Schwink

Reprinted from the Newsletter of the Mid-Iowa Cactus & Succulent Society

The insects which are known in this country as fungus gnats and in Europe as Sciara flies are common and can be very destructive to succulents, especially seedlings.

They are small insects and many people are not very aware of their presence, sometimes mistaking them for other small insects such as fruit flies. As their name indicates, the food of these insects includes fungi and they can be very destructive to mushrooms. They feed on vegetable matter of many kinds, including plant roots. When they feed on small succulent seedlings, especially on the roots of these seedlings, they can seriously damage the seedlings and frequently kill them. Most of the damage is done by the larvae.

Identifying these very small insects requires magnification. I use an inexpensive 6x hand-held magnifier purchased from Toys R Us. Since these insects are also very active, most of us need an insect which is not moving (a dead insect) to make the identification. The most useful character for identifying adult fungus gnats is their long legs, especially their long coxae. The coxae are the proximal segments of the legs, the segments closest to the thorax. I make the identification using insects adhering to the sticky traps which I use. These traps are for sale by many gardening supply businesses.

I want to describe some of the control methods which I have used or at least tried, read about, or think would be worth trying.Regardless of whatever else is done, heating the seed-starting mix prior to using it is highly recommended. This kills eggs of fungus gnats and is also effective against other pests, both insects and diseases, which may be present in the mix. Keeping the germinating seeds and the seedlings covered for as long as practical is also very effective. I like using humidity domes for this purpose, while many people use plastic bags. The increased humidity present when this is done usually facilitates germination and this is also very important of the seedlings. Incorporating insecticide into the seed-starting mix prior to sowing the seeds—This is something that I tried only once, with disastrous results. The insects were controlled, but so were the seedlings. Seedlings are often more sensitive to strong insecticides than are older plants. Sticky traps—I use these routinely and they do trap many insects, but many of the insects are not trapped and these insects do much damage. These traps are easy to use and their help in killing some of the insects is desirable. I look upon them chiefly as a monitoring tool which I can examine regularly and use to determine the numbers and kinds of insects trapped.

Dryness—Keeping the soil in the pots as dry as practical does help, but most seedlings are not tolerant of dryness of a degree that will deter these gnats. A thick gravel mulch also helps, but this too is not very practical with seedlings. A completely inorganic potting mix should also be of help, but I do not like using such a mix for seedlings.

Diatomaceous earth—Since the larvae of fungus gnats live in the soil and are very active, this product may be effective if mixed into the seed-starting mix or sprinkled on top. I have never read about this product being used against fungus gnats, but think that it may be worth trying. Bacterial control—There are bacteria for sale which have been selected for their ability to control fungus gnats. However, although these may be effective when first used, with continued use the insects can develop resistance to the bacteria.

Predatory nematodes—These are widely sold for control of different insects, including fungus gnats. As with many insect control methods, including biological controls, enough gnats can survive to do considerable damage. I do think that these nematodes are worth trying, since they do not apparently harm the plants.

Mothballs—These have been recommended as a repellent for many insects and may be worth trying as a method to keep the adult gnats from laying their eggs in the seedling mix.What I like best—I keep a spray bottle of a pyrethrin preparation handy when I examine my seedlings and when I take care of them. When I see a flying insect near the seedling pots, especially an insect that has flown from one of the seedling pots, I spray the insect. Even though what I use is an emulsion and may damage the seedlings if they are sprayed directly with it, the occasional droplet that may land on the plants when I direct the spray at the insects seems to do no harm. To be certain that the insect is dead after the spray has knocked it down, I squash it with my finger if I can find it. All insecticides should be considered potentially toxic, and thus I wash my hands after squashing the insect with my finger. Now that I have used this method for a period of time, I seldom see a fungus gnat and seldom catch one on the sticky trap in the room where I start my seedlings. Also, there is now very little if any fungus gnat damage to the seedlings.

I am grateful to Keeth for this article since it addresses a common problem that is difficult to overcome.


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