Shoo, fly, don't bother my plants

March 2, 2011 

Most Alaska gardeners who grow their own tomatoes from seed start them sometime around April 1. As the date grows near I always get questions about white flies.

White fly is the common name of an insect related to the aphid, which should give you some indication where this is going. They look like tiny, 1.4 inch white moths. They lay eggs on the bottom sides of leaves. A single adult deposits between 200 and 400 of them.

A week or so later, the nymphs hatch from the eggs and start to live off the plant by sucking sap from the leaves. It doesn't take them long to develop into flying adults, which lay more eggs. The math of this all is staggering and populations increase fast.

It is not that they do much damage directly to the plants, though eventually their effects are noted with yellowing leaves. And, it is not so much that they can carry tomato viruses that can infect our precious plants. No, it is the nuisance they become when populations establish themselves. Clouds of tiny bits of Kleenex tissue fly into your face every time you go near a plant. They fly into the air at the least disturbance during warm, daylight hours.

I've done a bit of research on the matter and here is what I have to report.

If you have an outdoor greenhouse, any white flies, larvae or eggs left over from last year on dead, frozen leaves or soil are dead. You don't need to spray the soil with a white fly pesticide. You don't need to take the soil out and put it new soil. Any white flies that you see this summer in your greenhouse will have been introduced on plants brought into it this year.

I know this will shock many, given traditional advice about white flies. There are really only two ways in Alaska to develop a problem with white flies.

The first is to buy a plant that has eggs, nymphs or adults already on it.

The second is to accept a plant as a gift that does as well. Realizing this, I now I shudder to think that I might have been responsible for the white flies my neighbor had to face last summer, as we trade plants. Forgive me Greg!

Short of a total freeze for a week, it is very, very hard to get ride of white flies. They become resistant to pesticides very quickly. This makes sense given the numbers and rate of births. It is evolution in action. They also don't just breed on tomato plants, they like all sorts of things.

It is very hard for a commercial nursery, for example, which doesn't shut down for a week in the winter to freeze them out. Similarly, those of us that have indoor greenhouses don't like to toss all the plants we have collected for years and then freeze the place. Not to mention frozen pipes.

So, if you absolutely don't want white flies this year, grow all your own plants in a place where there are no white flies. Do not accept plants from friends and do not buy plants from nurseries.

That doesn't sound like fun to me. It is much easier to buy started plants in many cases and friends' plants help increase the summer's varieties. So what can you do to at least minimize the chances of getting them?

If you don't start your own plants, at least try and get them young. There is less chance infection will be present. Some suggest washing plants before you expose other plants to them. This also suggests buying young plants as the older ones have too many leaves.

Consider isolating purchased or gifted plants for a couple of weeks until you are absolutely sure they are not infected. When getting a gift plant, you are not out of bounds asking politely about any exposure to white flies.

You can also use, according to directions, AzaMax. Apply it to your purchase as a spray and a drench. It is a neem-based product with impacts on white flies from death to slowed metabolism.

Biological control comes in the form of a small white fly eating wasp called the Encarsia Formosa. Some years they are available locally from indoor grow shops and some nurseries. Green lacewings, usually an aphid predator, also love white flies.

If you do end up with white flies, you can vacuum up adults. Do it in the early morning when they are very sluggish. Freeze the vacuum bag for a day or two when done. White flies are attracted to yellow and you can buy traps that catch them in a sticky oil. You can also make your own with yellow cardboard or coffee can or tennis ball can tops, coated with Vaseline.

Finally, consider this. White flies generally do not really harm tomato plants much in Alaska. Aside from their high nuisance value, they are not as bad as they could be. Still as we enter the tomato growing, buying and giving season, it pays to inspect your plants and get a hold of the problem before it starts.


Jeff Lowenfels is a member of the Garden Writers Hall of Fame. You can reach him at teamingwithmicrobes.com or by calling 274-5297 during "The Garden Party" radio show from 10 a.m. to noon Saturdays on KBYR AM-700.

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