grow 516: Fly larvae in tomatoes


Plan ahead for pest-free tomatoes


Dear Janet,

Last year, my three tomato plants produced an average amount of tomatoes. Then, in mid-season, to my horror, I sliced open some tomatoes only to find little maggot-looking worms!

The tomatoes were unblemished, so I don't know how these little critters got in there.

Please help. These tomatoes were bought at a reputable garden shop, planted in full sun and watered regularly.

I'm afraid to have home-grown tomatoes again.


Dear J.R.,

What you probably saw was the larvae of a fly we call pepper maggot, a native of eastern Canada and the U.S.. Once, it lived out its life around the weed, horse nettle. When pepper and tomato fields came to this region, the fly found it could live on these relatives of horse nettle, too.

It never reached "major pest" status. Infestations of pepper maggot are patchy. Even on farms with a history of this pest, just one portion of a field or one field among many may be infested. There may be no infestation at all some years. As further evidence that this is not a pest we see much or expect to become a regular, significant problem. 

The flies spend winter as pupae (the fly equivalent of cocoons) in the soil near the plant they preyed upon the previous year. They usually emerge early in July. Within a week the flies have found horse nettle, pepper, tomato or eggplant with fruits just 4/10 to 1-2/10 inches in diameter. They lay eggs just under the skin of these fruits. The wound is tiny, the size of the fly's ovipositor -- egg laying organ. Even later you might see only a depressed dimple on the surface. Unlike other tomato insects, these maggots develop and do their eating on the inside -- no need to chew their way in.

The maggots eat for 2 to 3 weeks, then chew their way out and drop into the soil to wait out the off season.

Preventing infestation can take several paths.

First and most important, clean up after any maggot problems. Don't leave infested fruit laying around. Remove any nearby horsenettle weeds. (It's not terribly common; I see it once every few years.)

You can cover the plants with fine netting such as floating row cover during adult fly time from early July to early August. If you didn't rotate your plantings, it's possible that flies may emerge from soil at the base of a covered plant. In that case the cover won't do any good.

You can buy HB predatory nematodes (from mail order companies such as Gardens Alive) and in August add them to the soil in your tomato patch. They destroy pupating larvae so any flies that bothered your plants this year don't survive to repeat the performance.

You can plant only late-bearing varieties of tomato. Flies lay eggs only on the fruit, so if there is no fruit during July, there will be no fly eggs.

You can spray the plants with Malathion during July, repeating as directed on the label. Start spraying when you see the first adult fly.

The adults are brightly colored yellow striped flies. There are three yellow stripes down the back and the eyes are iridescent green. (Go to to see a color photo.)

Or, try the do nothing, it's not likely to happen again strategy. It may work, or maybe you'll invoke my dad's old saying, "What's the problem? Cut it out or close your eyes and chew -- it's a little extra protein is all."

Short reports:

More reports of plants lost or badly burned over winter...

...redbud trees partially or completely killed and foxgloves vanished, even from beds where they had been happily resident for 15 years.


Green thumbs up

to those who say "How nice" or "We like to look at your garden" to their gardening friends and neighbors. Simple attention like that is a great stimulus to the gardener to work even harder. Sometimes it's all the gardener needs to persevere in a trying season.


Green thumbs down

to the capriciousness of winter kill, to freeze dry evergreen leaves and needles within inches of the ground but spare all those maple seeds to become seedlings. Was my brain winter damaged, too, that I took the leaves off those beds and left the ground layer bare, ripe for sprouting?

Originally published 5/17/03