Mark,
Sevin is ineffective against leafminers in vegetables as it doesn't penetrate the outside of the leaf to the interior where they feed. The fly doesn't feed on the foliage either but lands to penetrate the leaf with its ovipositor to lay their eggs, so it likely doesn't come into enough contact with Sevin dust to kill it. Even if it did it would have already laid its eggs by the time the Sevin had an effect.
Other than row covers to prevent the fly from reaching the plant, there isn't a chemical I can think of for the home gardener to use. Kaolin clay sprayed may work but you would have to apply it at the right time and it is difficult to get an even distribution because it is hard to keep in suspension.
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barkeater
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