What do you get when you mix genetically modified corn and caddisflies? That's not the preamble to a joke.

Last month, researchers funded by the National Science Foundation published a study in the Proceedings of the National Academy of Sciences providing the first documented evidence that toxins from genetically modified corn may get into streams and harm insects that are an essential food for fish.

The corn is known as BT corn, and it is designed to manufacture a toxin that provides protection against agricultural pests -- essentially, the plant that grows from BT corn seed is a pesticide. The seed is manufactured by a number of large biotechnology companies, and up until last summer, growing BT corn was prohibited in Maine.

But a group of dairy farmers in the state who wanted to grow the corn to feed their cows made the case that BT corn can be grown more cheaply than conventional corn and thus offered them competitive advantages -- advantages already enjoyed by farmers in other states where the corn wasn't prohibited.

They pressed their case with the state's Board of Pesticides Control, which was the agency responsible for permitting the corn's planting because it is considered a pesticide. Their case was met with resistance by the state's growing number of organic dairy farmers, who asserted that pollen drifting from the genetically modified corn would contaminate organic corn used for feed, jeopardizing essential organic certification for their operations.

The board made a Solomon's judgment in July and declared it would permit BT corn to be planted in the state -- but only under a set of strict rules to be devised by late this year designed to protect organic farmers.

But while they were working on devising those rules this fall, the study about BT corn and caddisflies was released.

The data in the study is worrisome enough that the board, which was due to discuss the new rules later this week, should reverse course and reconsider its permitting of BT corn use in Maine.

When the board first considered the request for permission to plant BT corn, the major issue of contention was the genetic contamination of organic corn by genetically modified corn. That's because the EPA had previously performed tests to determine the corn's effect on water resources -- and found no significant effects.

But those EPA tests were problematic -- they didn't look at insects more closely related to the ones the BT corn's pesticide targeted. So EPA's researchers potentially missed an entire biological community that could be effected by the corn. That's what the most recent study looked at. Where the EPA looked at the crustacean species known as Daphnia, the more recent study looked at caddisflies, one of the most important food sources for fish.

And what it found, according to the National Science Foundation, was that the corn's 'plant parts are washing into local streams.' BT corn pollen was also 'being eaten by caddisflies.' In laboratory tests, 'consumption of BT corn byproducts increased the mortality and reduced the growth of caddisflies,' and thus 'the toxin in BT corn pollen and detritus can affect species of insects other than the targeted pest.'

Maine's farmers are an important part of our state's economy, social fabric and our landscape. Giving them more of a competitive edge is something we should do -- if possible. But in this case, there's a competing value that's potentially at risk if BT corn is planted and damages our water resources. Maine's rivers and streams, the species that depend on them and the fishing they provide are an equally important part of our economy, social fabric and landscape. The introduction of a technology that benefits one, but threatens another, must be carefully weighed.

At the very least, the Board of Pesticides Control should revoke the BT corn permits until they can be reconsidered in light of further study.

We need to know the degree of toxicity posed by BT corn to caddisflies and other aquatic insects. We need to know how long the toxins persist in streams and how far the toxicity may travel once it gets into a stream. The University of Maine's Mitchell Center for Environmental and Watershed Research has strong staff expertise in stream ecology and would be ideally suited to pursue this work.

In the meantime, BT corn should not be planted in Maine.