In the last decade, there’s been a lot of controversy about Bt corn—that is, corn that’s been genetically modified to produce the toxins of Bacillus thuringiensis (Bt), in order to protect against the European corn borer and other insect larvae that prey on corn. As most green gardeners know, Bt is a soil bacterium with a long history of use as an organic insecticide.
Most of the controversy stems from the discovery that Bt corn can harm the caterpillars of the monarch butterfly, at least under laboratory conditions. But let’s not throw the baby out with the bathwater here. First of all, monarch caterpillars eat only milkweed in the wild—never corn. And while milkweeds near Bt corn fields are occasionally contaminated with Bt corn pollen, it doesn’t seem to have affected the monarchs much. In fact, monarch populations increased by 30% in 1999, the same year the Bt toxicity study was produced, despite the fact that 30% of all corn grown in the U.S. that year was of the Bt variety. The population increase has been attributed to reduced chemical pesticide use.
Second, we’d like to emphasize here that not all Bt strains are created equal. One, Bacillus thuringiensis israelensis (Bti), is the active ingredient in mosquito control dunks and other related insecticide preparations popular among organic gardeners. This has led to the suspicion among some observers that Bti may be the culprit behind the recent unfortunate decline in butterfly populations on the East Coast of the U.S.
However, the truth is that Bti isn’t a broad-spectrum insecticide; it’s specific to fly larvae, including mosquitoes and gnats. The Bt variety that kills caterpillars is the Kurstaki strain (Btk), and it was Btk genes that were transplanted into Bt corn. A third Bt strain, San Diego/Tenebrionis, is specific to beetles.
In other words, as long as beetle- or fly-specific strains are used to target specific insects, Bt is safe for butterflies. It’s only when Btk is spread that they’re in danger, so be sure to keep that strain of Bt far, far away from your butterfly garden.
{ 0 comments… add one now }