Spraying the bacterium on corn, however, is not nearly as effective as growing corn already imbued with the Bt insecticidal proteins. When the bacteria are sprayed onto plants, they are sensitive to radiation and heat, do not reach the whole crop, and are not as effective against older larvae. Corn plants grown with the Bt proteins are much more effective against insects. For example, corn grown with the Bt insecticidal proteins kills 99 percent of the European corn borer larvae compared to corn sprayed with BT only kills between 60 and 95 percent of the larvae with each spraying (which also suggests that the effectiveness of spraying varies).

Although Bt corn has spread rapidly the last few years, reaching 30 percent of cornfields throughout the U.S., there are a number of ill effects. One concern among environmentalists is that monarch butterflies (Danaus plexippus) eat milkweed plants that grow close to cornfields. An experiment conducted by John Losey, a Cornell University professor of entomology, indicated that Bt not only kills the European corn borer, but also kills the monarch butterflies (Losey, 1999). After four days of eating Bt pollen artificially spread on milkweed leaves, over 44 percent of the butterflies died. Others have corroborated Losey's findings (Hansen and Obrycki, 2000).