Safety of Bt Plants and Nontarget Effect

permission pending from Green, D. 2002
Bt plants have advantages and disadvantages. Bt toxins should be safer than other insecticides, because they are very specific and minimally persistent (DeMaagd and et al., 1999). However, the potency of Bt toxins causes problems. Topics such as Bt specificity, direct and indirect contact with Bt toxins, and Bt toxin proliferation in the soil have all been studied.

The Effects of Bt plant pests on their Predators
Studies about indirect contact with Bt plants have produced varying results. Bt plants were tested to see what effects eating Bt-fed pests have on insects. The corn pest predator grass thrip ( A. obscurus ) showed no significant difference in mortality after eating insects that ate Bt plants. Nor was there any significant difference in development time of grass thrips reared on these insects that ate Bt corn versus insects that did not eat Bt corn. The safeness of Bt corn is still questionable since the insects eating the Bt toxins may not have had enough toxins in its system to affect the grass thrip (Zwahlen, C., and et al ., 2000).  

Despite the evidence that the grass thrip study provided, there has been counter evidence on the indirect effect of Bt plants on crop pest predators. Lacewings that ate European corn borers that fed on Bt corn developed slower and had a higher mortality rate (Lutz, K., 1999). Generally, plants that produce the crystal protein Cry1Ab have been noted to increase the mortality of the lacewing when the lacewing eats prey off that plant. This crystal protein is supposed to only affect Lepidoptera larvae (Schuler, T.H. and et al., 2001).

The lacewing is not the only insect to have adverse effects when eating Bt-fed prey. Ladybird beetles that eat Bt-fed aphids have reproductive problems (Lutz, K., 1999). The longevity of minute pirate bugs ( O. tristicolor ) decreased when they were fed Bt -pests versus minute pirate bugs that were not fed pests that ate Bt crops. The Big-eyed bug ( G. punctipes) adults' longevities decreased when they were fed Bt leaves and prey (Ponsard, S. and et al ., 2002).

Non-target Insects Eating Bt plants
The issue of how Bt plants affect beneficial insects that come into direct contact with them involves the specificity of Bt plants. Bt plants are more specific than traditional insecticides used on crops (DeMaagd, R. A. and et al. , 1999). (See Bt Plant Specificity for more info on specificity) Bt plants should not affect beneficial insects according to the specificity of Bt plants. Beneficial insects do not distinguish between Bt plants and non-transgenic plants (Schuler, T.H. and et al. , 2001). Thus, they are vulnerable to any adverse effects Bt plants may have. Bt plants have harmfully affected lacewing mortality when lacewings ate BT maize (Schuler, T.H. and et al. , 2001). Also, when insects eat both Bt prey and Bt leaves their longevities decrease versus animals that eat only Bt leaves (Ponsard, S. and et al ., 2002).

Bt plants, insects, and the soil
Some Bt plants express the Bt protein in the roots in order to target rootworms. Therefore, soil ecology has been an area of interest for Bt plant implementation. The effects of Bt plants on the soil have been positive thus far. The number of microarthropods in the soil does not change when Bt plants are planted in the soil. Nemaotdes do not change in number when Bt plants expressing the crystal protein Cry3Bb1 are introduced into their soil. Three reasons may be the cause for this effect. First, the Cry3Bb1 protein may not be released into the soil. The crystal protein may not escape from the plants roots into the soil, thus keeping the soil Bt toxin free. Second, the Cry3Bb1 protein may not be present in the soil in lethal amounts. The crystal proteins may escape, but are not in high enough concentrations to kill any organisms. Last, the soil microarthropods and nematodes may not be sensitive to the Cry3Bb1 protein. There may be a crystal protein that affects the soil organisms, Cry3bb1 may not be it. (Al-Deeb, M. A., and et al., 2003).

Bt plants Effects on Insect resistance in Crop Pests

See Insecticide Resistance and Controlling Negative Effects pages.

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This website was created by
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as part of an undergraduate assignment at Davidson College.

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