What are the alleged disadvantages of bt crops?

The most serious disadvantage is that long-term effects of genetically modified organisms is not known. BT varieties of GM crops are the result of taking DNA from a bacteria and inserting it into the seeds of corn, cotton, potatoes or other crops. This makes it possible for the plants themselves to produce substances that repel insects. Having that ability is good for insect control, but could have negative long-term effects on animals and humans who eat the crops.

More immediate concerns are possible contamination of non-GMO varieties and the crops becoming invasive in the environment. There are reports that genetically-modified crops have contaminated non-GMO varieties and that invasiveness has occurred.

Another possible disadvantage of BT crops, as well as GM crops called Round-up Ready, is that the seeds are patented. The worst-case scenario is that contamination (which has reportedly occurred and is likely to continue to occur) would eventually wipe out other varieties and leave the control of crop production in the hands of a few companies: those who own the patents to GM varieties.