Most GM crops fall into one of two categories: either engineered to resist chemical herbicides, or engineered to produce insecticides themselves. When herbicides are used on resistant crops, over time the weeds develop resistance, leading to the use of even more chemicals. Crops engineered to produce insecticides on the other hand produce toxins that are not only harmful to pests but other insects such as butterflies, moths and insect pollinators.
That would be a reason to be against specific uses of GMO technology. It is not a reason to be categorically anti-GMO.
The general arguments against GMO are entirely without merit, in my opinion. Moreover, their consequences are monstrous, imposing costs on the third world just so first worlders can practice a deplorable philosophical fetish (or, more banally, inflame anti-GMO passions as a kind of agricultural protectionism.)
I don't buy that argument. GMOs enable the use of low toxicity herbicides, like glyphosate (demonizing propaganda notwithstanding). And herbicides allow low impact farming practices like no-till, or substitute for terrible practices like slash-and-burn in the tropics.
Do you know what strains of Bt have been incorporated into the various corn cultivars?
Are you familiar with the Aizawa strain of Bt?
Did you know that Aizawa Bt is extremely toxic to honeybees?
Do you think the widespread adoption and cultivation of Bt-engineeered corn crops, which of course are prolipherated through pollination to neighboring fields and into the wider ecosystem could possibly have any negative consequences whatsoever?
Do you think the near-extintion of critically important pollinating species is a legitimate reason to question incorporating highly damaging toxin-producing genes from bacteria into corn?