One of the things you have to conclude, as any rational person should, is that the science is mixed. If you had enough money, scientists could probably come up with whatever conclusion you were after, especially in the life sciences in the fields of biology, where we don't fully understand the mechanisms at work. Biology, ecology, and the health of people, animals, plants, and agricultural systems are very complex, and our understanding of them is in its infancy. That's a statement that good scientists around the world would agree with.
If we take that as a working hypothesis, what is the best and most sensible response? It is the precautionary principle that your colleague has alluded to. If we don't need something, stay away from it. If we really need it, as we do in the case of producing biofuels on marginal land, go for it. That is what I would tell your constituents if they asked me that question.