Handpicking of tomato hornworms is the best option if one has the time, and predator insects are a good back-up, but probably won't substitute for hand-picking.

I agree, pdq, that bt should be used with discretion, but I don't read evidence that bt is the primary culprit for the decrease in butterfly populations over all. I'm not particularly well informed, though, and don't always know how to evaluate the research. Would appreciate links or leads to research that strongly supports your statement.

I've never used bt, but that is mostly because I never got around to it.

The moth larva that threaten most home garden crops are far from endangered species, and the application of bt by home gardeners is mostly likely to be done in a very selective manner, and only on targeted crop plants.

I can certainly understand concern if bt is used in large commercial fields and applied in a manner that leads to widespread overspray onto non-commercial food plants for species such as the Monarch.

I know there are concerns about the effects of bt corn on the Monarch population. The USDA cites research that indicates the pollen from bt corn does not get spread in sufficient concentration to threaten Monarchs, but I think the jury is still out on that because those studies are single issue studies, and do not take into account that when there are a number of factors contributing to the decline of a species, no single one of them may be individually significant, but taken all together, all of them are significant.

There are also many different strains of bt that target different species in their larval stages. The only time I seriously considered using bt was when I was growing flowers for market, and considered treating my lawn with a strain to control japanese beetles. It didn't take much research to realize that unless everyone in the neighborhood did the same, I was wasting my time. I ended up using my roses (which really aren't a viable flower crop for a local farmer's market for many reasons,) as a trap crop, and focused on keeping the roses well-tended and well fed to produce lots of blooms from which I handpicked the beetles, minimizing damage to other, more prolific and more durable blooms, especially zinnias and dahlias.

I have never had any significant problem with tomato hornworm. (Maybe, in part, because I live in tobacco country, and monoculture pest control practices to control tobacco horn worm have also reduced the tomato horn worm population.) Cabbage moth larva have always been the beasties I have had to combat. My veggie garden has usually been small enough that I could use row cover to protect plants from most of the damage.

I suppose it comes down to this; the large scale use of any pesticide or herbicide in commercial practice can pose a significant environmental threat, regardless of whether the control is organic, natural, or not.