Garden Pests - Cabbage Loopers

Pests in the garden are no fun, but they're part of the garden's life cycle and accept that I'll be dealing with some pest or another in each garden, each year. Yesterday, I had a client with cabbage loopers. His landscaper recommended buying bT, but I'm not a fan of adding any inputs to the garden (whether or not someone certifies them as organic) , in particular bacteria.

Then this morning I got the following email from a rooftop gardener in NYC, so I know cabbage loopers are making the rounds this year:

Hi Amy, I hope this email finds you well. Had a frustrating morning of discovering my bibb lettuce was gnawed away, holes in my sage and my mint halfway eaten though. I saw a few little green caterpillar culprits, so after a little searching it seems like these enemies may be cabbage loopers? 

Any advice on how to best get rid of them? My garden is entirely a rooftop container garden if that helps!  Thanks so much!

And so for anyone dealing with loopers currently, here is an easy solution (and my response to the above email):

How long have you had the rooftop garden? Did you plant anything in this pot before the lettuce? (*) Cabbage loopers typically like members of the brassica family - cabbage, broccoli, sometimes kale. (editorial note: I also found a paper siting looper issues for lettuce production in AZ!) Did you notice white moths/butterflies llying around about a week or two ago?

For Cabbage loopers you have them, so now you'll have to pick them off and squish them. From there, you can cover your pots with Floating Row Cover, which will prevent them from landing on your crops. Not super pretty, to wrap all your plants in spun white cloth, but effective. Keep me posted! amyp

*I asked about what she had in this pot before, because crop rotation will often help prevent pests in the garden. If she ALWAYS uses this pot for cabbages, for instance, that could be the invitation for loopers.