Several other species of parasitoid wasps attack caterpillar cabbage pests. Use of B.t. Stay away from using botanical sprays such as rotenone or pyrethrum when you do not need them. Use of a spreader-sticker is recommended with B.t. on crucifers. I have heard that in some areas they have already caused extensive damage to seedlings.
Crop rotation works fairly well, at least to delay the attack.
They are pretty good fliers and usually will eventually find your cucurbits. Start looking for them now and keep it up at least until flowering. Seedlings are particularly susceptible. As soon as they push through the soil the beetles attack them, eating off the stems and cotyledons. If the plants survive, later feeding on the leaves, vines, and fruits by adults and then larval mining of the roots.