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. In addition, the beetles are vectors of the organisms that cause bacterial wilt, which can be more damaging then the insect. The beetle is abut 6mm in length, yellowish green with 3 black longitudinal stripes. The adult passes the winter in shelter of plant debris.