1. A method of screening a lepidopteran for resistance to a Bacillus thuringiensis (B.t.) insecticidal Cry1 protein that is toxic to lepidopterans, wherein said lepidopteran is of a species known to develop resistance to said Cry1 protein, wherein said method comprises collecting said lepidopteran from a field of a B.t. crop, obtaining a gut cell membrane sample from said lepidopteran, adding a plurality of detectable anti-alkaline phosphatase antibodies to said sample wherein said antibodies bind to an approximately 68 kDa alkaline phosphatase enzyme, detecting a level of bound antibody, and comparing said level of bound antibody to a control amount of bound said antibodies determined from a gut cell membrane preparation from a control insect of known susceptibility to said Cry1 protein, wherein resistance to said Cry1 protein by said lepidopteran is indicated if said level is less than that of a non-resistant insect, wherein said non-resistant insect is of the same genus and species as said lepidopteran.