This invention relates in part to modifying BtBooster (BtB) peptides, in part to increase their stability in insect midgut digestive juices. Some preferred embodiments of BtB have removed proteinase cleavage sites resulting in increased stability of the modified BtB in the insect gut, while retaining the ability to enhance B.t. proteins for improved insect control. In some preferred embodiments, the protease-stable BtB is used in combination with B.t. spores and/or crystals comprising a Cry protein. Also reported herein is the significant and increased enhancement of Bt toxins against relatively Bt-tolerant insects (Helicoverpa zea, Spodoptera exigua and Agrotis ipsilon), when used with BtBs. We also describe increased toxin enhancement with cadherin fragments that are stabilized against over-digestion by insect midgut proteinases. We also report enhancement of Bt Cry1F toxin by cadherin fragments.