A binding site for Bacillus thuringiensis Cry1Ab toxin is lost during larval development in two forest pests

The insecticidal activity and receptor binding properties of Bacillus thuri ngiensis Cry1A toxins towards the forest pests Thaumetopoea pityocampa (pro cessionary moth) and Lymantria monacha (nun moth) were investigated. Cry1Aa , Cry1Ab, and Cry1Ac were highly toxic (corresponding 50% lethal concentrat ion values: 956, 895, and 379 pg/mu l, respectively) to first-instar T. pit yocampa larvae. During larval development, Cry1Ab and Cry1Ac toxicity decre ased with increasing age, although the loss of activity was more pronounced for Cry1Ab. Binding assays with I-125-labelled Cry1Ab and brush border mem brane vesicles from T. pityocampa first- and last-instar larvae detected a remarkable decrease in the overall Cry1Ab binding affinity in last-instar l arvae, although saturable Cry1Ab binding to both instars was observed. Homo logous competition experiments demonstrated the loss of one of the two Cry1 Ab high-affinity binding sites detected in first-instar larvae. Growth inhi bition assays with sublethal doses of Cry1Aa, Cry1Ab, and Cry1Ac in L. mona cha showed that all three toxins were able to delay molting from second ins tar to third instar. Specific saturable binding of Cry1Ab was detected only in first- and second instar larvae. Cry1Ab binding was not detected in las t-instar larvae, although specific binding of Cry1Aa and Cry1Ac was observe d. These results demonstrate a loss of Cry1Ab binding sites during developm ent on the midgut epithelium of T. pityocampa and L. monacha, correlating i n T. pityocampa with a decrease in Cry1Ab toxicity with increasing age.