A field-collected colony of the diamondback moth, Plutella xylostella, had 31-fold resistance to Cry1C protoxin
of Bacillus thuringiensis. After 24 generations of selection with Cry1C protoxin and transgenic broccoli expressing a Cry1C
protein, the resistance that developed was high enough that neonates of the resistant strain could complete their entire life cycle on transgenic broccoli expressing high levels of Cry1C.
After 26 generations of selection, the resistance ratios of this strain to Cry1C protoxin were 12,400- and 63,100-fold,respectively,
for the neonates and second instars by a leaf dip assay.
The resistance remained stable until generation 38 (G38) under continuous selection but decreased to 235-fold at G38 when
selection ceased at G28. The Cry1C esistance in this strain was seen to be inherited as an autosomal and incompletely recessive factor
or factors when evaluated using a leaf dip assay and recessive when evaluated using Cry1C transgenic broccoli. Saturable binding of I-Cry1C was found with brush border membrane vesicles (BBMV) from both susceptible and Cry1C-resistant strains.
Significant differences in Cry1C binding to BBMV from the two
strains were detected. BBMV from the resistant strain had about sevenfold-lower affinity for Cry1C and threefold-higher binding site concentration than BBMV from the susceptible strain.
The overall Cry1C binding affinity was just 2.5-fold higher for BBMV from the susceptible strain than it was for BBMV from
the resistant strain. These results suggest that reduced binding is not the major mechanism of resistance to Cry1C.

Correspondent Address
Department of Entomology, Cornell University,
New York State Agricultural Experiment Station,
Geneva, New York 14456, USA.

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