CYTA ENABLES CRYIV ENDOTOXINS OF BACILLUS-THURINGIENSIS TO OVERCOME HIGH-LEVELS OF CRYIV RESISTANCE IN THE MOSQUITO, CULEX-QUINQUEFASCIATUS

Cry proteins produced by Bacillus thuringiensis are selective biodegra dable insecticides used increasingly in bacterial insecticides and tra nsgenic plants as alternatives to synthetic chemical insecticides, How ever, the potential for development of resistance and cross-resistance in target insect populations to Cry proteins used alone or In combina tion threatens the more widespread use of this novel pest control tech nology, Here we show that high levels of resistance to CryIV proteins in larvae of the mosquito, Culex quinquefasciatus, can be suppressed o r reduced markedly by combining these proteins with sublethal quantiti es of CytA, a cytolytic endotoxin of B. thuringiensis. Resistance at t he LC95 level of 127-fold for a combination of three CryIV toxins (Cry IVA, B, and D), resulting from 60 generations of continuous selection, was completely suppressed by combining sporulated powders of CytA in a 1:3 ratio with sporulated powders of a CryIVA, CryIVB, and CryIVD st rain. Combining the CytA strain with a CryIVA and CryIVB strain also c ompletely suppressed mosquito resistance of 217-fold to the latter tox ins at the LC95 level, whereas combination of CytA with CryIVD reduced resistance in a CryIVD-selected mosquito strain from greater than 1,0 00-fold to less than 8-fold. The CytA/CryIV model provides a potential molecular genetic strategy for engineering resistance management for Cry proteins directly into bacterial insecticides and transgenic plant s.