CROSS-RESISTANCE AMONG CRYIV TOXINS OF BACILLUS-THURINGIENSIS SUBSP ISRAELENSIS IN CULEX-QUINQUEFASCIATUS (DIPTERA, CULICIDAE)

Strains of Culex quinquefasciatus Say, selected for resistance to 1, 2 , 3, or 4 of the insecticidal toxins produced by the bacterium Bacillu s thuringiensis subsp. israelensis Berliner, were tested with individu al B. thuringiensis subsp. israelensis toxins, as well as combinations of toxins, to determine the pattern of the resulting resistance and c ross-resistance. In each strain, except the Cq4AB (C. quinquefasciatus selected with CryIVA + CryIVB), resistance ratios at the LC95 were ge nerally highest toward the strain's respective selecting toxin(s). The strain Cq4D, selected with CryIVD, demonstrated significant cross-res istance to CryIVA + CryIVB, and vice-versa. Selected strains also reve aled resistance toward the component toxins present in their respectiv e selecting mixture. For example, the Cq4ABDCytA strain, which was sel ected with all 4 of the naturally occurring insecticidal toxins in B. thuringiensis subsp. israelensis (CryIVA, CryIVB, CryIVD, CytA), revea led only a low level of resistance toward this mixture, whereas much h igher levels of resistance were detected toward individual CryIV toxin s as well as combinations of 2 or 3 CryIV toxins. All of the selected lines remained sensitive to the native 4-toxin mixture in B. thuringie nsis subsp. israelensis, suggesting that the CytA toxin, which was pre sent only in the 4-toxin mixture and differs in sequence and mode of a ction from the CryIV toxins, map plap an important role in suppressing resistance to CryIV toxins.