The insect growth regulator pyriproxyfen (a juvenoid) eþ ectively inhibits the hatching of eggs of the tobacco or cotton whiteýy, Bemisia tabaci, as well as causing pupal mortality. Since 1991, this insecticide has been one of the main agents for controlling B tabaci on Israeli cotton. Seasonal trends of susceptibility to pyriproxyfen in üeld populations were monitored from June (prior to treatment) through late summer at diþ erent locations in Israel. After seven years of pyriproxyfen use within an insecticide resistance management strategy that limits this insecticide to a single application per season, susceptibility has been maintained in many areas. In other locations where pyriproxyfen had been used against geographically isolated populations of B tabaci, moderate to high levels of resistance have been observed. Ecological and agronomic factors that may contribute to geographical variation in selection for resistance are discussed.
The dynamics of pyriproxyfen-susceptible and -resistant populations of B tabaci following a single application of pyriproxyfen were investigated under simulated üeld conditions in the laboratory. The susceptible population was suppressed very eþ ectively, whereas eþ ects of pyriproxyfen against the resistant population were much more transient. Diþ erences in the productivity of susceptible and resistant strains in the absence of pyriproxyfen treatment could reýect a ütness cost accounting for observed reductions in resistance levels between seasons in the üeld. They may also explain why, following a recent reduction in the use of pyriproxyfen in cotton üelds, resistance in 1998 declined to levels observed in 1995/6.