Resistance has previously curtailed the use of almost
every type of insecticide and poses a serious
threat to the future of pyrethroids. Cross resistance
does not appear to be a problem between pyrethroids
and organophosphates or methylcarbamates. However,
previous selection of houseflies with DDT for a
recessive factor conferring knockdown resistance
(kdr) carries with it a cross-resistance to pyrethroids.
Houseflies on Danish farms developed pyrethroid
resistance when pyrethrins and pyrethroids replaced
chlorinated hydrocarbon and organophosphorus insecticides.
One pyrethroid-resistant field strain was
subsequently selected in the laboratory with bioresmethrin
to a resistance factor of 1400-fold. Despite
no previous exposure, this strain was more than
60,000-fold resistant to decamethrin. Thus, the most
potent of all insecticides on a normal strain has almost
no effect on this resistant strain. This is the
most dramatic example available of pyrethroid resistance.
Some of the housefly resistance mecha nisms are considered in Figure 4. Low levels of field
resistance are also known in local areas with mosquitoes,
Australian cattle ticks, some lepidopterous
pests on cotton, and a few other pest species. It may
be possible to forestall pyrethroid resistance by restricting
their use to minimal doses and numbers of
applications and by more fully utilizing integrated
pest management systems. Routine application of
pyrethroids at high doses, as done in the past with
other insecticides, practically guarantees that resistance
will reach levels where economic use of pyrethroids
is no longer possible.