A confirmed case of DDT-resistance in Xenopsylla cheopis in India.

The authors describe a technique, developed in their laboratory at Poona, Bombay State, for estimating the susceptibility of the oriental rat-flea, Xenopsylla cheopis, to insecticides. Using this technique, they compared the susceptibility to DDT, gamma-BHC and dieldrin of a field population of X. cheopis from a locality in Poona District which had been sprayed with DDT 28 times, with that of a similar population from a locality in North Satara District which had been sprayed only three times. The Poona strain proved to be highly resistant to DDT and moderately resistant to BHC, which has never been used in Poona District, but remained susceptible to dieldrin, although slightly less so than the Satara strain. The authors consider this a clear-cut case of the development of physiological resistance as a result of continued DDT spraying, and suggest that the rat-fleas in any area where DDT has been used on a large scale for disease control should be tested regularly for susceptibility so that, in the event of an outbreak of plague, the insecticide most likely to destroy the vector could be applied from the outset.