Biological Resistance Against DDT

The other reason for the decline mentioned above about cotton yield in Cannette Valley was that DDT became less effective in killing the insects. Larger doses became necessary. The cotton bollworms in Cannette Valley were found in 1965 to be 30,000 times more resistant to DDT than they had been in 1960. In fact, as early as 1946, houseflies resistant to DDT had been noted, and by 1948 the list of resistant insects 

Some DDT-resistant houseflies were found to contain an enzyme DDT-dehydrochlorinase that removes hydrogen chloride (HCl) from and convert DDT to DDE (dichlorodiphenylethylene). Recall the case of a resistance factor against penicillin (Chap. 7). A resistance factor in the case of penicillin was an enzyme to decompose penicillin. A similar strategy is used here. Other insects seem to convert DDT to 2-OH-2,2-bis (chlorophenyl)-l,l,l-trichloroethane (=dicofol). 

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