Biological changes following use of dieldrin are summarized. Also included are the results of a field study wherein a previously unsprayed ranch with known initially insecticide-susceptible house flies was sprayed five times over a 3-year period. Resistance generally increased over this period as well as the following year, except for a temporary decrease following overwinterings. Reproductive potential was generally unchanged the first 2 years, decreased the third, but increased above the prespray potential in the fourth year. Significant differences in percentage survival occurred yearly within each life history stage, during those years that evaluations were made.

Two extensively studied insecticide-susceptible laboratory populations had several times the reproductive potential of three field populations.

These studies suggest that results obtained while investigating reproductive potential and other characteristics of one population are not necessarily applicable to another. Differences in success of larvicidal and adulticidal chemical control measures may be expected because of corresponding differences in survival rates of the various life history stages.

Whether biological changes accompanying insecticide resistance can be predicted in the laboratory in advance of field control measures is questionable because of (1) differences between laboratory and field environments, (2) possible use of inadequate population samples in establishing laboratory cultures, (3) difficulty in reproducing field insecticide exposures in the laboratory, and (4) the time involved in evaluating results.

Biological changes resulting from repeated treatments generally are more apparent when the control program is well along and particularly when control results are adverse. These changes are generally difficult to evaluate if based on field data alone.