Three consecutive years (1993-95) of monitoring responses of Bemisia tabaci (Gennadius) adult populations from cotton fields in the Imperial Valley, CA to various insecticides failed to detect progression to higher levels of resistance. Instead, regressions of LC_50s generated by bioassays of field populations against time for each year indicated a lower mean response for 1995 compared to 1993 for bifenthrin and endosulfan. Negative slopes significantly different from zero (P<0.05) for both insecticides in 1994 and for endosulfan in 1995 indicated higher susceptibilities at the end of each season despite instances of intensive insectide use. This same pattern was observed for other insecticides representing different classes, thus suggesting the involvement of ecological stress factors such as declining crop quality, overcrowding and/or rampant dispersion leading to physiologically-weakened adult whiteflies.

Declining levels of LC_50s between 1993-95 are thought to be due to various agricultural and agronomic factors, including large acreages of alfalfa that act as insecticide refuges for whiteflies and maintain a high frequency of susceptible genotypes; largescale dispersion and subsequent matings of susceptible and resistant genotypes that result in lower frequencies of putative resistant genes; the prevalent use of insecticide mixtures that potentially eliminate individuals resistant to single insecticides; and the recent addition of several effective insecticides representing different classes to the insecticide arsenal available to growers and pest control advisors.