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Title: TRENDS IN RELATIVE SUSCEPTIBILITIES OF WHITEFLIES TO INSECTICIDES THROUGH THE COTTON SEASON IN THE IMPERIAL VALLEY, CA

Author
item Castle, Steven
item Henneberry, Thomas
item PRABHAKER, NILIMA - UNIV OF CA, RIVERSIDE, CA
item TOSCANO, NICK - UNIV OF CA, RIVERSIDE, CA

Submitted to: National Cotton Council Beltwide Cotton Conference
Publication Type: Proceedings
Publication Acceptance Date: 6/1/1996
Publication Date: N/A
Citation: N/A

Interpretive Summary: Three consecutive years of monitoring responses of whitefly adult populations from cotton fields in CA to various insecticides failed to detect progression to higher levels of resistance. Regressions of LC50s generated by bioassays of field populations against time for each year indicated a lower mean response for 1995 compared to 1993 for insecticides. .Negative slopes significantly different from zero for insecticides in 1994 and for endosulfan in 1995 indicated higher susceptibilities at end of each season. 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.

Technical Abstract: Three consecutive years of monitoring responses of Bemisia tabaci adult populations from cotton fields in CA to various insecticides failed to detect progression to higher levels of resistance. Regressions of LC50s 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 for both insecticides in 1994 and for endosulfan in 1995 indicated higher susceptibilities at end of each season. 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 LC5Os 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; large scale dispersion and subsequent matings of susceptible and resistant genotypes that result in lower frequencies of putative resistant genes: prevalent use of insecticide mixtures that potentially eliminate individuals resistant to single insecticides; and addition of several effective insecticides representing different classes to insecticide arsenal available to growers and pest control advisors.