A dramatic decline in the efficacy of fenpropathrin was documented over a three year period (1993-1995) in an areawide whitefly suppression program conducted near Gila Bend, Arizona by the Arizona Cotton Research & Protection Council. High efficacy and extended control activity were demonstrated after the initial treatment of fenpropathrin in 1993. More than 37% of the 228 contiguous cotton fields treated required no additional insecticide treatments for three full weeks post fenpropathrin treatment. However, in 1994 and 1995, all fields required additional insecticide treatment during the 2nd or 3rd week following the initial treatment with fenpropathrin. In addition, the percent of fields not showing a reduction in adults three weeks after treatment increased from 17 to 55 to 68 % in 1993, 1994 and 1995 respectively.

A significant reduction of whitefly eggs and nymphs was found three weeks after fenpropathrin treatment in 1993 but no significant reductions were found in 1994 and 1995. The supplemental use of organophosphates and pyrethroids for control during the three weeks post treatment increased dramatically, from 0.115 to 0.560 to 1.920 lbs AI/a in 1993, 1994 and 1995 respectively.

These results verify that the high levels of suppression of all life stages demonstrated by initial application of fenpropathrin in 1993 deteriorated significantly during both the 1994 and 1995 cotton program seasons.