Changes in Attractancy and Chemical Composition of the Screwworm (Diptera: Calliphoridae) Chemical Attractant, Swormlure-2, under Field Conditions1

Abstract

Swormlure-2 (SL-2), the chemical attractant of the adult Cochliomyia hominivorax (Coquerel), was 17 to 56% more attractive during about the first 5 days of field exposure than SL-2 exposed only 1 or 2 days (considered fresh). Attractancy of SL-2 declined steadily after 5 days of exposure; after about 11 days, it was half as attractive as fresh SL-2. Sex ratios were not affected by changes in attractancy. An average of 40.5% of the SL-2 evaporated during about the first 5 days of exposure. Group I chemicals (iso-butanol, sec-butanol, dimethyl disulfide, acetic acid and butyric acid) exhibited high and then rapidly declining rates of evaporation as exposure time increased. Group II chemicals (phenol, valeric acid, p-cresol, benzoic acid, and indole) exhibited low but relatively constant rates of evaporation; they comprised a larger, and eventually the greatest, proportion (52.7%) of the SL-2 vapor as exposure time increased. Fly capture rates (attractancy) and group I dosage were positively correlated, but fly capture rates and group II dosage were not. Loss of attractancy, however, may have resulted from the increased proportion of group II in SL-2 vapor. Loss of attractancy, therefore, appeared to result from changes in composition, but not decreases in dosage of SL-2 vapor. Temperature and wind were the main weather-related factors affecting evaporation.