Chemical studies on proteinaceous attractants for fruit flies, including the identification of volatile constituents

Abstract

The chemical characteristics of protein attractants for the Queensland fruit fly, Dacus tryoni, have been investigated. Highly effective commercial fruit fly attractants were found to contain a large proportion of high molecular weight material. Lists are provided of the amino acids present in free and bound form in these hydrolysates. The products found by gas liquid chromatography-mass spectrometry analysis of methylene chloride extracts and head-space volatiles were mainly the primary and secondary products of amino acid degradation, including a number of aldehydes and significant amounts of ethylene. None of these aldehydes nor any of a number of others that may have been present in smaller quantities was found to be active when tested individually for attractancy. Nor did ethylene show any evidence of attractancy under the conditions of these tests. Strecker reagents (such as isatin), which stimulate the formation of amino acid degradation products, caused a temporary and relatively small elevation in attractancy of the commercial hydrolysates. Three enzymatic hydrolysates of a relatively pure protein (bovine serum albumin) were prepared and studied; one, a tryptic digest, was exceptionally attractive to the fruit fly. Analysis of methylene chloride extracts and head-space of these 'model' preparations revealed a number of significant differences between them, but the most important difference in relation to attractancy was found to be the presence of ammonium salts in the trypsin hydrolysate. Subsequent experiments showed that high attractancies could be attained by combinations of amino acid mixtures and ammonium bicarbonate, even though neither had high attractancy (under the particular conditions of these experiments) when tested independently. This apparent synergism was probably due to the attractancy of the ammonia coupled with a feeding stimulus provided by the amino acids.