Pheromone-mediated copulatory responses of the screwworm fly,Cochliomyia hominivorax

Meta-analytical evidence that males prefer virgin females

Males are often predicted to prefer virgin over non-virgin females because of the reduced risk of sperm competition. Does this prediction hold across studies? Our systematic meta-analysis of 138 studies, mainly conducted in invertebrates, confirms that males generally prefer virgin females. However, males preferred virgin females even in species with last male sperm precedence, suggesting that sperm competition alone does not drive male preferences. Furthermore, our results suggest that males may reject mated females even when no alternative exists. Preference for virgins is unlikely to influence female reproductive success since virginity cannot be selected for, but strong preference for virgin females could swamp or reinforce selection on other traits. Our results add to growing evidence that males are not indiscriminate in mating. However, given the unexplained heterogeneity in effect sizes, we urge caution in assuming that males will prefer virgins and recommend considering the natural context of mating decisions.

Physiological and molecular correlates of the screwworm fly attraction to wound and animal odors

The screwworm fly, Cochliomyia hominivorax (Coquerel), was successfully eradicated from the United States by the sterile insect technique (SIT). However, recent detection of these flies in the Florida Keys, and increased risk of introductions to the other areas warrant novel tools for management of the flies. Surveillance, a key component of screwworm control programs, utilizes traps baited with rotting liver or a blend of synthetic chemicals such as swormlure-4. In this work, we evaluated the olfactory physiology of the screwworm fly and compared it with the non-obligate ectoparasitic secondary screwworm flies, C. macellaria, that invade necrotic wound and feed on dead tissue. These two species occur in geographically overlapping regions. C. macellaria, along with other blowflies such as the exotic C. megacephala, greatly outnumber C. hominivorax in the existing monitoring traps. Olfactory responses to swormlure-4 constituents between sex and mating status (mated vs unmated) in both species were recorded and compared. Overall, responses measured by the antennograms offered insights into the comparative olfactory physiology of the two fly species. We also present detailed analyses of the antennal transcriptome by RNA-Sequencing that reveal significant differences between male and female screwworm flies. The differential expression patterns were confirmed by quantitative PCR. Taken together, this integrated study provides insights into the physiological and molecular correlates of the screwworm’s attraction to wounds, and identifies molecular targets that will aid in the development of odorant-based fly management strategies.

Identification of compounds in an HPLC fraction from female extracts that elicit mating responses in male screwworm flies,Cochliomyia hominivorax

When hexane extracts of mature screwworm females were chromatographed on a silica gel column, mating stimulant activity was concentrated in a fraction that eluted with hexane-ether (94∶6, v/v). Separation of this fraction with HPLC (acetonitrile-acetone; 60∶40, isocratic) resulted in a chromatogram of some 20 peaks. Only peaks 4-11 elicited mating responses. Peaks 5-10 had most of the activity, with peak 8 producing the highest response. Sixteen compounds were characterized from peak 8 by gas chromatography-mass spectrometry: six unbranched secondary acetates (C31H62O2); seven previously unreported methyl-branched secondary acetates (C32H64O2); one unbranched ketone (C31H62O); and one methyl-branched ketone (C32H64O). The isomeric acetates were not completely resolved from each other by capillary gas chromatography (CGC) on methyl silicone columns. The sixteenth compound was an aldehyde (C30H60O) that was present only in occasional peak 8 preparations. These compounds and several derivatives were characterized by capillary gas chromatography-mass spectrometry (CGC-MS). The position of the acetate group was ascertained by conversion to a keto group or by replacement of the acetate with a methyl group. Pheromone activity was not observed in peaks trapped either from CGC or by recombination of the trapped CGC peaks from HPLC peak 8. This apparent loss of activity from CGC peaks or from TLC cannot currently be explained.

Candidate sex pheromones of the New World screwworm Cochliomyia hominivorax

Five novel homologous acetate derivatives of long-chain secondary alcohols and a related ketone were tested for their efficacy as contact mating stimulants for Cochliomyia hominivorax Coquerel (Diptera: Calliphoridae). Full copulatory behaviour at a high percentage was found in tests with racemic 6-acetoxy-19-methylnonacosane at 2.5-20 microg using fertile males from three strains. Males of two strains responded nearly as well to 7-acetoxy-15-methylnonacosane, but an older strain first colonized in 1992 did not respond to this compound. Few or no copulatory responses were obtained to the other secondary alcohol acetates and a related ketone. These two acetate derivatives are the first sex pheromones identified in a calliphorid fly. The threshold of response was also tested, but could not be pinpointed.

Cuticular hydrocarbons and their role in copulatory behavior in Phormia regina (Meigen)

The cuticular hydrocarbons from adult Phormia regina (Meigen) were characterized by gas chromatography-mass spectrometry. Both sexes had similar components in nearly identical quantities, consisting of complex mixtures of saturated n-, monomethyl- and dimethylalkanes from 23 to 33 total carbons. Although no diet-, age-, or sex-specific differences were observed, cuticular hydrocarbons were shown to be involved in copulatory behavior. Hydrocarbon profiles of wild, compared to laboratory reared flies, showed no major differences. Behaviorally, males responded the same to dead decoys of either sex. Removal of the hydrocarbons, using hexane, from either male or female decoys, did not affect the number of mating strikes, but markedly reduced the number of copulatory attempts and the amount of time males spent mounted on either decoy. House fly, Musca domestica L., males when paired with a female M. domestica decoy produced copulatory attempts: whereas, when P. regina males were placed with M. domestica female decoys, there were no copulatory attempts. It is concluded that the cuticular hydrocarbons of P. regina function as species-specific but not sex-specific mating cues and elicit species-specific copulatory behavior in males.

The effect of different proportions of males and females over the Chrysomya albiceps (Wiedemann 1819) (Diptera, calliphoridae) biotic potential and longevity under laboratory conditions

Chrysomya albiceps specimens were derived from colonies kept under laboratory conditions. The oviposition period, total number of eggs-mass and the weight of the eggs-mass (average/female) presented significant differences between colonies regarding the sexual ratio of 1 male/1 female (situation 1), when compared to the other ratios (1 male/3 female, situation II), (1 male/5 female, situation III), (3 male/1 female, situation IV) and (5 male/1 female, situation V). It was ascertained that the increase in the proportion of females, resulted in higher weight and greater number of ovipositions and lengthening of the period of oviposition, leads to a decrease in their lifespan.

Analysis of genetic control of mating behavior in screwworm (Diptera: Calliphoridae) males through diallel crosses and artificial selection

The mode of genetic control of male screw-worm (Diptera: Calliphoridae) mating behavior was examined using diallel cross and artificial selection. Diallel crosses showed strong dominance effects, with hybrids being uniformly more successful in copulation than their more inbred parental strains. Weaker additive and reciprocal effects were also noted. Environmental (replicate) effects were highly significant. Regression of array variances and covariances indicated that epistatic interactions or unequal allele distribution during gametogenesis may have occurred and that high courtship propensity polygenes show dominance over low propensity genes. Artificial selection on males from outbred strains from Guatemala and Belize resulted in a decreased number of mating attempts for lines selected for reduced activity, but mating attempts in lines selected for high mating activity did not increase. A combination of inbreeding during the selection cycles as well as selection for recessive traits would explain this response. The two types of experiments were in general agreement, indicating significant dominance and environmental influence on male mating behavior with weaker additive and possible maternal effects.