The physiological age of female horn flies (Diptera: Muscidae) emigrating from a natural population.

Abstract

The dispersal of a naturally occurring population of horn flies, Haematobia irritans (L.), was examined, and age-grading techniques were applied to females as an indicator of the age structure of the dispersing population. Age structures of native and emigrant populations were found to be substantially different. Parous females were a principal component of the native population; their frequency in the population oscillated but seldom was <50%. Parous females were only 11.3% of the recent emigrants, and their frequency varied little during the season. Previtellogenic, nulliparous females were <15% of the native population on all but one occasion, when they were 30%. In contrast, the frequency of these young females among emigrant females was 36.2-59.1%. An hypothesis of independence of vitellogenic stage and population was rejected. Almost all (98%) parous females examined for spermatozoa were mated, and their frequencies were essentially identical for the native and emigrant populations. Insemination rates for nulliparous females were considerably lower than for parous females and were at different levels for the populations. Frequency of insemination increased from zero for previtellogenic nulliparous females to 100% for gravid females. Insemination and stage of vitellogenesis were significantly correlated. We attribute the difference in insemination rates for nulliparous females of native and emigrant populations to the different proportions of the previtellogenic nulliparous females in the two populations. The data support the conclusions that older females in an extant population did not have the same propensity to disperse as did younger females, or that the principal source of migrating flies was newly emerged adults from the larval habitat rather than the extant adult population.