The genus Philornis (Diptera: Muscidae) comprises Neotropical parasitic flies that parasitize bird nestlings while in their larval stage. The ecology of most species of these parasitic flies is largely unknown. Here, we contribute with data that shed some light on the environmental factors that are associated with variations in parasitism intensity of Philornis torquans, and examine whether increased intensity is followed by greater probability of mortality or reduced nestling growth. Intensive examination of nestlings of the bird community present in a 30 ha area was carried out weekly along two breeding seasons in Santa Fe, Argentina. Some nestlings of the most frequently parasitized bird species were followed twice a week, from hatching to fledging, to assess the impact of the parasites. High average maximum temperature and increased rainfall were significantly positively correlated with mean Philornis intensity. In turn, heavily parasitized nestlings were more likely to die: 10 larvae doubled the chances of mortality, and growth was affected in those that survived. The greater precipitation and warmer weather predicted for some areas of South America pose a potential impact on nestlings via this parasitism, and consequently on the population dynamics of native birds.

The study of myiasis is important because they may cause problems to the livestock industry, public health, or wildlife conservation. The ecology of parasitic dipterans that cause myiasis is singular, as they actively seek their hosts over relatively long distances. However, studies that address the determinants of myiasis dynamics are very scarce. The genus Philornis include species that may be excellent models to study myiasis ecology, as they exclusively parasitize bird nestlings, which stay in their nests until they are fully fledged, and larvae remain at the point of entry until the parasitic stage is over, thus allowing the collection of sequential individual-level infection data from virtually all the hosts present at a particular area. Here we offer a stratified multi-level analysis of longitudinal data of Philornis torquans parasitism in replicated forest bird communities of central Argentina. Using Generalized Linear Models and Generalized Linear Mixed Models and an information theory approach for model selection, we conducted four groups of analyses, each with a different study unit, the individual, the brood, the community at a given week, and the community at a given year. The response variable was larval abundance per nestling or mean abundance per nestling. At each level, models included the variables of interest of that particular level, and also potential confounders and effect modifiers of higher levels. We found associations of large magnitude at all levels, but only few variables truly governed the dynamics of this parasite. At the individual level, the infection was determined by the species and the age of the host. The main driver of parasite abundance at the microhabitat level was the average height of the forest, and at the community level, the density of hosts and prior rainfall. This multi-level approach contributed to a better understanding of the ecology of myiasis.

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