Spatiotemporal variability of horsefly abundance and richness in northwestern Patagonia

Variabilidad espaciotemporal de abundancia y riqueza de tábanos en el noroeste de la Patagonia

Horseflies (Diptera: Tabanidae) are a nuisance to humans and livestock in many regions around the world including Patagonia, Argentina. This is due to the blood-feeding habit of female horseflies, which generally need protein to produce eggs. Adult horseflies of several native species are remarkably abundant from late spring to early summer across a variety of environments in northwestern Patagonia, from the arid steppe towards the east, to the humid rainforests in the western portion of the Andes. In some locations, a large number of horseflies can be simultaneously swarming on a single person or a large animal such as a cow or a horse (Coscarón, 1998). In the Patagonian-Andean region, where tourism represents one of the most important economic activities, tabanids are viewed as a pest by the general public due to the disruption of outdoor activities (Coscarón & González, 1989; Coscarón, 1998). Although in Patagonia there are no records of Tabanidae species as vectors of disease-causing microorganisms to livestock, tabanids are known as suspected disease vectors (e.g., equine infectious anemia) in other Argentinean regions adjacent to Patagonia (De la Sota et al., 2005; Ricotti et al., 2016). On the other hand, several species of Tabanidae in Patagonia have significant ecological roles. For instance, some species feed on the nectar of native plant species potentially pollinating many of them (Lessard, 2014) and larvae of most species are important predators of soil and aquatic invertebrates (Coscarón, 1998; Massaferro et al., 2011). Lastly, larvae of some abundant species constitute food sources for other animals including invasive species such as the wild boar (Skewes et al., 2007). Thus, due to their high abundance, pest status and ecological roles, horseflies are a key component of the entomological fauna of the region.

Most research on horseflies in Patagonia has been conducted specifically on their taxonomy, morphology, and broad geographic distribution (Coscarón, 1998) but little is known on the ecology and behavior of Tabanidae in this particular region. There are several comprehensive studies and revisions on the taxonomy of horseflies that include species in Patagonia, which also provide information on the geographic distribution of some species (Wilkerson & Coscarón, 1984; Coscarón, 1985; González, 1999, 2017; Lessard, 2014; Gonzálezet al., 2022). In the patagonian region in Argentina there are over 40 described Tabanidae species, most of which belong to the Dasybasis Macquart genus (Coscarón, 1998; Massaferro et al., 2011), with some of the species being very abundant. The tribe Scionini also contains some common species, which can be a nuisance to humans and livestock in the shrublands and western humid rainforests. For instance Osca lata Guérin-Méneville (syn. Scaptia lata) is known to feed on humans and animals and is highly abundant in the Valdivian rainforest (Coscarón & González, 1989).

Despite these important contributions to the knowledge of Tabanidae, studies in Patagonia assessing population dynamics, the ecology and behavioral aspects of horseflies are surprisingly scarce. Additionally, basic information, such as relative species abundance, habitat preference, phenology and feeding habits is lacking. Information on habitat preference is mostly based on opportunistic observations and locations of the collected specimens in museum collections but there is no systematic specimen sampling associated with habitat types. Documenting community composition and abundance fluctuations of adult horsefly populations over the flying period in relation to temperature variations and site characteristics is necessary to better understand the environmental requirements of the most common tabanid species.

The objective of this study is to know species richness and abundance of Tabanidae over three months, from late spring to mid-summer in northwestern Patagonia, Argentina, to qualitatively evaluate their spatial and temporal variability. Our intention is to conduct an initial assessment aimed at describing the fluctuations in horsefly populations throughout the flight period and among different locations, and associate these with temperature and relative air humidity fluctuations and among different sites with diverse vegetation and presence/absence of livestock.

A total of 483 individuals of four species were captured throughout the sampling season. Most individuals (87 %) belonged to the species Dasybasis antilope Brètheswhile 12 % to the species Scaptia australis Philippi, and the remaining 1 % to D. gagatina Philipi and D. andicola Philipi (Table II). The most abundant species (D. antilope) was captured in four of the five sites, although the capture was representative almost exclusively in one site (Baqueanos), whereas individuals of the second most abundant species (S. australis) were more evenly distributed among sites. There was a large variability in the number of captured horseflies among sites ranging from a total of 416 in Baqueanos (all D. antilope, except one individual) to two in Hotel. The site with the highest richness (i.e., Seccional; four spp.) was also one of the sites with the lowest abundance (nine individuals) together with Hotel, with only two individuals. Both, Melgarejo and Guillelmo had intermediate abundances mostly composed of S. australis, but in the warmest site (Melgarejo) the captures peaked in mid-December, whereas in the coolest site (Guillelmo) these peaked in early January.

Table II.
Tabanidae species captured in five sites in northwestern Patagonia, for three months 2014/2015/late Spring 2014 to middle Summer 2015.

Overall, variations in horsefly abundance weakly follow average temperature fluctuations during December and January (Fig. 1). However, after the drop in average temperatures of approximately 4 °C recorded in mid to late December (Dec 18 to Dec 26), captures decreased by nearly 70 % and recovered after two weeks following an increase in temperature (except in Melgarejo and Hotel). Despite this, in Baqueanos, there was also a similar decrease in captures by mid-January that was not associated with temperature drops. By early February, the abundance of horseflies captured decreased in Baqueanos, Guillelmo and Seccional (Fig. 1).

Figure 1.
Mean air temperature, mean relative humidity %, and total captured horseflies of four species during the trapping period across five different sites with contrasting characteristics in northwestern Patagonia.

Tabanid phenology, abundance and species composition seem to be influenced by site characteristics (Table I; Fig. 1) with temperature variability being another factor driving population numbers. Average recorded temperatures for all these sites are markedly different, with the most extreme difference being of 2.6 °C in overall average temperatures (12.3 vs. 14.9 °C, Table I), resulting in a shift of the moment when populations peak depending on site local temperatures. For instance, in the site with the highest temperatures (i.e., Melgarejo), captures peaked early, during mid-December 2014, while in the site with the lowest temperatures (i.e., Guillelmo) captures peaked a few weeks later, in mid-January 2015 (Fig. 1). Nevertheless, one of the sites where temperatures were found to be moderate (i.e., Baqueanos) captures were relatively constant throughout the flight season (Fig. 1).

This is the first study that provides novel information on the relative abundance and composition of tabanids in northwestern Patagonia. Horsefly abundance and richness are highly variable among sites showing an enormous variation in horsefly abundance among relatively close environments. Overall recorded richness was relatively low compared to the number of species cited for the entire patagonian region in Argentina (47 according to Coscarón, 1998). Although this study involves a modest sampling effort, it nevertheless constitutes a significant progress for the knowledge of the spatiotemporal variability of horseflies in the study area. Future studies should encompass a wider variety of environments during multiple sampling periods to better characterize the composition of a wider portion of the horsefly community in Patagonia.

Our results suggest an influence of site characteristics on the abundance and species composition of horseflies. Higher horsefly abundance of Dasybasis antilope in Baqueanos could be associated with several characteristics of the site. Livestock presence has been shown to be associated with increased horsefly abundance in certain areas, as large animals result more attractive from an olfactory and visual point of view to female horseflies (Baldacchino et al., 2013). In addition, the wetland is potentially a better location for larval development of this species. Despite that the preferred environment of D. antilope larvae has not been described in the literature, it is likely that wet soils are favored as it occurs with many Tabanidae species (Mullens, 2019), including other Dasybasis species, such as D. pruinivitta (González, 2004). Finally, Baqueanos had the largest clearing among sites (approximately 35 % larger than the other sites in average) where traps were placed. It is likely that trap increased visibility also contributed to the large number of captures. The marked influence of site location on tabanid abundance and species composition agrees with recent findings of research conducted in Uruguay (Lucaset al., 2020) where the environment was a determining factor related to the number of horseflies captures (largely represented by Tabanus campestris Brèthes, T. aff. platensis Brèthes and D. missionum (Macquart)).

In this study we documented a qualitative relation between horsefly abundance and average temperature. Although, monitoring populations over several years is needed to accurately evaluate the influence of weather variability on horsefly abundance, our observations provide preliminary information on this aspect. Weather events, such as the mean temperature drop by mid-late December suggest that low temperatures can negatively affect horsefly captures either by reducing insect activity or lowering local abundance due to higher mortality rates, as occurs with other Tabanidae species (Herczeget al., 2015). The rebound in captures two weeks later with higher temperatures, suggests new emergences and/or adults becoming active again after the cold period. The decrease in activity observed during mid-January is intriguing as is not associated to lower temperatures. This result suggests that additional factors, other than temperature and relative humidity fluctuations, may play a role in horsefly activity levels. Even though we only surveyed tabanid abundance and richness in five sites, some of them with low captures, results suggest that differences in temperatures between sites seem to shift the moment when populations peak in their abundance (within our sampling period), with the site with the lowest temperatures being most abundant a few weeks later than the site with the higher temperature.

Tabanids are known to be vectors of several livestock diseases worldwide (Baldacchino et al., 2014). In Argentina they are known vectors of equine infectious anemia virus (EIAV) in areas adjacent to Patagonia to the north (Ricottiet al., 2016). Global change, through temperature rise and increased livestock transport can favor the spread of these diseases to northwestern Patagonia in the near future. Furthermore, although not quantified in the region, large densities of horseflies attacking cattle can result in losses in weight gain and reduction in milk production (Perichet al., 1986; Baldacchinoet al., 2014). Therefore, data on species composition, relative abundance over time and species-habitat associations are much needed to face the likely expansion of diseases transmitted by horseflies to livestock in our study region. The current study is an initial contribution toward understanding spatiotemporal fluctuations in horsefly populations and activity in the Argentinean Patagonia.

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We dedicate this paper to the memory of Dr. Sixto Coscarón who made essential contributions to the knowledge of Tabanidae of South America. We thank him for identifying voucher specimens in this study. We thank the landowners who allowed us to install traps on their properties (H. Vereertbrugghen and C. Núñez). We are also thankful to M.G. Socías, M.E. Elgueta Donoso, and an anonymous reviewer for their suggestions that helped to improve this manuscript. The Argentinean National Park Service Administration granted permission to sample in the Nahuel Huapi National Park.