Darwin’s finches combat introduced nest parasites with fumigated cotton

Urban living can rescue Darwin's finches from the lethal effects of invasive vampire flies

Human activity changes multiple factors in the environment, which can have positive or negative synergistic effects on organisms. However, few studies have explored the causal effects of multiple anthropogenic factors, such as urbanization and invasive species, on animals and the mechanisms that mediate these interactions. This study examines the influence of urbanization on the detrimental effect of invasive avian vampire flies (Philornis downsi) on endemic Darwin's finches in the Galápagos Islands. We experimentally manipulated nest fly abundance in urban and non-urban locations and then characterized nestling health, fledging success, diet, and gene expression patterns related to host defense. Fledging success of non-parasitized nestlings from urban (79%) and non-urban (75%) nests did not differ significantly. However, parasitized, non-urban nestlings lost more blood, and fewer nestlings survived (8%) compared to urban nestlings (50%). Stable isotopic values (δ15 N) from urban nestling feces were higher than those from non-urban nestlings, suggesting that urban nestlings are consuming more protein. δ15 N values correlated negatively with parasite abundance, which suggests that diet might influence host defenses (e.g., tolerance and resistance). Parasitized, urban nestlings differentially expressed genes within pathways associated with red blood cell production (tolerance) and pro-inflammatory response (innate immunological resistance), compared to parasitized, non-urban nestlings. In contrast, parasitized non-urban nestlings differentially expressed genes within pathways associated with immunoglobulin production (adaptive immunological resistance). Our results suggest that urban nestlings are investing more in pro-inflammatory responses to resist parasites but also recovering more blood cells to tolerate blood loss. Although non-urban nestlings are mounting an adaptive immune response, it is likely a last effort by the immune system rather than an effective defense against avian vampire flies since few nestlings survived.

Nest Box Bacterial Loads Are Affected by Cavity Use by Secondary Hole Nesters

Among the environmental factors that affect bird nesting in nest boxes, the influence of microbial communities is relatively poorly understood. In this study, nest boxes used for breeding by secondary cavity nesters were sampled before the start of the breeding season to assess the bacterial loads of the nest box in relation to their previous year status. Different parts of the wooden nest box offer variable conditions for the development of bacteria. During the breeding season, the nest box entrance hole is wiped out by birds, delivering bacteria to their bodies, but during winter, it is exposed to unfavourable external conditions. The interior of the nest box, in turn, is also wiped by birds, but the conditions during winter are more stable there. Therefore, samples from the entrance hole and the interior of the nest box were taken at two different study sites: an urban parkland and a natural forest. We predicted that both the occupancy of the nest boxes during the previous breeding season by birds and the nesting sites would influence the bacterial load of the nest box. To verify this prediction, two categories of nest boxes were sampled at both study sites: nest boxes occupied by any of the two tit species (Great Tit or Blue Tit) in the previous season for breeding and nest boxes that had remained empty that year. The interior bacterial load of the nest box was higher in the nest boxes occupied in the previous breeding season, but only in the forest area. Furthermore, the bacterial load of both the entrance hole of the nest box and the interior was significantly higher in the forest study area in both occupied and unoccupied nest boxes. Our results show that the bacterial load of the nest box is positively related to the presence of nests in the previous breeding season and can vary between different sites.

A comparison of nest-site characteristics for two sympatric Estrildid finches (Uraeginthus spp.) in Tanzania

It is well known that birds have great diversity in nesting strategies, but we still have limited knowledge of the variation among species that share the habitat. Here, I will report and compare the nesting strategies between the two sympatric songbirds. Blue-capped (Uraeginthus cyanocephalus) and red-cheeked cordon-bleus (Uraeginthus bengalus) are socially monogamous, biparental songbirds (family Estrildidae) that sympatrically inhabit arid landscapes with trees and bushes in East Africa. Both species build domed nests with grasses that are often located near wasp nests. They also sometimes take over old weaver (family Ploceidae) nests. While these nesting strategies are already described as common behavioral traits in both species in the literature, interspecies variation in these nesting strategies in areas of sympatry is not reported. My initial field observation during their breeding season suggested that whether these nesting strategies were adopted or not varied somewhat between the sympatric cordon-bleus. Thus, I carried out a more formal investigation to test these differences. I found that red-cheeked cordon-bleus built their nests near wasp nests more frequently than blue-capped cordon-bleus, while I did not find any other significant differences between the nesting sites of the two species, such as the use of weaver nests, the types of nesting plants, or nest heights. These results suggest that the sympatric cordon-bleus share several nest-site characteristics, but that red-cheeked cordon-bleus have an affinity for nesting near wasp nests. Further studies will be required to elucidate the costs and benefits of these nesting strategies or the role that adjacency to wasp nests might play in the sympatry of the two species.

Evaluating Volatile Plant Compounds of Psidium galapageium (Myrtales: Myrtaceae) as Repellents Against Invasive Parasitic Diptera in the Galapagos Islands

Plant-based repellents represent a safe, economic, and viable alternative to managing invasive insects that threaten native fauna. Observations of self-medication in animals can provide important cues to the medicinal properties of plants. A recent study in the Galapagos Islands found that Darwin's finches apply the leaves of Psidium galapageium (Hooker 1847) to their feathers, extracts of which were repellent to mosquitoes and the parasitic fly Philornis downsi (Dodge & Aitkens 1968; Diptera: Muscidae). Introduced mosquitoes are suspected vectors of avian pathogens in the Galapagos Islands, whereas the larvae of P. downsi are blood-feeders, causing significant declines of the endemic avifauna. In this study, we investigated the volatile compounds found in P. galapageium, testing each against a model organism, the mosquito Anopheles arabiensis (Patton 1905; Diptera: Culicidae), with the aim of singling out the most effective compound for repelling dipterans. Examinations of an ethanolic extract of P. galapageium, its essential oil and each of their respective fractions, revealed a mixture of monoterpenes and sesquiterpenes, the latter consisting mainly of guaiol, trans-nerolidol, and β-eudesmol. Of these, trans-nerolidol was identified as the most effective repellent to mosquitoes. This was subsequently tested at four different concentrations against P. downsi, but we did not find a repellence response. A tendency to avoid the compound was observed, albeit significance was not achieved in any case. The lack of repellence suggests that flies may respond to a combination of the volatile compounds found in P. galapageium, rather than to a single compound.

Behavior of the Avian Parasite Philornis downsi (Diptera: Muscidae) in and Near Host Nests in the Galapagos Islands

The Avian Vampire Fly, Philornis downsi, has invaded the Galapagos Islands, where it causes high mortality of endemic and native landbird species, including most species of Darwin's finches. Control methods are under development, but key information is missing about the reproductive biology of P. downsi and the behavior of flies in and near nests of their hosts. We used external and internal nest cameras to record the behavior of P. downsi adults within and outside nests of the Galapagos Flycatcher, Myiarchus magnirostris, throughout all stages of the nesting cycle. These recordings showed that P. downsi visited flycatcher nests throughout the day with higher fly activity during the nestling phase during vespertine hours. The observations also revealed that multiple P. downsi individuals can visit nests concurrently, and that there are some interactions among these flies within the nest. Fly visitation to nests occurred significantly more often while parent birds were away from the nest than in the nest, and this timing appears to be a strategy to avoid predation by parent birds. We report fly mating behavior outside the nest but not in the nest cavity. We discuss the relevance of these findings for the adaptive forces shaping P. downsi life history strategies as well as rearing and control measures.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s10905-021-09789-7.

Optimizing Risk Management Strategies for the Control of Philornis downsi—A Threat to Birds in the Galápagos Islands

One of the most concerning threats to Galápagos bird populations, including some critically endangered species, is the invasive parasitic fly Philornis downsi. While long-term sustained solutions are under study, immediate actions are needed to reduce the impacts of this fly. Application of permethrin to birds's nests has been successfully done, but there might be potential long-term reproductive effects to birds. Cyromazine, an insect growth regulator, has been proposed as an alternative, but its risks and effectiveness are unknown. The goal of this study was to assist managers to determine which combination of chemical (permethrin or cyromazine) and mode of application (injection, spray, and self-fumigation) was likely to be most effective to control P. downsi while minimizing toxicity to small land birds in Galápagos, given data available and high levels of uncertainty in some cases. This study is presented as a semi-quantitative risk assessment employing the use of a multi-criteria decision analysis (MCDA) model. For the six potential alternatives resulting from the combination of chemical and mode of application, the criteria were given a score from 1 to 6 supported by available evidence from the literature and from expert opinion. In addition, three different scenarios with different sets of weights for each criterion were assessed with stakeholder's input. Considering the scenario with higher weight to effectiveness of the method against P. downsi while also weighing heavily to minimize the toxicity to birds, cyromazine spray followed by permethrin injection were the preferred strategies. Self-fumigation was the mode of application with highest uncertainty but with much potential to be further explored for its feasibility. The approach taken here to evaluate mitigation strategies against an important threat for avian species in Galápagos can also be used in other conservation programs when making real time decisions under uncertainty.

Trans-ovo permethrin exposure affects growth, brain morphology and cardiac development in quail

Permethrin is a commonly used, highly effective pesticide in poultry agriculture, and has recently been trialed in conservation efforts to protect Galápagos finch hatchlings from an invasive ectoparasite. Although permethrin is considered safe for adults, pesticides can have health consequences when animals are exposed during early life stages. The few studies that have examined permethrin's effects in embryonic chicks and rats have shown hydrocephaly, anencephaly, reduced cellular energy conversion, and disruption of developing heart muscle. To test whether trans-ovo exposure of permethrin affects early development in birds, we exposed Japanese quail (Coturnix japonica) eggs to cotton treated with 1% permethrin that was incorporated into nests in two amounts (0.2, 0.8 g), each with a paired untreated cotton control group. When measured on incubation Day 15, we found permethrin-treated developing birds were smaller and showed signs of microcephaly, although mortality rates were the same. Despite no difference in heart mass, ventricular tissue was less compact, cardiac arteries were reduced and heart rates were slower in permethrin-treated birds. Differences in heart development were also observed at 5 days of incubation, indicating that abnormalities are present from early in cardiac development. Future studies are needed to examine permethrin's effects on developmental pathways and to determine if these effects persist after hatching to affect offspring health. This study provides evidence that permethrin can cross the eggshell to cause non-lethal but adverse effects on embryonic development, and studies should look beyond hatching when monitoring the efficacy of permethrin on wild bird populations.

Urban living influences the nesting success of Darwin's finches in the Galápagos Islands

Urbanization is expanding worldwide with major consequences for organisms. Anthropogenic factors can reduce the fitness of animals but may have benefits, such as consistent human food availability. Understanding anthropogenic trade-offs is critical in environments with variable levels of natural food availability, such as the Galápagos Islands, an area of rapid urbanization. For example, during dry years, the reproductive success of bird species, such as Darwin's finches, is low because reduced precipitation impacts food availability. Urban areas provide supplemental human food to finches, which could improve their reproductive success during years with low natural food availability. However, urban finches might face trade-offs, such as the incorporation of anthropogenic debris (e.g., string, plastic) into their nests, which may increase mortality. In our study, we determined the effect of urbanization on the nesting success of small ground finches (Geospiza fuliginosa; a species of Darwin's finch) during a dry year on San Cristóbal Island. We quantified nest building, egg laying and hatching, and fledging in an urban and nonurban area and characterized the anthropogenic debris in nests. We also documented mortalities including nest trash-related deaths and whether anthropogenic materials directly led to entanglement- or ingestion-related nest mortalities. Overall, urban finches built more nests, laid more eggs, and produced more fledglings than nonurban finches. However, every nest in the urban area contained anthropogenic material, which resulted in 18% nestling mortality while nonurban nests had no anthropogenic debris. Our study showed that urban living has trade-offs: urban birds have overall higher nesting success during a dry year than nonurban birds, but urban birds can suffer mortality from anthropogenic-related nest-materials. These results suggest that despite potential costs, finches benefit overall from urban living and urbanization may buffer the effects of limited resource availability in the Galápagos Islands.

Timing of infestation influences virulence and parasite success in a dynamic multi-host-parasite interaction between the invasive parasite, Philornis downsi, and Darwin's finches

Recently commenced host–parasite interactions provide an excellent opportunity to study co-evolutionary processes. Multi-host systems are especially informative because variation in virulence between hosts and temporal changes provides insight into evolutionary dynamics. However, empirical data under natural conditions are scarce. In the present study, we investigated the interaction between Darwin’s finches and the invasive fly Philornis downsi whose larvae feed on the blood of nestlings. Recently, however, the fly has changed its behavior and now also attacks incubating females. Two sympatric hosts are affected differently by the parasite and parasite load has changed over time. Our study observed a reversal of trends described two decades ago: while, currently, small tree finches (Camarhynchus parvulus) experience significantly higher parasite load than warbler finches (Certhidea olivacea), this was the opposite two decades ago. Currently, fledging success is higher in warbler finches compared to small tree finches. Our data indicate that not only intensity but also timing of infestation influences hosts’ reproductive success and parasite fitness. During incubation, prevalence was higher in warbler finches, but once chicks had hatched, prevalence was 100% in both species and parasite load was higher in small tree finches. Furthermore, our results suggest faster development and higher reproductive success of P. downsi in small tree finch nests. A change in host preference driven by larvae competition could have led to the reversal in parasite load.

Sub-lethal effects of permethrin exposure on a passerine: implications for managing ectoparasites in wild bird nests

Permethrin is increasingly used for parasite control in bird nests, including nests of threatened passerines. We present the first formal evaluation of the effects of continued permethrin exposure on the reproductive success and liver function of a passerine, the zebra finch (Taeniopygia guttata), for two generations. We experimentally treated all nest material with a 1% permethrin solution or a water control and provided the material to breeding finches for nest building. The success of two consecutive clutches produced by the parental generation and one clutch produced by first-generation birds were tracked. Finches in the first generation were able to reproduce and fledge offspring after permethrin exposure, ruling out infertility. Permethrin treatment had no statistically significant effect on the number of eggs laid, number of days from clutch initiation to hatching, egg hatch rate, fledgling mass or nestling sex ratio in either generation. However, treating nest material with permethrin significantly increased the number of hatchlings in the first generation and decreased fledgling success in the second generation. Body mass for hatchlings exposed to permethrin was lower than for control hatchlings in both generations, but only statistically significant for the second generation. For both generations, an interaction between permethrin treatment and age significantly affected nestling growth. Permethrin treatment had no effect on liver function for any generation. Permethrin was detected inside 6 of 21 exposed, non-embryonated eggs (28.5% incidence; range: 693-4781 ng of permethrin per gram of dry egg mass). Overall, results from exposing adults, eggs and nestlings across generations to permethrin-treated nest material suggest negative effects on finch breeding success, but not on liver function. For threatened bird conservation, the judicious application of this insecticide to control parasites in nests can result in lower nestling mortality compared to when no treatment is applied. Thus, permethrin treatment benefits may outweigh its sub-lethal effects.

Trypanosomatids Detected in the Invasive Avian Parasite Philornis downsi (Diptera: Muscidae) in the Galapagos Islands

Alien insect species may present a multifaceted threat to ecosystems into which they are introduced. In addition to the direct damage they may cause, they may also bring novel diseases and parasites and/or have the capacity to vector microorganisms that are already established in the ecosystem and are causing harm. Damage caused by ectoparasitic larvae of the invasive fly, Philornisdownsi (Dodge and Aitken) to nestlings of endemic birds in the Galapagos Islands is well documented, but nothing is known about whether this fly is itself associated with parasites or pathogens. In this study, diagnostic molecular methods indicated the presence of insect trypanosomatids in P. downsi; to our knowledge, this is the first record of insect trypanosomatids associated with Philornis species. Phylogenetic estimates and evolutionary distances indicate these species are most closely related to the Crithidia and Blastocrithidia genera, which are not currently reported in the Galapagos Islands. The prevalence of trypanosomatids indicates either P. downsi arrived with its own parasites or that it is a highly suitable host for trypanosomatids already found in the Galapagos Islands, or both. We recommend further studies to determine the origin of the trypanosomatid infections to better evaluate threats to endemic fauna of the Galapagos Islands.

Behavioral Responses of the Invasive Fly Philornis downsi to Stimuli from Bacteria and Yeast in the Laboratory and the Field in the Galapagos Islands

Philornis downsi Dodge and Aitken (Diptera: Muscidae) is an avian parasitic fly that has invaded the Galapagos archipelago and exerts an onerous burden on populations of endemic land birds. As part of an ongoing effort to develop tools for the integrated management of this fly, our objective was to determine its long- and short-range responses to bacterial and fungal cues associated with adult P. downsi. We hypothesized that the bacterial and fungal communities would elicit attraction at distance through volatiles, and appetitive responses upon contact. Accordingly, we amplified bacteria from guts of adult field-caught flies and from bird feces, and yeasts from fermenting papaya juice (a known attractant of P. downsi), on selective growth media, and assayed the response of flies to these microbes or their exudates. In the field, we baited traps with bacteria or yeast and monitored adult fly attraction. In the laboratory, we used the proboscis extension response (PER) to determine the sensitivity of males and females to tarsal contact with bacteria or yeast. Long range trapping efforts yielded two female flies over 112 trap-nights (attracted by bacteria from bird feces and from the gut of adult flies). In the laboratory, tarsal contact with stimuli from gut bacteria elicited significantly more responses than did yeast stimuli. We discuss the significance of these findings in context with other studies in the field and identify targets for future work.

PREFLEDGING MORTALITY AND THE ABUNDANCE OF MOSQUITOES BITING NESTLING BARN OWLS (TYTO ALBA) IN FLORIDA, USA

Mosquito-borne diseases can have disastrous effects on avian populations; therefore, most studies of bird and mosquito interactions have focused on the mortality and morbidity associated with the diseases. However, the effect of mosquitoes feeding on birds, independent of disease, has not been well studied. We studied Barn Owls (Tyto alba) nesting in artificial nest boxes in sugarcane (Saccharum officinarum) fields in Florida, US. To reduce mosquito effects on nestlings, we used an insecticide spray in half of the nest boxes. Mosquito suction traps were fixed to the outside of eight nest boxes (four treated and four untreated) to collect mosquitoes over a 24-h period (one trap night) once weekly, from incubation until all nestlings fledged. Collected mosquitoes were counted, sorted into blood-fed and unfed females, and identified to species when possible. The dominant mosquito species captured were Culex nigripalpus, Mansonia dyari, and Mansonia titillans. The highest total number of mosquitoes and blood-fed mosquitoes captured in a suction trap in one trap night was 3,193 and 379, respectively. Overall, significantly fewer mosquitoes were captured from treated nest boxes compared to untreated boxes. Nestling age influenced the total number of mosquitoes captured, with the highest numbers associated with fledglings 22-42 d old. The highest numbers of blood-fed mosquitoes were captured when nestlings were 22-28 d old. Nestlings in insecticide-treated boxes had higher survival rates compared to those in untreated boxes during months with high mosquito numbers. Mosquitoes can impose energetic costs on nestlings by causing stress from irritation, dehydration, and the constant regeneration of blood cells. These costs, in addition to factors such as food shortage, temperature, and overall health of the nestling, can contribute to higher mortality rates during nesting periods with high mosquito numbers.

Nest building by Darwin’s finches as an overlooked seed dispersal mechanism

Caliochory, or seed dispersal by birds as nest material, has been reported for several species, but its effectiveness remains unclear in most cases. Darwin’s finches are traditionally regarded as seed predators, but the observation of two nests challenges this assumption by demonstrating that they can act as seed dispersers via caliochory. Darwin’s finches incorporate cotton-like materials into their nests, including seeds of Darwin’s cotton (Gossypium darwinii), a shrub endemic to the Galápagos (Ecuador). Bird nests typically break down after intense rainfall, so the seeds incorporated into nests might benefit from suitable conditions for germination. By simulating the germination conditions experienced over a 72-h period by cotton seeds in a naturally fallen nest, this study qualitatively confirms the long-term viability of at least a small fraction of the seeds at the surface of the nest. Darwin’s finches might therefore provide seed-dispersal services to Darwin’s cotton and possibly, other native and exotic plants of the Galápagos commonly incorporated into nests. However, larger confirmatory studies are needed.

Galápagos mockingbirds tolerate introduced parasites that affect Darwin's finches

Introduced parasites threaten native host species that lack effective defenses. Such parasites increase the risk of extinction, particularly in small host populations like those on islands. If some host species are tolerant to introduced parasites, this could amplify the risk of the parasite to vulnerable host species. Recently, the introduced parasitic nest fly Philornis downsi has been implicated in the decline of Darwin's finch populations in the Galápagos Islands. In some years, 100% of finch nests fail due to P. downsi; however, other common host species nesting near Darwin's finches, such as the endemic Galápagos mockingbird (Mimus parvulus), appear to be less affected by P. downsi. We compared effects of P. downsi on mockingbirds and medium ground finches (Geospiza fortis) on Santa Cruz Island in the Galápagos. We experimentally manipulated the abundance of P. downsi in nests of mockingbirds and finches to measure the direct effect of the parasite on the reproductive success of each species of host. We also compared immunological and behavioral responses by each species of host to the fly. Although nests of the two host species had similar parasite densities, flies decreased the fitness of finches but not mockingbirds. Neither host species had a significant antibody-mediated immune response to P. downsi. Moreover, finches showed no significant increase in begging, parental provisioning, or plasma glucose levels in response to the flies. In contrast, parasitized mockingbird nestlings begged more than nonparasitized mockingbird nestlings. Greater begging was correlated with increased parental provisioning behavior, which appeared to compensate for parasite damage. The results of our study suggest that finches are negatively affected by P. downsi because they do not have such behavioral mechanisms for energy compensation. In contrast, mockingbirds are capable of compensation, making them tolerant hosts, and a possible indirect threat to Darwin's finches.

Ectoparasites as developmental stressors: Effects on somatic and physiological development

Developmental stress can alter resource allocation in early life, and in altricial birds with rapid developmental trajectories and high resource demands, nestlings may adjust early resource partitioning to cope with challenging environments. We experimentally manipulated ectoparasite levels in nests and assessed whether ectoparasites affected somatic and physiological development in European starling (Sturnus vulgaris) nestlings. We hypothesized that mites act as developmental stressors in nestlings and predicted that nestlings from infested nests would exhibit either reduced somatic growth, or reduced physiological development, including impaired innate immunity, and would have elevated corticosterone concentrations. We either added ≈200 mites to nests during early incubation, or treated nests with a pesticide, permethrin, to reduce mites and possibly other arthropods. We assessed treatment effects on egg spottiness and mite abundance, and monitored offspring hatching and survival. We also measured somatic growth (mass, tarsus length, and feather growth), hematocrit, immune-related metrics (bacterial killing ability [BKA] and spleen mass), and baseline corticosterone concentrations in response to treatment. Compared with mite treatment, permethrin reduced egg spottiness and mite abundance in nests. Relative to nestlings in mite-reduced nests, nestlings in mite-enhanced nests had lower survival, hematocrit, and corticosterone concentrations. Early in development, nestlings from both treatments exhibited similar rapid somatic growth, yet mite-treated nestlings exhibited lower BKA. Nestlings in both treatments increased BKA across development, despite nestlings in mite-treated nests exhibiting lower mass as nest leaving neared. Overall, we found evidence that mites can act as development stressors, but contrary to our prediction, mites decreased corticosterone concentrations.

Alien Invasion: Biology of Philornis Flies Highlighting Philornis downsi, an Introduced Parasite of Galápagos Birds

The muscid genus Philornis comprises approximately 50 described species of flies, nearly all of which are obligate parasites of nestling birds. Philornis species are native to the Neotropics and widely distributed from Florida to Argentina. Most research on this group has focused on P. downsi, which was introduced to the Galápagos Islands in the late twentieth century. Although Philornis parasitism kills nestlings in several native host species, nowhere do the effects seem more severe than in P. downsi in the Galápagos. Here, we review studies of native and introduced Philornis in an attempt to identify factors that may influence virulence and consider implications for the conservation of hosts in the Galápagos.

Identification and Optimization of Microbial Attractants for Philornis downsi, an Invasive Fly Parasitic on Galapagos Birds

We investigated the role of olfactory cues from actively fermenting yeast (Saccharomyces cerevisiae) in attraction of adult Philornis downsi and identified two synergistically attractive yeast volatiles. Larvae of this invasive fly parasitize the hatchlings of passerines and threaten the Galapagos avifauna. Gas chromatography coupled with electroantennographic detection (GC-EAD), coupled gas chromatography-mass spectrometry (GC-MS), and field trapping experiments were used to identify volatile compounds from a yeast-sugar solution. EAD responses were consistently elicited by 14 yeast volatiles. In a series of field trapping experiments, a mixture of the 14 EAD-active compounds was similarly attractive to P. downsi when compared to the yeast-sugar solution, and we found that acetic acid and ethanol were essential for attraction. A mixture of 0.03 % acetic acid and 3 % ethanol was as attractive as the 14-component blend, but was not as attractive as the yeast-sugar solution. Philornis downsi showed positive and negative dose-responses to acetic acid in the ranges of 0.01 ~ 0.3 % and 0.3 ~ 9 %, respectively. Further optimization showed that the mixture of 1 % acetic acid and 3 % ethanol was as attractive as the yeast-sugar solution. Both mixtures of acetic acid and ethanol were more selective than the yeast-sugar solution in terms of non-target moths and Polistes versicolor wasps captured. These results indicate that acetic acid and ethanol produced by yeasts are crucial for P. downsi attraction to fermented materials on which they feed as adults and can be used to manage this invasive fly in Galapagos.

Darwin's finches treat their feathers with a natural repellent

Darwin's finches are highly innovative. Recently we recorded for the first time a behavioural innovation in Darwin's finches outside the foraging context: individuals of four species rubbed leaves of the endemic tree Psidium galapageium on their feathers. We hypothesised that this behaviour serves to repel ectoparasites and tested the repellency of P. galapageium leaf extracts against parasites that negatively affect the fitness of Darwin's finches, namely mosquitoes and the invasive hematophagous fly Philornis downsi. Mosquitoes transmit pathogens which have recently been introduced by humans and the larvae of the fly suck blood from nestlings and incubating females. Our experimental evidence demonstrates that P. galapageium leaf extracts repel both mosquitoes and adult P. downsi and also inhibit the growth of P. downsi larvae. It is therefore possible that finches use this plant to repel ectopoarasites.

An introduced parasitic fly may lead to local extinction of Darwin's finch populations

Introduced pathogens and other parasites are often implicated in host population level declines and extinctions. However, such claims are rarely supported by rigorous real-time data. Indeed, the threat of introduced parasites often goes unnoticed until after host populations have declined severely. The recent introduction of the parasitic nest fly, Philornis downsi, to the Galápagos Islands provides an opportunity to monitor the current impact of an invasive parasite on endemic land bird populations, including Darwin's finches.In this paper we present a population viability model to explore the potential long-term effect of P. downsi on Darwin's finch populations. The goal of our study was to determine whether P. downsi has the potential to drive host populations to extinction and whether management efforts are likely to be effective.Our model is based on data from five years of experimental field work documenting the effect of P. downsi on the reproductive success of medium ground finch Geospiza fortis populations on Santa Cruz Island. Under two of the three scenarios tested, the model predicted medium ground finches are at risk of extinction within the next century.However, sensitivity analyses reveal that even a modest reduction in the prevalence of the parasite could improve the stability of finch populations. We discuss the practicality of several management options aimed at achieving this goal.Synthesis and applications. Our study demonstrates the predicted high risk of local extinction of an abundant host species, the medium ground finch Geospiza fortis due to an introduced parasite, Philornis downsi. However, our study further suggests that careful management practices aimed at reducing parasite prevalence have the potential to significantly lower the risk of host species extinction.

The role of antiparasite treatment experiments in assessing the impact of parasites on wildlife

It has become increasingly clear that parasites can have significant impacts on the dynamics of wildlife populations. Recently, researchers have shifted from using observational approaches to infer the impact of parasites on the health and fitness of individuals to using antiparasite drug treatments to test directly the consequences of infection. However, it is not clear the extent to which these experiments work in wildlife systems, or whether the results of these individual-level treatment experiments can predict the population-level consequences of parasitism. Here, we assess the results of treatment experiments, laying out the benefits and limitations of this approach, and discuss how they can be used to improve our understanding of the role of parasites in wildlife populations.