Galapagos isn’t just about tame, cute, intriguing animals and exotic landscapes. The islands are home to many invasive species and diseases that threaten its wildlife, and you should be warned, this post may gross you out. Since 2004 on Floreana Island, Sonia Kleindorfer, Rachael Dudaniec and others have been studying one of the major problems threatening Galapagos land birds, the introduced parasitic fly, Philornis downsi (pictured above), which feeds on the blood of growing nestlings in Darwin´s finch nests. The fly is thought to have been introduced to the islands via fruit or live chicken imports from the Ecuadorian mainland, and was first discovered in finch nests in 1997. However, some Philornis flies were found in museum collections made in 1964, though it wasn’t noticed as a problem until about 30 years later. We are working together with Charlotte Causton from the Charles Darwin Research Station, who is coordinating the international program to address Philornis on the Galapagos Islands.
The adult flies lay their eggs in finch nests; egg laying by adult flies has rarely been observed. Our in-nest videos showed adult flies walking over finch eggs and laying fly eggs in nests with newly hatched finch chicks (see this). The fly larvae feed on the blood and tissues of nestlings, and frequently kill all nestlings in a nest before the chicks can fledge. The larvae reside in the nesting material, and feed on nestling blood for about 5 days before pupating at the base of the nest. The flies then emerge into the non-parasitic adult fly stage about 10 days later. These flies are very robust and are known to live up to 280 days in the laboratory (pers. comm. Charlotte Causton), so that’s a lot of nests that can be infested by a single female fly. Our group has found that in-nest mortality of chicks ranges between 20-100% across years (see here). We also showed that finch species differ in how many parasites they have in their nests, and in their ability to survive (see here). The parasitic larvae are surprisingly large and can reach up to 1.8 cm in length – so the death rate is perhaps not surprising when you imagine 30 of these feeding on a single little nestling.
The parasite is a huge problem for declining local finch populations and localised species like the endangered Mangrove Finch on Isabela Island (less than 200 pairs) and Medium Tree Finch on Floreana Island (around 5000 birds left in the world). This year, working on Floreana and Santa Cruz, we have found many inactive nests containing dead chicks that have died from Philornis parasitism. The chicks seem to die off one by one, with the healthiest chicks avoiding larvae by standing over their siblings. The sibling at the bottom of the pile dies first but also acts as a buffer for its siblings standing on top of it. When all the other siblings have died one by one, the single remaining chick is left to endure the full onslaught of hungry fly larvae, succumbing to death sometimes within a few hours. The Medium Ground Finch nestling pictured below is an example of this. This solitary nestling appeared fine when discovered at 8am one morning, but when we returned at 10.30am the nestling was dead with many holes from larvae found all over its body. This is one vicious fly.
A large part of our work here is monitoring nests and collecting them once they become inactive due to death by parasitism, predation or abandonment. We tear the nests apart carefully to count every single Philornis larva and pupa within the nest. This year we are finding about 30-40 per nest on average, but recently found one with 114!! For each nest we count the number of 1st, 2nd and 3rd larval instars as well as pupa and empty pupa cases (emerged flies). They are rather disgusting things, to say it mildly (see below).
Previous genetic analysis done by our group on the flies across islands found there is pretty high connectivity of fly populations across islands, and across the highland and lowland habitats (published here ). Genetics has also provided insight in to how many flies lay their eggs in a single nest (published here).
From our and others observations, it seems that things are beginning to changing over the last 12-15 years, with some evidence that these incredibly harmful parasites may be showing signs of co-evolution with their hosts (e.g. see here). Co-evolution in hosts and parasites may lead to less mortality among nestlings, which ultimately will act to ensure that fly populations are sustained with a sure supply of hosts in the long run. For example, our group (Kleindorfer et al.) has documented increasing hybridisation among Floreana tree finch species, and hybrid birds have lower numbers of parasites and fledge more nestlings (see here). If hybridisation is good for combating Philornis, this suggests the fly is directing Darwin finch populations along new evolutionary trajectories.
We can’t just wait for evolution to ‘do its thing’ though – death rates are way too high for that. This is why many researchers are currently working very hard at the Charles Darwin Research Station and in the United States to develop control methods for the fly. These range from chemical eradication by spraying nests with insecticide, to developing pheromonal compounds to attract adult flies to traps.
There is also work on the potential of using ‘parasitoids’ to control fly numbers, which are insect parasites of the parasitic fly itself. A lot of work is also being done to try and find weaknesses in the fly’s reproductive cycle that could indicate prime times for control measures. Breeding the flies is proving difficult though, and this important work continues by research groups around the globe.
Clearly one thing missing from the puzzle is funding, and unfortunately, money for research and implementing control measures may be the one thing that holds us from solving this issue – the biggest threat to the worlds’ fastest evolving group of birds. With all of this knowledge combined with careful conservation decisions and action, Darwin’s finches will continue to provide insight into how different life forms come into being.