Researchers Raise Maggots on Chicken Blood to Save Darwin’s Finches
By Meredith Swett Walker
In 1676, the great scientist Isaac Newton (of gravity fame) wrote a letter to another scientific titan, Robert Hooke (who had coined the biological term “cell”), in which he humbly stated, “If I have seen further, it is by standing on the shoulders of Giants.” Even in using that line he was standing on the shoulders of a twelfth-century philosopher named Bernard of Chartres, who originally came up with it.
Newton’s (appropriated) quote is used frequently when discussing science because it captures how science progresses. All scientific discoveries depend on years and years of work by earlier scientists who made the observations, designed the tools, hammered out the lab protocols, and built up the bits of knowledge that are the foundations for new research. And much of the time, this “foundation” work is tedious, overlooked, and unglamorous.
Raising maggots may be about as unglamorous as it gets, but that doesn’t mean it’s not important. In the latest issue of the Journal of Insect Science, Paola Lahuatte and her colleagues reveal how they used chicken blood to rear the larvae of the parasitic fly Philornis downsi in the lab. This protocol may be the first to effectively rear an avian blood-feeding fly from egg to adult in the absence of its host. More importantly, it may prove to be a crucial tool in the fight to save endemic birds in the Galapagos islands, including the critically endangered mangrove finch.
Adult Philornis downsi feed on fruit, but as larvae they parasitize baby birds, usually small songbirds. The female fly lays its eggs in bird nests. Once hatched, first-instar larvae crawl into the nostrils of the nestling birds, where they feed on both blood and tissue. Larger, second-instar larvae leave the confined space of the nostrils and hide out in nest material during the day. They emerge at night to continue feeding on the chicks.
Philornis downsi infestation takes a serious toll on nestlings. In some cases, all of the young in an infested nest are killed by the parasites.
Philornis downsi is not native to the Galapagos Islands. The species was likely introduced accidentally in the 1960s via imported fruit and has since wreaked havoc on the islands’ endemic bird populations. Parasitism is the leading cause of nestling mortality for at least one species of Darwin’s finch, and some believe it is the main cause of decline of landbirds in the Galapagos Islands.
In order to find an effective control for these invasive flies, scientists need to be able to raise them in the laboratory. With lab-reared flies, it would be possible to evaluate various methods of control, be they insecticides or biological control, without disturbing or harming the very endangered bird hosts. However, Philornis downsi is a specialized parasite that has adapted to complete its life cycle in the environment of a songbird nest with its specific light, temperature, and humidity conditions. Keeping captive songbirds in the lab to play host to these lethal parasites would not only be cruel, but also logistically difficult.
So Lahuatte and her colleagues set out to design a protocol that would artificially recreate the conditions and food that Philornis downsi larvae encounter in a songbird nest. They collected female flies from the field and found that they would readily lay eggs on the surfaces of the containers in which they were housed. Eggs were then collected and watched for hatching. At hatching, first-instar larvae were transferred to small, open-ended glass tubes with a cotton plug at each end. One plug was moistened with pre-boiled water to maintain humidity and the other was saturated with chicken blood for food.
As the larvae developed into their second and third instars, they were moved to larger, closed-end test tubes. Moistened cotton was again used to maintain humidity, and a section of plastic drinking straw with a cotton plug saturated with a blood mixture was added to provide nourishment. The researchers evaluated three different larval diets, each based on chicken blood. The first diet was blood alone; the second was a mixture of blood, hydrolyzed protein, and milk powder; and the last combined blood, hydrolyzed protein, and brewer’s yeast.
Lahuatte and colleagues were able to raise about 10 percent of the eggs they collected to adulthood, which is a significant improvement over a previous attempt at the Charles Darwin Foundation that produced only three adult flies from almost 500 larvae. Mortality was highest (77 percent) among first-instar larvae. During their first instar, larvae in the field are living in the nestling bird’s nostrils and this is a difficult environment to recreate. The researchers suggest that modifying the environmental conditions or diet during this stage may improve survival.
Once larvae reached the second and third instars, their survival was much higher. The diets evaluated did not appear to affect survival. However, larvae fed the diet containing milk powder had heavier pupae. Heavier adults emerge from heavier pupae and these larger adult flies may have reproductive advantages.
One method that has been proposed to control Philornis downsi is the “sterile insect technique” or SIT. This method produces large numbers of artificially sterilized insects (usually males) that are released into the wild population. Females that mate with sterile males produce no offspring, thus reducing the size of the next generation. SIT has been used to successfully eradicate populations of screwworm flies and has been useful in the control of certain species of fruit fly. However, the ability to rear large numbers of flies in the laboratory is essential for this technique. Lahuatte and her colleagues have taken the first major step in making SIT control of Philornis downsi possible.
Ms. Lahuatte did this work when she was an undergraduate at the University Central of Ecuador in Quito. She is currently continuing her work as a junior researcher at the Charles Darwin Foundation.
“I have always been fascinated by insects and the conservation of species, and the Philornis project covered both of these aspects,” she said. “I felt that I was following my dreams.”
Her commitment to this work was evident when she was asked about her least favorite part of the project. Was it mixing up chicken blood concoctions or handling flesh-eating maggots? The answer was neither. Lahuatte’s least favorite part was “when so many larvae died after I got them to two-three days old. That was very frustrating for me because I had become so familiar with them and was so invested in getting them through to adulthood.”
Eradicating or controlling Philornis downsi in the Galapagos would be a major step forward in the conservation of these islands’ important endemic bird species. If control is achieved, it will be due to the dedicated, unglamorous work of scientists like Lahuatte, her co-authors, and others. It will likely take a lot of lab-reared flies to save Darwin’s famous finches.
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Meredith Swett Walker is a former avian endocrinologist who now studies the development and behavior of two juvenile humans in the high desert of western Colorado. When she is not handling her research subjects, she writes about science and nature. You can read her work on her blogs http://picahudsonia.com and https://citizenbiologist.com or follow her on Twitter at @mswettwalker.