High-carb diet gives migrating locusts a head start

Abstract

Some feats of migration are truly astounding. Arctic terns journey pole to pole each year, the great-great-great-grandchildren of the monarch butterflies that embarked initially in autumn return to their northern homes after their ∼5000 km odyssey and some desert locusts have even crossed the Atlantic, reaching the Caribbean and South America from North Africa. Stav Talal and colleagues from Arizona State University (ASU) explain that most creatures fuel these epic voyages with carbohydrate and energy-rich lipids, often obtained directly from an animal's diet. However, locusts, which tend to feast on vegetation, have to supply their own lipid fuels created from the carbohydrates in their diet and little was known about the impact that a locust's diet might have on its ability to sustain a long flight. In 2017, Arianne Cease, Jon Harrison and ASU colleagues discovered that Mongolian locusts (Oedaleus asiaticus) raised on a high-carb diet flew longer than locusts fed a low-carb diet. But how well would the true champions of the locust world, the migratory locust (Locusta migratoria), fly when they too were provided with a high-carb diet?‘We reared the locusts in crowded cages from hatchlings, sometimes with more than 1000 locusts in one cage, to produce the migratory form of the insects’, says Talal, who, with Geoffrey Osgood (ASU), fed the locusts one of three separate diets – a high-carb, low-protein diet; a medium-carb, medium-protein diet; and a low-carb, high-protein diet – before building locust-sized flight tunnels to measure the insects’ flight endurance. Filming more than 100 locusts, the team recorded that the locusts on the high-carb diet had the most stamina, while the locusts that had been reared on the low-carb diet were much weaker. ‘Six out of 34 locusts fed the high-carb diet flew continuously for 12 h, whereas only one locust out of 35 from the medium-carb diet flew for 12 h’ says Shivam Parmar (ASU), adding that none of the locusts on the low-carb diet managed to fly for the full 12 h. But how had the diets affected the locusts’ body composition and how did that change after their epic flights?Sure enough, when Parmar and Talal compared the locusts, the insects that had dined on the high-carb food were carrying the most lipid, accounting for up to 37% of their body mass, before embarking on their day-long flights. But by the time they had completed their marathons, all of the insects, regardless of diet, had used up a significant proportion of their lipid stores. However, even though the low-carb diet locusts were still carrying 75 mg of lipid – which is sufficient to keep a locust airborne for a full 12 h – they stopped flying after just 5.5 h. So why were they giving up so soon?The team suspects that several factors could be responsible for the low-carb locusts giving up early. They suggest that the locusts may simply be guarding their carbohydrate reserves, terminating a flight when their carb levels fall too low, or a high-carb diet might somehow protect flying insects from the damaging effects of oxygen that they use to power their flight. Another possibility is that the locusts fed a low-carb diet dry out faster when flying, or their muscle composition might be subtly different, limiting their endurance.Whatever the reason, it is clear that a high-carb diet is essential for locusts intent on a high endurance flight, unless they get some tailwind assistance, which probably accounts for the intrepid transatlantic voyage of the locusts that made it all the way to South America.