Flight fuel related differences between solitary and gregarious locusts (Locusts migratoria migratorioides)

Flight fuel relations of crowded and isolated Locusta migratoria migratorioides were investigated in younger (12-16 days after fledging) and older (27-30 or 27-32 days after fledging) adult males. No phase polymorphism dependent differences were found in resting haemolymph carbohydrate levels of the younger locusts. In the older age group, resting haemolymph carbohydrate levels were slightly though significantly higher in the isolated than in the crowded locusts. Injection of various doses of synthetic adipokinetic hormones (AKHs) did not induce marked changes in haemolymph carbohydrate levels and no differences were found between crowded and isolated locusts. A 30 min flight led to the same decrease in haemolymph carbohydrate levels of isolated and crowded locusts, 43.3% and 44.6% of the resting levels, respectively. We concluded, therefore, that the results do not seem to indicate that isolated locusts rely more heavily on carbohydrates as flight fuel than crowded locusts. Hyperlipaemic responses to flight were less intense in isolated than in crowded locusts, bur phase polymorphism dependent differences in flight-induced increase of haemolymph lipid levels were not parallel in 12-16-day-old and 27-32-day-old males. In the younger age group the difference was mainly in the duration of flight needed to induce full response which appeared already after 20 min of flight in the crowded locusts, but only after 45 or 60 min of flight in the isolated ones. In contrast, the older isolated locusts showed markedly lower haemolymph lipid elevations than the crowded locusts even after 30, 45 or 60 min of flight. The hypothesis is forwarded that isolated locusts have a rather coarse adipokinetic strategy focused on a single long-distance migratory flight, whereas gregarious locusts possess a fine adipokinetic balance for reiterative migratory flights and saving fuel reserves for unpredictable long-distance migrations.