Separating food and water deprivation in locusts: effects on the patterns of consumption, locomotion and growth

In a factorial experiment, fifth-instar Locusta migratoria (L.) (Orthoptera: Acrididae) were given either dry food (lyophilized grass) and drinking water, food only, water only, or neither food nor water. Food consumption and insect weight were measured daily, and the behaviour of each locust was recorded for 5 h on each of four consecutive days and for 2.5 h on the fifth. Consumption declined progressively in locusts given food only, and those given water only were not observed to drink after the first day of food deprivation. The decline in food consumption on the first day was accounted for by a decrease in the average duration of feeds, which remained constant thereafter. The further decline in consumption over subsequent days was due to a progressive decline in the number of feeds. Although food availability did not slow weight loss relative to locusts given neither food nor water, the availability of water without food did. The proportion of time locomoting increased in all deprivation treatments, but the pattern of change across the five observation days differed markedly between treatments. Locusts given food but no water increased locomotion from 20% of the time budget (the value for controls) to 30% on the first day of deprivation, and by the second day had reached a plateau of approximately 65%, which was maintained until the experiment was terminated on day 5. In contrast, locusts given water but no food approached the 65% level of locomotion on the first day, which was statistically greater than the 55% observed in those deprived of both food and water. This increase was due both to an increase in the number of locomotion bouts initiated and an increase in the average duration of locomotion bouts. On the second and third days, all deprivation treatments maintained locomotion at around 65%. By day 4, locomotion had decreased to approximately 15% in locusts deprived of both food and water, but not in those deprived of food only or water only. Unlike those given only food, locusts given only water showed a reduction in locomotion of c. 15% on the fifth day.