Band movement and thermoregulation in Schistocerca cancellata.

At high density, juvenile locusts create marching hopper bands. Understanding the roles of temperature and vegetation on the movement of these bands shall allow to better forecast and control them. Following a hopper band in North Argentina in November 2019, we explored the thermoregulation behaviours of the South American locust, Schistocerca cancellata. Gut-content samples informed about the feeding status at different time of the day. Hoppers' body temperature was above cold air temperature in the mornings during basking and group-basking activities and before the onset of marching behaviour. Marching by walking or hopping was dominant at body temperatures close to 40 °C. Jumping, stilting, shading and perching on plants were seen as thermoregulatory behaviours to avoid ground temperatures above 50 °C. Feeding was observed throughout the day with continuous high gut contents despite an intermittent pattern of feeding-resting-marching. Speed and daily travelled distance of the front of the hopper band was depending on the type of encountered vegetation. Daily behavioural patterns, thermoregulatory behaviours, walking speed and daily travelled distances of S. cancellata were similar to the ones observed for the Desert locust, S. gregaria, in Africa. High air temperatures recorded during the observation times could explain the continuous feeding patterns. These species may have evolved behaviours of alternating consuming a bit and marching as a migration strategy to avoid staying where no food is available after the havoc left behind large hopper bands. Recommendations made for the control of Desert locust hopper bands can be extended to South American locust ones.

High carbohydrate diet ingestion increases post-meal lipid synthesis and drives respiratory exchange ratios above 1.

Locusts have been reported to elevate metabolic rate in response to high carbohydrate diets; this conclusion was based on metabolic rates calculated from CO2 production, a common practice for insects. However, respiratory exchange ratio (RER, CO2 production divided by O2 consumption) can rise above 1 as a result of de novo lipid synthesis, providing an alternative possible explanation of the prior findings. We studied the relationship between macronutrient ingestion, RER and lipid synthesis using South American locusts (Schistocerca cancellata) reared on artificial diets varying in protein:carbohydrate (p:c) ratio. RER increased and rose above 1 as dietary p:c ratio decreased. Lipid accumulation rates were strongly positively correlated with dietary carbohydrate content and ingestion. RERs above 1 were only observed for animals without food in the respirometry chamber, suggesting that hormonal changes after a meal may drive lipid synthesis. Schistocerca cancellata does not elevate metabolic rate on low p:c diets; in fact, the opposite trend was observed.

Plant carbohydrate content limits performance and lipid accumulation of an outbreaking herbivore.

Locusts are major intermittent threats to food security and the ecological factors determining where and when these occur remain poorly understood. For many herbivores, obtaining adequate protein from plants is a key challenge. We tested how the dietary protein : non-structural carbohydrate ratio (p : c) affects the developmental and physiological performance of 4th-5th instar nymphs of the South American locust, Schistocerca cancellata, which has recently resurged in Argentina, Bolivia and Paraguay. Field marching locusts preferred to feed on high carbohydrate foods. Field-collected juveniles transferred to the laboratory selected artificial diets or local plants with low p : c. On single artificial diets, survival rate increased as foods became more carbohydrate-biased. On single local plants, growth only occurred on the plant with the lowest p : c. Most local plants had p : c ratios substantially higher than optimal, demonstrating that field marching locusts must search for adequate carbohydrate or their survival and growth will be carbohydrate-limited. Total body lipids increased as dietary p : c decreased on both artificial and plant diets, and the low lipid contents of field-collected nymphs suggest that obtaining adequate carbohydrate may pose a strong limitation on migration for S. cancellata. Anthropogenic influences such as conversions of forests to pastures, may increase carbohydrate availability and promote outbreaks and migration of some locusts.