RAPID: Testing macronutrient imbalance as a key factor limiting range expansion in herbivores

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Insect outbreaks are common in nature and locust outbreaks are particularly dramatic due to their tremendous ecological and agricultural impacts. Occasionally, insect populations explode throughout broad geographic ranges, and then collapse back into their smaller permanent breeding zones within months or years. Neither the mechanisms of population expansions nor declines are well understood, but our knowledge of the declines is particularly poor. This is a major challenge in organismal biology and ecology, with significant implications for human society. This research project will take advantage of an ongoing outbreak of the South American locust (Schistocerca cancellata) to test the hypothesis that the ability for S. cancellata to attain a balance of nutrients optimal for growth limits their capacity to maintain persistent high populations over broad regions of South America. Locusts are a major challenge for food security globally, with outbreaks causing 80-100% crop losses. In the future, working collaboratively with government plant protection agencies, this research can be directly applied to strategies to improve livelihoods, human and environmental health, and global food security. Moreover, this award will support postdoctoral and student training, and cross-cultural exchange. The South American locust (Schistocerca cancellata) is experiencing its largest population upsurge in 60 years, expanding beyond its permanent breeding zone in northwest Argentina into a much larger part of Argentina and beyond into Bolivia and Paraguay, causing national emergencies in those three countries. One hypothesis for why S. cancellata do not persist at high densities in their invasion zone, which is supported by preliminary data, is that S. cancellata in their expansion zones are unable to obtain food of sufficiently high carbohydrate:protein ratio to maximize their fitness. This poor nutrition then leads to population decreases in their expansion zone. This carbohydrate insufficiency hypothesis will be tested in this project by: 1) assessing the plant macronutrient contents and feeding behavior of S. cancellata in the field at sites differing in median plant macronutrient ratios; 2) measuring the short-term effects of dietary macronutrient content on the performance of juvenile S. cancellata in the field, and 3) measuring the whole-lifetime effects of dietary macronutient content on the performance of S. cancellata in the lab under controlled environmental conditions. Together these experiments will test the mechanisms potentially responsible for fluctuations in locust populations in the recession and invasion zones.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.