Nathan S. Hasegawa

Thesis Advisor: Dr. Andrew J. Bernoff (Harvey Mudd College - Department of Mathematics)

Second Reader: Dr. Heather Zinn Brooks (Harvey Mudd College - Department of Mathematics)

Contact: nahasegawa@hmc.edu

Collision Avoidance and Vegetation as Drivers of Collective Motion in Australian Plague Locusts

Locusts are agricultural and ecological pests that cause billions of dollars in crop damage each year. Due to global climate change, outbreaks may become more severe and frequent, and outbreaks have already appeared in regions that were not previously susceptible, increasing the potential for damage. In this thesis, we study hopper bands of the Australian plague locust (Chortoicetes terminifera), dangerous agglomerations of juvenile locusts that are several kilometers wide and can advance hundreds of meters per day, destroying vegetation in their path. While mathematical models of hopper bands have traditionally focused on social interactions between locusts, recent work has found that vegetation slows down hopper bands and influences their observable properties. Mathematical models are critical for estimating biological parameters that are difficult to obtain in the field, such as the amount of food a hopper band leaves behind. Yet recent data suggests that locusts slow down to avoid collisions with other locusts, and estimates of biological parameters from past modeling studies can be improved by accounting for this. My thesis will extend models of hopper bands moving through vegetation by introducing collision avoidance between locusts. Using agent-based and PDE models, we will examine how collision avoidance between locusts influences the shape, speed, and destructiveness of hopper bands. We will also study field data to estimate how long it takes a locust to move around another locust that is in the way, which may allow us to improve existing estimates of other biological parameters. These results may advance scientific understanding of collective structures in locusts and could benefit locust control efforts, which are essential to maintaining food security and economic productivity and will become more crucial in the future due to global climate change.