Parasitic helminthes remain important causative agents of human, plant and animal diseases. Helminthes seek out food sources and navigate toward potential hosts using olfaction of simple chemical cues in a process called chemoattraction. While several studies have examined how nematodes, including Caenorhabditis elegans, behave in response to a chemoattractant, how the characteristics of the chemoattractant affect worm behavior has yet to explored. In this manuscript, we develop a mathematical

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... odel to examine how characteristics of common chemoattractants affect movement and behavior in the model nematode C. elegans. Specifically, we model a scenario where a toxic, engineered bacteria designed to express a chemoattractant influences the behavior of a population of worms. Through the model we observe that, under static conditions, the diffusion rate of the chemoattractant is critical in influencing choice of C. elegans. Here, the higher diffusion rate, the more the worms are attracted to the chemoattractant. We then show that if the worms learn that the chemoattractant is associated with toxicity, choice index is counterintuitively more strongly reduced with increasing diffusion rate. Finally, our model predicts a tradeoff between pulse period and attractant strength when the chemoattractant is dynamically pulsed in the environment. Our results reveal unique tradeoffs that govern chemoattraction in worms and may have implications in designing novel strategies for preventing or treating infections with parasitic worms.