Innate behaviours shape generalist herbivore host choice and feeding patterns independent of plant defences

Abstract Plant–herbivore interactions are typically examined through plant defence and herbivore performance, yet individual herbivore behavioural mechanisms underlying these interactions are rarely considered alongside plant traits. We characterized key behavioural differences between the two closely related polyphagous lepidopteran chewing herbivores, Spodoptera exigua and Spodoptera littoralis , and quantified the contribution of herbivore traits, plant chemistry, and environmental factors to these behaviours and their consequences for plant and herbivore performance. The two herbivore species exhibited strikingly different behavioural strategies: S. exigua is relatively immobile, attaches tightly to leaves and is more cryptic on foliage. In contrast, S. littoralis is highly mobile, easily detaches from plants, is better camouflaged on soil, preferentially feeds on lower leaves, and exhibits reactive anti‐predator responses. By quantifying herbivore behaviours, plant chemical defences and morphology within an environmental context, we show that innate herbivore behaviour is the primary factor structuring interaction outcomes for both species, even though they carry performance costs in the presence of plant defences. Further, these behaviours occur independent of larval stage, across native and introduced populations. Together, our findings show that species‐specific behavioural syndromes, rather than plant defence traits, can structure plant–herbivore interactions. This can have important implications for plant–herbivore coevolutionary trajectories if plant defence capacity does not limit herbivory and herbivore behaviour is not modified by plant defence status. Read the free Plain Language Summary for this article on the Journal blog.