Hairy Roadblock: Stem Trichomes Impede Herbivore Movement in a Species and Instar‐Dependent Manner

ABSTRACT Plants have evolved to deploy a vast set of defence mechanisms against insect herbivores, including physical barriers like leaf trichomes with pre and post ingestive effects. Yet, the role of stem trichomes in plant‐herbivore interactions, particularly mobility dynamics, is poorly understood. To test this, we quantified the climbing speed of third and fifth instar soybean looper (SBL, Chrysodeixis includens ) and fall armyworm (FAW, Spodoptera frugiperda ) in the presence (intact) and absence (shaved off) of soybean ( Glycine max ) stem trichomes. The results show that SBL was significantly slowed down due to stem trichomes, but FAW was not affected. When compared between instars, fifth instar larvae were twice as fast as the third instar of both herbivores. Interestingly, significantly more fifth instar SBL successfully climbed to the top of the plant, compared to fifth instar FAW. Anecdotal observations also revealed a unique behaviour of FAW and SBL larvae where they ‘girdle’ the stems to remove the stem trichomes as they climb up. Our findings clearly demonstrate that soybean stem trichomes are an effective barrier that slows down the herbivores by restricting their mobility. However, they are not as effective against a generalist herbivore like FAW, and the effects are larval stage dependent.