Hairy roadblock: Stem trichomes impede herbivore movement in a species and instar-dependent manner

Plants have evolved to deploy a vast set of defense 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 behavior of FAW and SBL larvae where they “girdle” the stems to
remove the stem trichomes as they climb up. Our findings clearly
demonstrate that 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.