Trichome density and herbivore behaviour on tomato is influenced by herbivory, plant age, and leaf surface

Leaf trichomes in plants act as the first line of physical defence against herbivory, in addition to many other reported functions. Although trichomes have been found to vary intraspecifically and can be induced by herbivory, their interactive effects under additional factors, such as plant age and abaxial vs. adaxial leaf surfaces, are less understood. Here, using five common tomato varieties, we explored the effects of these factors and their interactions on trichome type and density. We quantified the densities of Type VI glandular trichomes, non-glandular trichomes, and total trichomes on abaxial and adaxial leaf surfaces, and total leaf trichomes with and without herbivory by Spodoptera exigua at both vegetative and reproductive stages. Further, we also tested whether the time taken to initiate feeding by S. exigua larvae could be influenced by the number of trichomes on the adaxial and abaxial surfaces. The results showed that there is significant variation in trichome density among varieties and leaf surfaces. Also, there were differences in herbivory-induced trichome production, with variable responses across varieties and growth stages. Bioassay results showed that insects took longer to initiate feeding on the abaxial leaf surface than on the adaxial surface, potentially due to the higher density of non-glandular trichomes on the abaxial side. Collectively, we report that the regulation and development of trichomes on the leaf surface of tomatoes is governed by multiple factors, with potential consequences for herbivore feeding, suggesting how physical defences play a significant role in insect-plant interactions.