Does the gall-forming cecidomyiid enhance host plant leaves and their phytochemicals? A case study of Bruggmanniella litseae (Diptera: Cecidomyiidae)

Gall-inducing insects manipulate host plant physiology and morphology, offering a unique system to explore insect–plant interactions. In this study, we investigated the effects of Bruggmanniella litseae (Diptera: Cecidomyiidae) galls on the performance of its host plant, Litsea acuminata, by examining variation in leaf size and five phytochemical traits across different gall abundances. Our results show that galled leaves are significantly larger than ungalled ones, with leaf size increasing with gall abundance—up to a threshold of approximately 4000 mm²—suggesting possible host manipulation to meet the nutritional requirement of the gall inducer. Among the phytochemicals examined, polyphenol content increased with gall abundance. This may support larger leaf size via polyphenol-enhanced auxin (IAA) synthesis, promoting cell differentiation and growth. Contrary to expectations, no significant different was found between gall abundance and folivory, despite elevated polyphenol levels, which are typically associated with herbivore defense. Overall, higher gall densities of B. litseae are associated with increased leaf size and polyphenol levels, which may represent a strategy to optimize host photosynthesis and maintain nutritional homeostasis for gall development.