Gall observations and DNA sequencing uncover structural features and habitat modulators of a hidden arthropod community

Hidden ecological interactions within natural microcosms, such as insect-induced plant galls, can be sophisticated and challenging to unravel. Beyond identifying the diversity of taxa involved and their temporal dynamics, understanding how habitat features (e.g., canopy height and agrichemical use) affect community structure remains poorly understood. Here, we combined observational data and DNA sequencing of leaf galls induced by Phylloxera spp. in a pecan system to investigate structure features of associated gall arthropod communities in relation to different vertical strata of the canopy over time and to explore community responses to agrochemical pressure. We dissected a total of 1150 leaf galls collected in August and September from the lower, middle, and upper pecan canopy. Interestingly, most of the content found consisted of Hymenopteran immatures (476 galls) or frass, and rarely Phylloxera. DNA sequencing data revealed that diversity was higher in middle and upper canopy galls that were under agrochemical pressure, suggesting a relationship between canopy height and the diversity of interactions in these communities. Furthermore, arthropod community composition was influenced by temporal dynamics (age of gall/time of collection). In July, community structure was distinct between decaying and fresh gall tissue but became more similar in August. Surprisingly, the two primary taxa found in the galls were mites and Hymenoptera, with minimal Phylloxera DNA. At least 30 taxa were identified from the galls. Therefore, pecan galls reveal a surprisingly diverse ecological space. Their cryptic arthropod communities provide insights into how environmental conditions can modulate enclosed community structure.