What drives gallers and parasitoids interacting on a host plant? A network approach revealing morphological coupling as the main factor

1. Ecological interaction networks, those built from linking interacting species in a biological community, can be structured according to neutral factors, like abundance, or depend on species distribution in space, time, or functional aspects of interacting species. 2. Gall inducers are a specialized guild of herbivores producing a structure serving as shelter, food, and defence against natural enemies. Interactions between galling insects and their parasitoids can become complex; studies analysing processes influencing how these interactions occur are uncommon. 3. We tested for ecological processes predictive of network structure: neutral (abundance), temporal, or morphological coupling factors. 4. We used a single host plant, and six Cecidomyiidae species from subtropical forests of southern Brazil. There were 14 samples across two years; out of 1762 galls, 741 parasitoids emerged (45 species). The network had high specialization (39 exclusive parasitoid species) and low connectance (21%). 5. Gall‐parasitoid morphological coupling (gall thickness vs. parasitoid ovipositor length) was the best factor explaining network structure, with little effect for time, or abundances. Modularity was significant, with two modules, one with only Pisphondylia brasiliensis and its parasitoids. This insect induces thick multichambered galls with the lowest parasitism rate, attacked by parasitoids with longer ovipositors. Thinner galls had higher parasitism rates and more attacking parasitoid species, implying an important adaptive role for gall shape/size in natural enemy interactions. 6. Interaction networks are important tools to help understand how ecological function impacts community structure, but higher‐scale studies may be needed to reach more encompassing conclusions about galler‐parasitoid interactions.