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.

Galls are specific changes induced by insects on plant organs mainly through increases in plant cell number and/or size. Gall diversity is easy to recognize in the field because gallers are mostly species-specific, and thus each gall morphotype can be a proxy for a galling species. Insect galls are virtually unknown in Seasonal Deciduous and Semi-Deciduous forests of southern Brazil. Here, galls and host plants were surveyed between 2015 and 2017 in four forest fragments of Rio Grande do Sul State in these two vegetation types, in secondary-growth and areas under restoration. We recorded 89 gall morphotypes, with gallers belonging to Lepidoptera and Diptera, with the latter represented mainly by Cecidomyiidae. Galls were associated to 46 plant species in 27 families. Asteraceae, Piperaceae, Fabaceae, Myrtaceae and Lauraceae were the richest families in terms of galls, whilst Piper aduncum and Mikania glomerata were superhosts. Most galls occurred in leaves and shoots. The most common shapes were fusiform, globoid and lenticular. Forty-eight gall morphotype records are new for both Rio Grande do Sul and Brazil, an expressive number considering only two seasonal forest types sampled and few sampling points, showing how important surveys still are for these little know fauna both in taxonomic and ecological terms.

ABSTRACT Parasitoid ovipositor size importantly affects host choice; those attacking exposed hosts usually have shorter ovipositors compared to those needing drilling deeper through substrates such as plant tissue and gall tissue. Most studies treat this theme at the interspecific level, and the aim of this work was to test for intraspecific variation and phenotypic matching in ovipositor size for Galeopsomyia sp. attacking galls. Galls were sampled from Guapira opposita(Nyctaginaceae), a host plant to five species of Bruggmannia gallers (Diptera, Cecidomyiidae) in southern Brazil: Bruggmannia elongata, B. robusta, B. acaudata, and two undescribed species of Bruggmannia (sp. 1 and sp. 2). On forest transects, all galls from the 30 first galled plants found were sampled and kept in sealed plastic bags until parasitoid emergence. A total of 15 samples were done along two years. We measured galler last instar larvae length and gall thickness for each galling species, and ovipositor length for all Galeopsomyia sp. individuals emerging from the galls. There were no differences in larval length among galler species. However, Bruggmannia sp. 1 and sp. 2 galls were significantly thicker than those of B. acaudata, and the latter thicker than both B. elongata and B. robusta galls. Wasp ovipositor size differed significantly between those coming from Bruggmannia sp. 1 and sp. 2 galls relative to all others. Host extended phenotype size is thus fundamental to determine parasitoid size, but in this case host (larval) size does not change among species, although gall thickness was different. Thicker galls were attacked by larger parasitoids with longer ovipositors, denoting phenotypic matching. Thicker galls appear to be selecting larger parasitoid individuals, which in the long run can lead to important evolutionary change as well.

Neste estudo descrevemos a riqueza de morfotipos de galhas induzidas por artrópodes em uma área de Floresta Estacional Semidecidual no Rio Grande do Sul, Brasil. Nós encontramos 71 espécies de artrópodes galhadores, a maioria delas Cecidomyiidae (Diptera) associada a 51 espécies de plantas hospedeiras de 27 famílias. Asteraceae foi a família hospedeira mais representativa (18 morfotipos). A maior parte das galhas foi induzida em ramos (56%) e a forma mais comum foi fusiforme (43,6%). Dezenove novos morfotipos de galhas foram registrados para o Rio Grande do Sul e para o Brasil, mostrando a importância desse tipo de estudo para galhadores em regiões pouco exploradas, como o Sul do Brasil.