Explaining large‐scale patterns of variation in body size has been considered a central question in ecology and evolutionary biology because several life‐history traits are directly linked to body size. For ectothermic organisms, little is known about what processes influence geographic variation in body size. Changes in body size and sexual size dimorphism (SSD) have been associated with environmental variables, particularly for Bruchinae insects, which feed exclusively on seeds during the larval stage. However, the effect of important seed traits on body size variation has rarely been investigated, and whether SSD varies substantially among populations within bruchine species is poorly known. Using the seed‐feeding beetle Acanthoscelides macrophthalmus infesting its host plant Leucaena leucocephala, we investigated whether specific seed traits (hardness, size, water content, carbon/nitrogen ratio, and phenolic content) were determinant in generating geographic variation in body size and SSD of A. macrophthalmus. We also examined the relationships between body size and SSD with latitude and altitude. The body size of both sexes combined was not related to latitude, altitude, and any of the physical and chemical seed traits. However, the female body size tended to vary more in size than the males, generating significant variation in SSD in relation to latitude and altitude. The females were the larger sex at higher latitudes and at lower altitudes, precisely where seed water content was greater. Therefore, our results suggest that water content was the most important seed trait, most severely affecting the females, promoting geographic variation in SSD of A. macrophthalmus.

Bruchine beetles are highly host-specific seed feeders during the larval stage. Although some specific parasitoid families have been recorded attacking bruchine beetles, most studies have been done at small spatial scales. Therefore, the current knowledge about the diversity and the geographic distribution of parasitoid species parasitizing bruchines is scarce, especially at a wide geographic area that extends over large distances through a latitudinal cline (i.e. large-scale spatial structure). The present study determined the species richness and evenness of parasitoids attacking the bruchine beetle Acanthoscelides macrophthalmus feeding on Leucaena leucocephala seeds, examined their geographic distribution, and characterized the large-scale spatial structure in parasitoid species composition. A total of 1420 parasitoids (all Hymenoptera) belonging to four families, five subfamilies and eight species were collected (genera: Horismenus, Paracrias, Urosigalphus, Stenocorse, Chryseida, Eupelmus). Most parasitoid species showed wide spatial distribution, high evenness in species abundance and the species richness estimators were close to stabilization (approximately eight species). Overall, greater similarity was observed in the species composition of plant populations near to each other than those farther apart, revealing a large-scale spatial structure in parasitoid species composition.

Rensch’s rule states that males vary more in size than females when body size increases. The main cause of Rensch’s rule has been credited to sexual selection. However, different degrees of plasticity between the sexes have also been proven to be useful for describing variations in sexual size dimorphism, particularly within an intraspecific context. For insects, in general, this rule has rarely been tested within species. Here, we tested whether Acanthoscelides macrophthalmus (Schaeffer, 1907) (Coleoptera: Chrysomelidae: Bruchinae) followed Rensch’s rule when individuals emerged from seeds immediately after fruit collection and when they were reared for one generation, by measuring three morphological traits. Rensch’s rule was not followed for any of the morphological traits. Variations in body size were similar in males and females for bruchines that first emerged from seeds and for those that were reared for one generation. These findings suggest that environmental conditions (e.g., temperature, humidity, and seasonality) are unlikely to drive differential plasticity in males and females of this seed-feeding beetle. It is possible that changes in the body size of A. macrophthalmus have a genetic basis. However, regardless of whether variations in body size have a genetic basis, our findings provide no support for Rensch’s rule.