Morphological traits, allometric relationship and competition of two seed-feeding species of beetles in infested pods

Abstract Pair-wise competition produces asymmetric consequences for the interacting species, resulting in reduction of species fitness at the individual scale; however, little is known of the effects of competition on the allometric patterns of insects. In this study, we explored how competition, by means of pod infestation, affects the development of female and male individuals in the co-occurring bruchine beetles Merobruchus terani and Stator maculatopygus. We found differences between M. terani and S. maculatopygus in all morphometric traits, but no significant differences between males and females in either species. We also found, with an increasing degree of pod infestation, a positive trend in the pronotum, elytron and body weight of M. terani and a negative trend in morphological traits and body weight of S. maculatopygus. A negative allometry was maintained, suggesting that with increasing body weight, the body structures did not increase proportionally. On the other hand, we found that increasing the degree of pod infestation produced a wider variation in the individuals' body size than in low levels of infestation. Finally, we discuss how pod infestation can trigger competition between species, with both positive and negative impacts, even though the species function similarly in resource exploitation.

Morphological Traits of Two Seed-Feeding Beetle Species and the Relationship to Resource Traits.

Morphological traits are useful to investigate insect sex-related differences in body size and to reveal differences in resource use. It has been suggested that as the resource increases, so does the body size of organisms interacting with the resource, highlighting the crucial role of resource quality and quantity in determining the morphological traits of organisms interacting with the resource. Here, we describe morphological traits of two species of Bruchinae, Merobruchus terani (Kingsolver 1980) and Stator maculatopygus (Pic 1930), consuming seeds of Senegalia tenuifolia (Fabaceae: Mimosoideae). We evaluated the influence of monthly sample and sampling sites on tibia and femur length and biomass. In addition, we tested two predictions in which body size related to resource amount and body size related to longevity. Males of M. terani were heavier than females, whereas the two sexes of S. maculatopygus did not differ in biomass. Both species had larger body sizes in the late ripe-fruit stage. With respect to sampling sites, biomass of M. terani did not differ, whereas S. maculatopygus did differ in biomass. Merobruchus terani showed a positive relationship with seed traits, whereas S. maculatopygus showed no relationship. At the same time, fruit traits showed a negative effect on morphological traits for both beetle species. The longevity experiment, performed using only M. terani, showed an equal longevity and seed consumption rate for both sexes. Our study indicates that different species, interacting in the same system and performing similar functional behaviors, respond differently to the same resource.