Intraspecific competition is an ecological process that can affect the structure of populations with important consequences for population persistence. In this study, we investigated (in the larval stage) the effect of intraspecific competition in the seed-feeding beetle Acanthoscelides macrophthalmus on its population structure and persistence. By using Leslie matrices, which are useful to project future population sizes of structured populations, we conducted demographic analysis when competition was (more than one larva per seed) and was not (one larva per seed) present. The hypothesis that competition reduces the finite rate of population growth, the survival of the immatures and the fecundity of females, influencing population persistence, was tested. To understand whether intraspecific competition is a common process in this system, the oviposition pattern was also studied. We found that in the presence of competition, the larvae from the third instar were smaller than those in the absence of competition and, in the presence of competition, fewer larvae reached the adult stage. Larval survival was lower in the presence of competition, particularly from the third larval stage until the pupal stage, and the fecundity of females was also reduced in the presence of competition, which affected the population structure. Although we showed that populations may either grow geometrically or go extinct in absence and presence of competition, respectively, the finite rates of population increase did not differ from 1.0 when compared to the estimated confidence intervals. Therefore, although larval competition may affect the structure of this seed-feeding beetle, there is no clear evidence that this ecological process can negatively affect its persistence. Furthermore, most seeds with eggs had just one egg per seed, which suggests that competition should occur with low frequency under natural conditions.
