Competition and climate affect body size and sexual size dimorphism in pine sawyer beetles

The importance of interspecific competition in structuring communities of plant-eating insects has been somewhat controversial. If interspecific competition is ever important for phytophagous insects, it is likely to be observed in the insect guild that feeds in the phloem of trees. We tested for signals of interspecific competition in co-occurring species of pine sawyer beetles Monochamus Megerle (Coleoptera Cerambycidae), generally the largest phloemophagous insects in pines. We evaluated patterns of body size and its correlation with other life-history traits. By studying pine sawyer beetles in different areas (Northeast USA, Southeast USA and Portugal) we assessed the interaction with climate. As predicted under the hypothesis of interspecific competition, there were always clear size differences among coexisting species. As predicted if competition is asymmetric, smaller species were less abundant and emerged earlier than the larger species. In the larger and numerically dominant species, sexual size dimorphism was more pronounced and the sex ratio was skewed towards females. In the smaller species, males emerged earlier (protandry), whereas the larger species lacked differences in emergence time or displayed protogyny. Effects of climate only seem to have been expressed in the larger dominant species of each area, whereas the effects of competition in smaller species probably overshadow the effects of climate.