Establishment and new hosts of the non-native seed beetle Stator limbatus (Coleoptera, Chrysomelidae, Bruchinae) on acacias in Europe

Stator limbatus is a phytophagous beetle native to warm regions of North and Central America, feeding on Fabaceae seeds and one of the most polyphagous species within the subfamily Bruchinae, here reported for the first time in Europe and on new hosts. Adult beetles emerged from Acacia spp. seeds collected in the islands of Corsica (France), and Sardinia (Italy). The wide presence in Sardinia and Corsica supports the hypothesis that this alien species was introduced several years ago. In both islands, S. limbatus emerged from Acacia mearnsii seeds, with infestation rates of up to 74.2 and 90.8% in 2019 and 2020, respectively. This seed beetle also emerged from two previously unreported host species, Acacia saligna and A. pycnantha, showing highest infestation rates of 4.0 and 95.1%, respectively. Both Acacia species are reported as new host associations with S. limbatus. Overall, seed infestation rates recorded in 2019 and 2020 indicate that S. limbatus is well established and that Mediterranean bioclimatic conditions are suitable for its population increase in size. This study lays the foundations for further research on known and potential host species and the spread and distribution of S. limbatus in Europe.

Smaller beetles are better scramble competitors at cooler temperatures

The role of temperature is central to both organic evolution and ecological processes. However, how temperature affects selection on body size is unknown. We tested whether small seed beetles ( Stator limbatus ) have an advantage over large beetles during scramble competition for mates, and whether this advantage varies with temperature. Within lines of beetles artificially selected to be large versus small, small males have a significant advantage over large males in scramble competition for females because the former takeoff more quickly and thus reach females before larger males. Selection favouring small male body size is significantly (and substantially) more intense at cooler temperatures. The adaptive significance of small male body size thus depends on ambient temperature.

Inbreeding depression in two seed-feeding beetles, Callosobruchus maculatus and Stator limbatus (Coleoptera: Chrysomelidae)

Inbreeding depression is well documented in insects but the degree to which inbreeding depression varies among populations within species, and among traits within populations, is poorly studied in insects other than Drosophila. Inbreeding depression was examined in two long-term laboratory colonies of the seed beetle, Callosobruchus maculatus (Fabricius), which are used frequently as models for experiments in ecology, evolution and behaviour. Inbreeding depression in these laboratory colonies are compared with one recently field-collected population of a different seed beetle, Stator limbatus Horn. Inbreeding reduced embryogenesis, egg hatch and larval survival in both species, such that eggs produced by sib matings were >17% less likely to produce an adult offspring. Inbred larvae also took 4–6% longer to develop to emergence in both species. Inbreeding depression varied among the measured traits but did not differ between the two populations of C. maculatus for any trait, despite the large geographic distance between source populations (western Africa vs. southern India). Inbreeding depression was similar in magnitude between C. maculatus and S. limbatus. This study demonstrates that these laboratory populations of C. maculatus harbour substantial genetic loads, similar to the genetic load of populations of S. limbatus recently collected from the field.

Egg-size manipulations in the seed beetle Stator limbatus: consequences for progeny growth

Studies that have demonstrated consequences of variation in egg size for progeny growth and development are often confounded by genetic correlations among life-history characters; relationships between egg size and progeny life-history characters do not always reflect cause and effect. Thus, experimental approaches to manipulating egg size in order to quantify the consequences of egg-size variation for progeny growth and development have been developed. I used egg-size plasticity in response to oviposition environment to manipulate egg size in a seed beetle, Stator limbatus, to test the hypothesis that progeny developing from larger eggs survive better, develop faster, and attain a larger adult body size than progeny developing from smaller eggs. Females exposed to Cercidium floridum during egg maturation laid eggs that were substantially larger than those laid by females exposed to Acacia greggii during egg maturation. The larger eggs laid by females exposed to C. floridum took significantly longer to hatch but had shorter total egg-to-adult development times than eggs laid by females exposed to A. greggii (when reared to adulthood on A. greggii). There was no evidence from the between-treatment comparisons that egg size affected egg-to-adult survivorship or the size at which progeny emerged as adults. Within-treatment correlations between egg size and progeny life-history characters were generally consistent with the between-treatment analyses, except that female progeny developing from larger eggs tended to emerge as larger adults than female progeny developing from smaller eggs. This result is interpreted as a consequence of heritable variation in body size rather than a cause-and-effect relationship between egg size and progeny body size.