Summary1. Closely related species often differ greatly in the quality and breadth of resources exploited, but the actual mechanisms causing these differences are poorly understood. Because in the laboratory specialized species often survive and perform as well or better on host species that are never utilized in nature, negative ecological interactions restricting host range must exist. Here, we focused on reproductive interference, which has been theoretically predicted to drive niche separation between closely related species with overlapping mating signals. 2. We examined the interspecific sexual interactions in relation to ecological specialization and generalization in two sibling ladybird species, Harmonia yedoensis and Harmonia axyridis. Harmonia yedoensis is a specialist predator that preys only on pine aphids, which are highly elusive prey for ladybird hatchlings, whereas H. axyridis is a generalist predator with a broad prey and habitat range. 3. We experimentally showed that conspecific sperm fertilized the vast majority of eggs regardless of mating order (i.e. conspecific sperm precedence) when a female of H. yedoensis or H. axyridis mated with both a conspecific and a heterospecific male. Moreover, we demonstrated that mating opportunities of H. yedoensis females strongly decreased as heterospecific density increased relative to conspecific density. In contrast, in H. axyridis, female mating success was high regardless of conspecific or heterospecific density. 4. Our results suggest that the generalist H. axyridis should be dominant to the specialist H. yedoensis in terms of reproductive interference. Our results support the hypothesis that asymmetric reproductive interference from the dominant species may force the non-dominant species to become a specialist predator that exclusively utilizes less preferred prey in nature.

PCR amplification bias is a well-known problem in metagenomic analysis of arthropod communities. In contrast, variation of DNA degradation rates is a largely neglected source of bias. Differential degradation of DNA molecules could cause underrepresentation of taxa in a community sequencing sample. Arthropods are often collected by passive sampling devices, like malaise traps. Specimens in such a trap are exposed to varying periods of suboptimal storage and possibly different rates of DNA degradation. Degradation bias could thus be a significant issue, skewing diversity estimates. Here, we estimate the effect of differential DNA degradation on the recovery of community diversity of Hawaiian arthropods and their associated microbiota. We use a simple DNA size selection protocol to test for degradation bias in mock communities, as well as passively collected samples from actual Malaise traps. We compare the effect of DNA degradation to that of varying PCR conditions, including primer choice, annealing temperature and cycle number. Our results show that DNA degradation does indeed bias community analyses. However, the effect of this bias is of minor importance compared to that induced by changes in PCR conditions. Analyses of the macro and microbiome from passively collected arthropod samples are thus well worth pursuing.

1. To elucidate the factors responsible for the evolution of host specialisation, prey capture performance in hatchlings of two sibling ladybird species, Harmonia yedoensis and H. axyridis, feeding on four aphid species was examined. Harmonia yedoensis is a specialist predator that preys only on pine aphids in the field, whereas H. axyridis is a generalist predator with a broad prey range.2. In H. yedoensis, sibling cannibalism in each clutch was intense and predation against pine aphid as well as other aphid species was moderately successful. In contrast, the predation success rate of H. axyridis against pine aphid was quite low.3. Moreover, it was experimentally shown that increased maternal investment enhanced prey capture performance against pine aphid in H. yedoensis but not in H. axyridis hatchlings, despite their increased body size due to maternal investment.4. In addition, morphological and behavioural analysis showed that hatchlings of H. yedoensis had longer legs and a larger head capsule size and could walk faster than H. axyridis.5. In summary, the interactive effects between a large amount of maternal investment and morphological specialisation of the first instars may enable H. yedoensis to capture the pine aphid efficiently, a highly elusive prey for ladybird hatchlings. The ability of H. yedoensis to utilise the three other aphid species in addition to the pine aphid suggests that a trade-off in prey capture performance is not the main factor in the host specialisation of H. yedoensis.

In nature, closely related species often utilize different host species, but it is still unclear what factors contribute to the evolution and maintenance of such diversified host selection. In this review, I describe how negative interspecific mating interactions (reproductive interference) can shape host selection by animals, focusing mainly on phytophagous and predatory insects. First, I explain an important premise of this hypothesis, which is that the adult reproductive site is the same as the feeding site for the offspring. Next, I describe several mathematical models and well-studied empirical systems to show that reproductive interference can sufficiently drive and maintain different host selection between phylogenetically related species. Then, I argue for the first time that reproductive interference can cause an oviposition preference in insects that is not optimal for the survival and development of the offspring, as a result of maternal adaptive behavior that maximizes the mother's own fitness. Furthermore, I argue that in insects, reproductive interference probably shapes oviposition behavior before the female alights on the host (e.g., habitat preference), without affecting post-alighting decision making. I would like to emphasize that these two arguments represent the novel approach to clarify the unrevealed pattern of complex insect oviposition behavior.