Sirex noctilio is an economically important invasive pest of commercial pine forestry in the Southern Hemisphere. Newly established invasive populations of this woodwasp are characterized by highly male‐biased sex ratios that subsequently revert to those seen in the native range. This trend was not observed in the population of S. noctilio from the summer rainfall regions in South Africa, which remained highly male‐biased for almost a decade. The aim of this study was to determine the cause of this persistent male bias. As an explanation for this pattern, we test hypotheses related to mating success, female investment in male versus female offspring, and genetic diversity affecting diploid male production due to complementary sex determination. We found that 61% of females in a newly established S. noctilio population were mated. Microsatellite data analysis showed that populations of S. noctilio from the summer rainfall regions in South Africa are far less genetically diverse than those from the winter rainfall region, with mean Nei's unbiased gene diversity indexes of 0.056 and 0.273, respectively. These data also identified diploid males at low frequencies in both the winter (5%) and summer (2%) rainfall regions. The results suggest the presence of a complementary sex determination mechanism in S. noctilio , but imply that reduced genetic diversity is not the main driver of the male bias observed in the summer rainfall region. Among all the factors considered, selective investment in sons appears to have the most significant influence on male bias in S. noctilio populations. Why this investment remains different in frontier or early invasive populations is not clear but could be influenced by females laying unfertilized eggs to avoid diploid male production in populations with a high genetic relatedness.

Introduced species face numerous biological barriers before they can establish in a new environment. Understanding how they overcome these obstacles is crucial for the development of effective risk assessment and regulation. Reproductive biology is known to influence establishment capacity in plants and is widely used for risk assessment. This biological field should receive more attention, and particularly in the case of insects, as they display a wide range of reproductive traits and have a great impact on the economy and environment. Among insects, the order Hymenoptera is of interest for its diversity, both in terms of reproductive traits and introduction history, as invasive species and biological control agents. We review the main reproductive strategies of Hymenoptera, spanning parthenogenesis, sex determination and mating strategies, and evaluate their effect on invasive potential. For instance, thelytoky could decrease the strength of Allee effects while Arrhenotoky could increase adaptive potential. A species with complementary sex determination could be more affected by inbreeding than other species, while paternal genome elimination could lead to high levels of homozygosity. Finally, some reproductive behaviours could decrease inbreeding, facilitate mate location or adaptation by encouraging admixture.The two invasive species Apis mellifera scutellata and Leptocybe invasa and the biocontrol agent Aphdius ervi serve as case studies to illustrate the effect of reproductive traits on species capacities to become established in a new area.

Chemical signals are important mediators of interactions within forest ecosystems. In insects, pheromone signals mediate intraspecific interactions such as mate location and acceptance. The evolution of pheromones in insects has been mostly studied from a theoretical perspective in the Lepidoptera. With this study, we aimed to broaden our understanding of pheromone communication in bark beetles. We first demonstrated that the enantiomeric ratios of ipsdienol produced by male I. avulsus, showed little variation. Subsequently, with field trapping trials we characterized the influence of the enantiomeric ratio of ipsdienol (pheromone component of I. avulsus) on I. avulsus captures and observed a great amount of variation in the receiver preference function. Most importantly, we demonstrated that responding individuals responded indiscriminately to all the enantiomeric ratios produced by the emitting individuals. These observations are consistent with the asymmetric tracking model which postulates that if the limiting sex is the emitting sex, responding individuals should not discriminate between emitted ratios. Consequently, responding individuals do not constrain the evolution of the signal. Our data suggest that, in I. avulsus, the composition of the aggregation pheromone signal might be more responsive to external selection forces, such as predation and metabolic constraints, as suggested by the asymmetric tracking model.

While male mate choice in insects is a widely accepted concept, there is still limited evidence showing that lek formation is compatible with the evolution of male mate choice. In the woodwasp Sirex noctilio, males form leks that are used by females to select a mate. However, males have been observed to ignore certain females, suggesting the presence of male mate choice despite the presence of a lek mating system. In this study we demonstrate that males only attempt to mate with certain females. To understand the criteria used by males and females to select a mate, we also tested the effect of age, size, and male to female size ratio on the number of mating attempts made by males and on female receptivity. We demonstrate that size and age play a role in both male and female mate choice. Our results suggest that males must reach sexual maturity after emergence and are neither receptive nor attractive to females during the first few days of their lives. We also show that older females become less attractive to males, suggesting that female S. noctilio switch to a strict host location phase sometime after emergence. Our results show that male and female size, and the ratio between them, play a role in mate choice. While larger males are more motivated to mate, their large size can physically prevent them from mating with small females. Small females are also more attractive and more receptive to males, consistent with the presence of convenience polyandry in S. noctilio.