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.