Mated Female Dispersal and Solitary Gallery Initiation in Ips typographus: Evidence From a Range Expansion Gradient

Abstract

Understanding colonisation strategies in Scolytinae beetles is critical for predicting their invasion dynamics, especially during early establishment under low-density conditions. In Ips typographus, a major Eurasian forest pest, a proportion of females are mated before emerging from their natal tree and are able to produce, alone, a viable brood. Here, we combine field surveys along a density gradient from northern France to southern England with laboratory experiments to investigate the potential for solitary reproduction. We show that the proportion of mated dispersing females increased over the season but declined with distance from outbreak centres. Sex ratios also shifted seasonally, from male-biased early flights to female-biased later ones. At invasion fronts, we observed and described previously undocumented solitary gallery foundations, occurring at low but consistent frequencies. These solitary galleries exhibited distinctive morphologies, typically lacking nuptial chambers and occasionally presenting characteristic turning notches. Solitary females could successfully establish egg-laying galleries under natural conditions. Our findings provide the first empirical evidence of solitary reproduction in I. typographus in the wild, suggesting that this strategy—enabled by pre-emergence mating and re-emergence—may enhance establishment under mate-limited conditions. This reproductive flexibility could facilitate persistence in marginal habitats and has important implications for risk assessment and management of emerging bark beetle populations.