Drivers behind female-biased sex ratios in the alder bark beetle system

Drivers behind female-biased sex ratios in the alder bark beetle system


Taylor Holt- MSc Student

Dr. Allan Carroll- PhD

The study system

ALDER BARK BEETLES (ALNIPHAGUS ASPERICOLLIS) are small, hardwood-infesting bark beetles in the lower mainland of British Columbia. They have female pioneer beetles that initiate attacks on red alder, an important tree in riparian ecosystems.


These beetles are unique to other Scolytines as a portion of the population moves to the branches to overwinter, rather than staying underneath the bark in the bole, where the rest of the developing larvae reside. This results in multiple dispersal events throughout the summer months as the beetles develop and emerge in an asynchronous fashion. We are interested in population differences in branch overwintering versus bole-emerging beetles.


A dispersal and branch collection trial was carried out over the field season of 2022 to determine overwintering locations and sex ratios in the population. We discovered that the population is highly female-biased (70%) in both branch overwintering and bole-emerging populations of beetles.

“Measuring at only 3 mm long, this small but mighty bark beetle has the capacity to take down healthy red alder trees in only a handful of years- causing untimely deaths of red alder and wreaking havoc in riparian ecosystems.” 

– Taylor Holt

Go girls?

There are many evolutionary drivers behind why a population may skew from the standard 1:1 sex ratio- we aim to determine what is driving a female bias in the alder bark beetle system.


A comparative anatomy study was carried out over the winter of 2023 to elucidate certain life history traits occurring in the population to determine why the population is female-biased. Beetles collected over the 2022 season were dissected to collect data on feeding and mating status as well as reproductive maturity. This gives us information on when the beetles are feeding, growing, mating, and developing!