COS 118-2 - Population structure of the Rainbow Scarab Phanaeus vindex (MacLeay 1819) in Michigan

Background/Question/Methods

The rainbow scarab Phanaeus vindex is native to North America and provides substantial ecosystem services to pasture systems. Until now, little is known about its population size, sex ratio, male minor/major ratio, and mobility. These are crucial data for the species’ conservation as pastures become increasingly rare and the landscape is fragmented by urbanization. Here, we provide some of these missing data and estimate population size, viability, and the beetles’ mobility within a pasture. For two years, beetles were trapped alive every week using ten life pitfall traps baited with fresh cow dung and set for 24h. Beetles’ elytra were marked with individual tattoo patterns using a small nail drill before being released.

Results/Conclusions

We captured a total of 238 rainbow scarabs comprising of 124 females and 114 males, which is not significantly different from a balanced sex ratio of 0.5 (Χ² (1, N=238) = 0.517, p < 0.5). The sex ratio changed over the months with more males than females captured in August and vice versa in the other sample months (not/marginally significant). Of the 114 males, 52 were major and 62 were minor morphs. Male major/minor ratio fluctuated but none of the months was significantly different from a 0.5 ratio. In 2019, we recaptured two of 108 beetles and in 2020 six of 130 beetles. Beetles travelled distances of up to 178 m between release and recapturing locations. One large female was recaptured 42 m away from her marking site after 338 days and survived the Michigan winter indicating a long life span and philopatry. The low number of recaptures indicates either a large population, substantial mobility, or high mortality – or a combination of these factors. We will continue and expand the research to neighboring pastures to gather additional data about the species' populations, inter-pasture mobility and the fragmentation and connectivity of their habitats in the changing suburban landscape of SE-Michigan.