Seasonal activity and phoresy rates of Nitidulid beetles (Coleoptera: Nitidulidae) captured in stands with oak wilt infections in northern Michigan, USA

Abstract

Nitidulid beetles (Coleoptera: Nitidulidae), the overland vectors of the Bretziella fagacearum fungus that causes oak wilt, were monitored in infection centers in Quercus rubra stands in northern Michigan, USA using baited, wind-oriented traps for 2 years. First nitidulid captures, accounting for <1.5% of total annual captures, occurred in late April in both years (<50 cumulative degree days [DDs]; base 10°C). A total of 1,188 and 735 beetles representing 19 and 18 species were captured in 2018 (18 traps; 3 sites) and 2019 (16 traps; 4 sites), respectively. Ground traps (1.5 m high) captured more beetles and species than traps on canopy branches of red oaks. Most nitidulids (81-86%) were captured in May and June, but frequent precipitation and cool spring temperatures extended activity into early July in 2019. In 2018, 336 beetles representing 12 species were screened for B. fagacearum spores, but only 20 beetles from 4 species bore viable spores. Mycelial mats on red oaks killed were 4-fold more abundant in 2019 than in 2018. Of the 225 beetles screened in 2019, 56 beetles representing 6 species had viable spores. Nearly all (96%) spore-bearing beetles in both years were captured in late May or June. In bimonthly xylem samples collected from healthy trees, large earlywood vessels, presumably more vulnerable to infection than latewood, were present from May to June in 2018 and until early July in 2019. Results are consistent with mid-May to mid-July high-risk periods designated in current state and regional guidelines for oak wilt management.

Keywords: Bretziella fagacearum; fungal vector; nitidulid beetle; oak wilt; overland spread; xylem development.

Fig. 1.

Mean (±SE) number of nitidulid beetles per trap captured on each trap check, along with degree day accumulation (base 10°C) at A) 3 sites in 2018 (6 traps per site) and B) 4 sites in 2019 (4 traps per site). Different letters above columns indicate significant differences in nitidulid captures among dates (P < 0.05).

Fig. 2.

Number of the top 5 most abundant nitidulid species along with all other species captured per trap in A) 2018 (18 traps in 3 sites) and B) 2019 (16 traps in 4 sites).

Fig. 3.

Principal component analysis (PCA) of nitidulid species captured in 3 canopy traps and 3 ground traps monitored in each of 3 sites in 2018 (18 total traps). Ordination hulls represent 95% confidence intervals.

Fig. 4.

Monthly mean, minimum, and maximum daily temperatures A) and bimonthly precipitation totals B) in 2018 and 2019 between mid-April and mid-November recorded by the MSU Enviroweather station at Williamsburg, Michigan, near the research sites.