Solitary bee larvae modify bacterial diversity of pollen provisions in the stem-nesting bee, Osmia cornifrons (Megachilidae)

Abstract

Microbes, including diverse bacteria and fungi, play an important role in the health of both solitary and social bees. Among solitary bee species, in which larvae remain in a closed brood cell throughout development, experiments that modified or eliminated the brood cell microbiome through sterilization indicated that microbes contribute substantially to larval nutrition and are in some cases essential for larval development. To better understand how feeding larvae impact the microbial community of their pollen/nectar provisions, we examine the temporal shift in the bacterial community in the presence and absence of actively feeding larvae of the solitary, stem-nesting bee, Osmia cornifrons (Megachilidae). Our results indicate that the O. cornifrons brood cell bacterial community is initially diverse. However, larval solitary bees modify the microbial community of their pollen/nectar provisions over time by suppressing or eliminating rare taxa while favoring bacterial endosymbionts of insects and diverse plant pathogens, perhaps through improved conditions or competitive release. We suspect that the proliferation of opportunistic plant pathogens may improve nutrient availability of developing larvae through degradation of pollen. Thus, the health and development of solitary bees may be interconnected with pollen bacterial diversity and perhaps with the propagation of plant pathogens.

Keywords: Megachilidae; bees; brood cell; development; larvae–development; microbiome; plant pathogen; pollen.

Figure 1

Samples and study design. (A) The exposed cells of Osmia cornifrons. (B) Experimental overview shows sampling time during our experiment: points; (t = 0 = day 0; t = 1 = day 3; t = 2 = day 6; t = 3 = day 9; t = 4 = day 12; and t = 5 = day 15). Only Day 3, 9, and 15 were selected for bacterial diversity analysis.

Figure 2

The number of unique bacterial ASVs summarized by the three treatment groups and sampling time points. Significance is denoted by (A–C). Significant differences in bacterial richness across time points were found using a Kruskal Wallis test for larvae only (H = 4.69, p = 0.096) and pollen with larvae (H = 11.53, p = 0.003), but not for pollen without larva (H = 0.358, p = 0.836).

Figure 3

The proportional abundance of dominant bacterial ASVs of pollen and Osmia cornifrons summarized across sampling days. Proportional abundances are displayed for three treatment groups, larvae only, pollen with larvae, and pollen without larvae.

Figure 4

A comparison of bacterial diversity between pairs of pollen masses that were combined and then separated returning one larva to each pair. All figures (observed features, Peilou’s evenness, and beta diversity) are summarized by three experimental time points (day 3, day 9, and day 15). Moderately significant is denoted by one star (*) and significant differences are denoted by three stars (***).