Wildflower strips enhance wild bee reproductive success

1. Intensification of agriculture has resulted in a simplification and fragmentation of agroecosystems. Yet, its impact on the reproductive success and population dynamics of wild bees, and how adverse effects can be mitigated, remains poorly understood.

2. We established populations of seven solitary bee species varying in body size in experimentally sown wild flower strips (WFS), existing semi-natural habitats (SNH; forest edges) and isolated sites lacking WFS and SNH in the local surrounding (350 m radius) to test whether (i) the wild bee species use planted flowers of wildflower strips (WFS) to provision offspring with pollen, (ii) whether vicinity to WFS reduces their foraging trip duration, (iii) parasitism rate, and (iv) increases bee reproductive success of nesting populations. Furthermore, we tested whether the effect size depends on the body size of the bee species considered.

3. We show that wild bees nesting in WFS provisioned their offspring primarily with pollen from plants of WFS. This led to shorter foraging trips of all bee species nesting in WFS compared to bees nesting in isolated (unrestored) sites and overall increased reproductive success (i.e. more viable offspring). Reproductive success of bees nesting in WFS was also higher, and parasitism rates lower, compared to bees nesting at forest edges. Smaller-potentially less mobile-bee species benefitted more than larger ones from WFS plantings in terms of reproductive success.

4. Synthesis and applications.Our findings demonstrate that diverse WFS can enhance the reproductive success of multiple solitary wild bee species, thereby mitigating negative impacts of agroecosystem simplification through the provision of suitable floral resources and reduced foraging times required for offspring provisioning. Our study further indicates that a relatively fine-meshed network of wildflower plantings and nesting habitats is required to most effectively enhance reproduction and populations of solitary bees in agricultural landscapes.