Attractiveness of sown wildflower strips to flower-visiting insects depends on seed mixture and establishment success

Establishing wildflower strips has been suggested as an effective measure
to promote pollination services, pest control or general insect
biodiversity, but little is known about the integration of these different
objectives when selecting flower seed mixtures. In ten agricultural
landscapes in the Netherlands, we established a wildflower strip (0.4 –
4.9 ha) with half of each strip sown with a mixture targeting
longer-tongued pollinators and the other half sown with a mixture
targeting shorter-tongued pollinators and natural enemies. We determined
establishment success of sown wildflowers and evaluated the attractiveness
of the established flower communities to multiple functional groups of
flower visitors: bumblebees (long-tongued pollinators), hoverflies
(short-tongued pollinators and natural enemies), and butterflies and total
flower-visitor richness (indicators of wider biodiversity values).
Bumblebees clearly preferred the pollinatortargeted seed mixture and were
positively associated with cover of Fabaceae and negatively with Apiaceae.
Hoverflies consistently preferred the natural enemy mixture and were
positively associated with Apiaceae. The other target groups displayed no
clear responses to seed mixture type but instead were associated with
local flower richness within strips. Across sites, responses of
flower-visitors to sown mixture types did not depend on wildflower strip
size, proportion of surrounding semi-natural habitat, or flower variables.
However, all flower-visitors except butterflies increased with increasing
cover or richness of (sown) flower species across sites. Our results
suggest that, although species-rich wildflower strips may benefit several
species groups, maximising different objectives involves trade-offs
between functional groups that prefer short- or long-corolla flowers.
Furthermore, our study suggests that sowing a wildflower mixture does not
necessarily result in a vegetation with the same composition as the seed
mixture as species may establish poorly or not at all. Selection of flower
species for seed mixtures should therefore, in addition to insect target
group, take the establishment characteristics of plant species into
account.