Taxon-specific response of natural enemies to different flower strip mixtures

Flower strips are a prominent agri-environmental scheme with the central
objective of promoting biodiversity and maintaining associated ecosystem
services. The promotion of natural enemies by increasing resource
availability through flower strips is a promising approach for integrated
pest management. In a three-year field study, two annual and two perennial
flower strip mixtures, as well as a grass mixture, were tested regarding
their attractiveness for natural enemies at three different study sites in
Germany. Natural enemies were sampled annually in nine sampling rounds at
a ten-day rhythm using sweep netting and pitfall traps. To assess
available floral resources, we estimated the species-specific flower cover
and classified species into flower types. Flower strip mixtures differed
in their attractiveness to natural enemies. Treatment effects on arthropod
activity density were most pronounced in the 2nd and 3rd years. Overall,
perennials were more attractive than annual flower strip mixtures for most
natural enemies, however, the response to flower strip mixtures varied
significantly among taxa. For example, perennial flower strips showed a
two- to fourfold increase of plant-dwelling spider and parasitoid wasp
activity density as well as higher numbers of juvenile stages of predatory
bugs and rove beetles compared to annual flower strips. In contrast,
annual flower strip mixtures showed the highest attractiveness for ground
beetles. Moreover, different natural enemy taxa were associated with
varying flower strip characteristics such as flower type proportion,
herbivore availability, and plant species richness. Synthesis and
applications We found taxon-specific responses of natural enemies to
different flower strips, which provide guidance for the floral composition
of flower strips in order to maximize their effectiveness for natural pest
control. We suggest the use of perennial mixtures with a high proportion
of flower heads (Asteraceae) and disk flowers (esp. Apiaceae, Rubiaceae,
Brassicaceae) in particular, but also flag blossoms (Fabaceae) as these
flower types were positively associated with several natural enemy groups.
However, since we found mixed responses to flower types, specific flower
strip composition will ultimately depend on its objective.