Arable farming is facing increasing demand to improve productivity, but to do so in a sustainable manner, including mitigating declines in global biodiversity. Silvoarable agroforestry systems have the potential to be a ‘win-win’ solution. For example, these systems can enhance natural enemy and pollinator abundances whilst suppressing pests, compared to arable (Staton et al. 2019).
The benefits of agroforestry systems are reasonably well understood for some taxonomic groups, such as ground beetles and aphids. However, bees, wasps (including parasitoids) and hoverflies are relatively poorly studied in agroforestry, despite their importance for the natural control of pests and pollination of crops. Furthermore, bees and wasps are amongst the taxa most affected by insect declines globally (Sánchez-Bayo and Wyckhuys 2019). The aim of this study was therefore to assess whether agroforestry systems could help conserve these threatened beneficial insects.
Our study sites comprised three working farms in England, all of which contained a silvoarable agroforestry field and a monocropped arable field under the same management and rotation. Two farms were organic, the third was conventional and minimum-till. Crop rotations were based around cereals, plus oilseed rape at the conventional farm. Agroforestry alleys were 24 m wide, tree rows were 3-4 m wide and principally comprised apple trees on MM106 rootstocks, planted between 2009 and 2015. Agroforestry field sizes were approximately 5.6, 3.8 and 12 ha respectively at the three sites (the latter field set within a 52 ha agroforestry system), while the corresponding arable field sizes were 6.1, 3.0 and 7.6 ha respectively.
Sampling was undertaken in 2018 and 2019. At each site, two agroforestry alleys were selected for sampling. Within these, 16 sampling points (eight in each alley including its adjacent tree rows) were selected at set distances from the tree rows. This procedure was repeated in an arable field at each site, to act as a control. At each point, a set of three UV-bright pan traps was deployed to capture flying insects. Each trapping day lasted for at least five hours, and was repeated nine times over the two-year period. All bees, wasps and hoverflies captured in the traps were assigned to taxonomic groups (Fig. 1).
Taxonomic richness and diversity (Shannon index) of bees, wasps and hoverflies collectively were significantly higher in agroforestry than arable fields (mixed models, t=3.007, p-value=0.003; t=3.414, p- value<0.001). Six of the taxonomic groups were significantly more abundant in agroforestry than arable fields (Fig. 1). The strongest effects were seen for the two predatory wasp taxa. We hypothesise that social wasps were attracted by apples in the tree rows, but could also aid biocontrol by hunting insect pests. Two of the three parasitoid wasp taxa were significantly more abundant in agroforestry than arable fields. However the braconid wasps were less abundant in the agroforestry fields. This effect was driven by an early-season influx of braconids into the arable field at one site, possibly attracted by a high prey resource.
Two of the five pollinator groups, comprising bumblebees and the small halictid bees, were significantly more abundant in agroforestry than arable fields. This is consistent with a study in the UK showing higher solitary bee abundance in agroforestry than monocultures, although in contrast to that study, we found no significant difference for hoverfly abundance (Varah et al. 2020).
Our findings of benefits to bees, wasps and hoverflies could be explained by the higher availability of flower and nesting resources in agroforestry tree rows compared to arable fields. Pan traps are less effective in flower-dense areas (O’Connor et al. 2019), therefore, our results may underestimate the benefits of agroforestry. In addition, field sizes were relatively small at two of the sites (3.8 and 5.6 ha), and set within relatively diverse landscapes (particularly within a 2 km radius), which could allow insects to rapidly colonise the arable fields from surrounding semi-natural habitat. Stronger effects could therefore be predicted at larger-scale farms.
In conclusion, we find strong evidence for higher taxonomic richness and diversity of bees, wasps and hoverflies, and abundance of predatory wasps and pollinators, in agroforestry compared to arable fields. This suggests that agroforestry systems can play a role in the conservation of these threatened insects, while improving the sustainability of agriculture by providing natural pest control and pollination services.