Beneficial arthropod abundance assessed by sweep-netting is negatively associated with landscape-wide insecticide use

In a new study, Bakker, Bianchi and van der Werf assess how the use of insecticides and semi-natural landscapes impact beneficial arthropods in the Netherlands.

Beneficial arthropods, like predators, parasitoids and pollinators, provide important ecosystem services such as biological pest control and pollination, and are therefore vital for ecosystem health and global food production. However, concerns have risen on the widespread decline of arthropods — and agricultural intensification is considered a key driver.

Arthropod populations can be influenced by farm management and properties of the landscape in which they live. However, the impacts on arthropod communities of specific farm practices (e.g., pesticide use) and landscape factors (e.g., the amount of semi-natural habitat) are difficult to disentangle.

For instance, insecticide use may impact non-target organisms through the presence of active ingredients on plants, soil and water. This can cause (sub)lethal toxic effects on non-target organisms, reducing availability of prey for predators or even hosts for parasitoid wasps. Furthermore, semi-natural habitats provide an important function within the agroecosystem, e.g. by providing overwintering sites, food resources (due to presence of alternate prey) or flower resources, reproduction sites, or dispersal corridors. However, there is strong variability in the responses of pests, their natural enemies and pollinators to semi‐natural habitats in landscapes, and it is unclear to what extent impacts of insecticides on beneficial arthropod populations can be mitigated by these habitats.

In our study, we assessed the impacts of local and landscape-wide insecticide use on beneficial arthropods. Through a two-year landscape study in the Netherlands, we assessed how natural enemies and pollinators were influenced by local and landscape-wide insecticide use and by semi-natural habitats in the surrounding landscape.

Figure 1: Examples of three categories of field margins bordering arable fields: A) mown grass-dominated vegetation; B) unmown grass-dominated vegetation, and C) flower strips. Photo: Lieneke Bakker

We sampled beneficial arthropod populations by sweep netting and placing yellow sticky traps in field margins of conventional and organic arable fields, which were located in landscapes with different proportions of semi-natural habitat. We used two local indicators of insecticide use; i) management in the adjacent field (conventional vs. organic) and ii) the frequency of insecticides applied in the field adjacent to the sampling site. We also estimated the quantity of insecticide applied within a 1 km radius around the sampling site as an indicator at the landscape scale.

Figure 2: Arthropods caught with sweep netting (above) and yellow sticky traps (below). Photo: Lieneke Bakker

In four sampling periods across two years we collected a total of 2803 predators (e.g., ladybeetles, lacewings, spiders and rove beetles), 24004 parasitoid wasps, and 895 pollinators (e.g., bees and hoverflies). We found that all three groups of beneficial arthropods assessed by sweep-netting were negatively associated with insecticide use at the landscape scale. However, arthropod sampling by sticky traps did not show such a relationship. Furthermore, pollinator abundance was negatively associated with conventional farming in the adjacent field and with the frequency of insecticide applications, while predator and parasitoid abundance did not show significant associations with local insecticide use indicators.

The consistent negative association between the numbers of predators, parasitoids and pollinators in our sweepnet samples can be explained by the ‘action at a distance’ mechanism, which posits that mobile arthropod populations in areas that are not directly affected by pesticides can still be affected indirectly through arthropod source-sink dynamics. That is, insecticide-treated fields act as sink habitats (i.e. patches in which death rates exceed birth rates and immigration rates are higher than emigration rates) and can deplete beneficial arthropod populations at the landscape scale when arthropods from the surrounding landscape (i.e. the source habitat) move to these insecticide-treated fields.

Thus, insecticide impacts on arthropod populations extend beyond the fields in which they are used. Awareness of the beyond-field impacts of insecticide use on arthropod populations may encourage land managers to adopt new technologies and practices, to reduce insecticide drift and the frequency of insecticide applications, thereby reducing the impacts of landscape-wide exposure on arthropods that provide services to agriculture.

Risk assessments of insecticides and pesticide use regulations  must account for potential spill-over effects and consider landscape-scale consequences associated with the source-sink dynamics of arthropods.

Read the full article Sweep netting samples, but not sticky trap samples, indicate beneficial arthropod abundance is negatively associated with landscape wide insecticide use in Journal of Applied Ecology.

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