Metabarcoding of canopy arthropods reveals negative impacts of forestry insecticides on community structure across multiple taxa

1. Insecticides used to combat outbreaks of forest defoliators can adversely affect non-target arthropods. Forest use insecticides typically suppress Lepidoptera larvae which are the keystone of the canopy community of deciduous oak forests. The abrupt removal of this dominant component of the food web could have far-reaching implications for forest ecosystems, yet it is rarely investigated in practice owing to several methodological shortcomings. The taxonomic impediment and the biased nature of arthropod sampling techniques particularly impede the assessment of insecticide impacts on diverse communities.

2. To tackle this issue, we propose an experimental approach combining pyrethrum knockdown sampling and species determination via DNA metabarcoding, using community subsampling to derive estimates of species abundances. We applied this protocol to investigate the short-term effects of the insecticides diflubenzuron (DFB) or Bacillus thuringiensis var. kurstaki (BTK) on canopy-dwelling arthropod communities in German oak woodlands.

3. Our approach allowed us to include most of the detected diversity and integrate species abundances in our analyses. By classifying arthropod species into assemblages based on their expected sensitivity rather than coarse taxonomic groupings, we could unveil substantial effects of DFB across multiple taxa five weeks after application.

4. Although strong effects on single species appear related to direct toxicity, substantial impacts of DFB on parasitoids and xylophagous beetles suggest that anti-defoliator treatments can have previously unsuspected indirect effects on some components of forest arthropod communities. The impacts of BTK on community structure were consistent with but much weaker than that of DFB.

5. Synthesis and applications. Comparing diversity patterns in the arthropod communities of sprayed and unsprayed oak canopies, our results show that selective insecticides can alter species diversity in presumably non-sensitive taxa. Even though the ecological significance of these impacts has yet to be assessed in an operational setting, their existence calls for increased regulatory scrutiny on indirect effects. As community approaches become more attainable with the rapid development of DNA metabarcoding, we suggest the inclusion of community level endpoints as regulatory requirements for the approval of forest use insecticides.