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.