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

Citation

Leroy, Benjamin et al. (2022), Metabarcoding of canopy arthropods reveals negative impacts of forestry insecticides on community structure across multiple taxa, Dryad, Dataset, https://doi.org/10.5061/dryad.s1rn8pk95

Abstract

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.

Methods

Sampling. Arthropods were sampled by pyrethrum knockdown using Swingfog SN-50 fogging machines and a 1% pyrethrum suspension in petroleum white oil. After 30 minutes exposure, all arthropods fallen on four 15 m2 tarpaulin sheets placed below the crown were collected and stored in ethanol.  Forty oak trees in 10 blocks distributed over three stands with high gradation of the defoliator Lymantria dispar (Lepidoptera: Erebidae) were sampled. In each block, one tree had been treated with Bacillus thuringiensis var. kurstaki, one with diflubenzuron, and two were not sprayed (controls). Spraying was conducted mid-May at the maximum legal application rate. All trees were sampled once five weeks after spraying. The abundance of Lymantria dispar was monitored on each tree by counting caterpillars below tree bands.

Species determination. Arthropods were sorted at high taxonomic rank (mostly order and sub-order; A) and split into two groups based on how easily they can be identified to species by available expert taxonomists (C1). Taxa belonging to the 'challenging' group were determined via DNA metabarcoding after samples were fractioned into 12 subsamples per tree (B). Sequence read abundances were filtered to remove artefactual and contaminant sequences and species abundances were estimated at the tree level by pooling the subsample level incidence data (C2).

Data processing protocols are provided in details in the associated publication.

Usage Notes

The dataset includes four files, each representing an intermediate step of the data processing pipeline (see letters corresponding to each file in the method description). 

(A) Sorting data (subsample level; only taxa later indentified by metabarcoding); file name: JAPPL-2021-00659_A_Data.csv

(B) Raw metabarcoding species data with sequence reads abundance (subsample level); file name: JAPPL-2021-00659_B_Data.csv

(C1) Derived metabarcoding data with species abundance estimates (tree level); file name: JAPPL-2021-00659_C1_Data.csv

(C2) Morphologically identified taxa with species abundance (tree level); file name: JAPPL-2021-00659_C2_Data.csv

Note that due to a pyrethrum dosing mistkaes, sampling failed on four blocks and these samples were excluded from the analyses as they were not comparable with the other samples. The excluded samples are only included in the dataframe A.

Each data file comes with a data structure file (prefix Str) detailing each variable.

Funding

Fachagentur Nachwachsende Rohstoffe, Award: 22012115