A project undertaken at University of Melbourne, and supervised by Charles Robin
It may seem obvious that chemicals used to control pest insects contribute to declines in insect biodiversity more generally. However, such declines may also be due to other causes such as habitat loss and fragmentation, climate change or other anthropogenic or natural causes. We sought evidence of insecticidal impact on non-pest insects by looking for specific ‘scars’ in the genomes of those species that are currently found in horticultural landscapes where insecticides are commonly used. We posited that because divergent pest species have frequently evolved insecticide resistance with identical amino acid substitutions in parallel, then the same set of substitutions may evolve in non-pest insects. If they do and if they can be shown to have arisen recently, then they provide specific evidence of the impact of chemical insecticides on non-target insect populations.
We collected insects using Malaise traps and sweep nets in three of the major horticultural areas of Victoria; the Yarra Valley, Sunraysia and the Murray-Goulburn. we used amplicon sequencing of CO1 and rRNA loci to classify specimens by taxonomic rank, comparing sequences against the Barcode of Life Database (BOLD) and the National Center for Biotechnology Information (NCBI) database. Simultaneously, we sequenced genes encoding insecticide target proteins, such as acetylcholine esterase (Ace) and a GABA receptor, examining them for canonical insecticide resistance mutations.
Our research asks how many insect species show signs that they have adapted to synthetic insecticides? We are collecting insects from horticultural areas in Australia and sequencing genes that are often associated with insecticide resistance in pest species. We will then ascertain whether such mutations have arisen recently and whether these signals of adaptation could be used to monitor the impact of insecticides on insect biodiversity.
We obtained sequences for well over 100 species from diverse insect orders. Insecticide resistance variants were found in at least ten species including four flies (a hover fly, the black pasture fly, a drosophilid, and a muscid), three butterfly species (the small white, the meadow argus, and the grass blue), two Hymenoptera and at least one aphid. While there is a formal possibility that these species have, by chance or pre-adaptation, ‘resistant’ variants as their ancestral state, the fact the variants are at low frequency within populations of the particular species is consistent with the hypothesis that they have arisen in the 60 years since the widespread use of insecticides.
As an illustration, consider a small black fly we collected abundantly. CO1 sequencing revealed 100% match with a Hydrellia tritici specimen in an online database. This species, also known as the black pasture fly, is endemic to Australia and New Zealand and relatively common. Though considered invasive elsewhere, it is not a pest. Among approximately 170 individuals collected, sequencing of the Rdl gene, which encodes a GABA receptor, revealed that approximately 24% of sequence reads contained a variant causing an A302S substitution previously shown to confer resistance to cyclodiene insecticides in numerous pest species.
These findings motivate further studies of recent selective sweeps at these loci in these species, and ‘museum-omic’ studies that sequence from historical pinned samples to establish recent origin of the variants. The approach could also be extended to survey other ecosystems such as those where insecticides are intensively used for mosquito control.
Our study supports the contention that chemical insecticides have been significant environmental pollutants in Australian horticultural environments that have profoundly changed the genomes of surviving species. Such ‘collateral damage’ inflicted by chemicals lays a context when seeking social license for alternate ways to control pests that damage agriculture and threaten human and animal health, such as genetic biocontrol methods. These new genetic technologies have the potential to be species -specific because they elicit their control via a species natural mating system.