The green peach aphid (Myzus persicae) is polyphagous, which means it can feed on the sap of over 50 different plants, including cultivated plants such as peach, oilseed rape, sugar beet, tobacco and potato crops. It has potential to cause great harm primarily by transmitting viruses to these host plants. Among other diseases, aphids are vectors of Sharka virus, which affects peaches, and of the viruses that cause beet yellows. The primary method used to control this pest is the application of synthetic plant protection products, but resistance to most of these products has developed in these insects.
However, the combined resistance observed in aphid populations varies depending on the host plant, which was intriguing to scientists. This finding prompted a study carried out by scientists from the Contracted Unit for Characterisation and Monitoring of Phenomena of Pesticide Resistance Development (CASPER), which is part of ANSES's Lyon Laboratory, in collaboration with scientists from INRAE's Institute for Genetics, Environment and Plant Protection (IGEPP) and the University of Montpellier. It gave rise to an article published in Evolutionary Applications in May 2022.
These scientists analysed the diversity and genetic structuring of populations of aphids collected in France, from three main crops: peaches, oilseed rape and tobacco. Clear genetic differentiation was demonstrated between the aphids collected from peach trees and those living on oilseed rape and tobacco crops. "Despite it being the same species of aphid, there were few gene flows between the three sub-groups corresponding to the three crops” affirms Benoit Barrès, Head of the CASPER Unit and co-author of the study.
Combined resistance also differs greatly between genetic groups. Aphids collected from peach trees were frequently resistant to neonicotinoids. Conversely, aphids taken from oilseed rape and tobacco crops were often carriers of gene variants (or alleles) conferring resistance to pyrethroids and carbamates but were not resistant to neonicotinoids. This means that resistance alleles had not been exchanged between these genetic groups.
The scientists also performed genetic analyses with aphids randomly collected from the air between 2001 and 2008. This showed the high speed at which resistance can emerge in the general aphid population: two years following the placing on the market of a new product, containing a pyrethroid and a carbamate, the frequency of aphid clones combining alleles conferring resistance to these two substances had strongly increased.
The low level of genetic exchanges observed between sub-groups of aphids is surprising. It may be due to their method of reproduction: during their life cycle, aphids can reproduce both sexually, leading to the mixing of the two parents’ genes, and asexually, where females give birth to offspring that are clones of themselves. Sexual reproduction only occurs on peach trees, while reproduction on other plants is purely asexual.
Whereas scientists used to believe that aphids had to regularly return to peach trees to reproduce, it now appears this is not necessarily true. "During the seven years of sampling, we observed the survival of asexually reproducing clones. This was why there was no mixing of genes between the groups” explains Benoit Barrès.
Moreover, groups of aphids living on oilseed rape or tobacco crops may be better suited to these host plants than their fellow aphids that usually live on peach trees, which may be an additional obstacle to genetic exchanges between the identified groups.
Even though the data used for the study are a few years old, they highlight the importance of taking into account the agronomic landscape, in particular the host plant species, when managing phenomena of resistance to plant protection products and more generally when carrying out effective strategies to control polyphagous insects.
"When we issued our opinion on available alternatives to neonicotinoids for controlling beet aphids, since these substances are now prohibited, we knew that the use of pyrethroids and carbamates was pointless. Indeed, aphids that feed on sugar beets have multiple genes of resistance to these insecticides. We now closely monitor the development of resistance to the insecticides that are still authorised and used in fields of sugar beets” concludes Benoit Barrès.