Scientists at Rothamsted Research Institute in Hertfordshire have uncovered the genetic mechanisms that allow aphids to adapt to a new host plant and provide natural resistance to neonicotinoid insecticides.

Insect host shifts are important because they may be a first step in the evolution of new species, potentially giving rise to new pests of agriculturally important crops. Plants can create a barrier to many potential insect host shifts in the form of secondary metabolites or allelochemicals. Many plants produce these in order to defend themselves from pests like aphids.

Consequently, insect pests can only be successful in using such plant species as food if they develop mechanisms to overcome sensitivity to the defence compounds that a plant is releasing.

Identifying the initial genetic changes involved in this process has proved elusive in the past, but the Rothamsted scientists, working in collaboration with researchers from the Liverpool School of Tropical Medicine and industry partners, have characterised novel genetic changes that underlie an insect host shift and the emergence of a new subspecies of crop pest with natural resistant to pesticides.

The team published their findings in the journal Proceedings of the National Academy of Sciences (PNAS) this month. Shifts in growing season and area, as well as species migration, wrought by climate change are expected to give rise to new crop pests and diseases. By understanding the mechanisms that allow insects to change their target plants, the researchers hope to inform future attempts to protect agricultural crops from a new and changing range of pests.

The peach potato aphid (Myzus persicae) is a major crop pest, one of the most common greenfly pests within the UK (and in other parts of the world). It can cause substantial crop yield losses through the transmission of up to 120 distinct plant viruses and/or directly through feeding and sucking the sap of plants. A subspecies of peach potato aphid (Myzus percicae nicotianae) has evolved to feed and survive on tobacco plants. Aphids of this subspecies show reduced sensitivity to the secondary metabolite nicotine (a potent natural insecticide produced by tobacco). Interestingly, and probably as a result, these aphids also show reduced sensitivity to neonicotinoids – a class of relatively recently developed synthetic insecticides.

Studying how these aphids manage to overcome the toxic effects of the tobacco-produced nicotine and understand how this relates to resistance to neonicotinoids, the researchers discovered the aphids had developed genes allowing them to digest nicotine into less harmful substances and that this process also gave the tobacco-adapted insects resistance to neonicotinoids.

Dr Chris Bass of Rothamsted Research, elaborated, "We are excited that for the first time we have been able to characterise the genetic mutations involved in the initial steps of the host shift of the peach potato aphid to tobacco. We found that a detoxification enzyme called CYP6CY3, which is naturally present in all aphids, is responsible for the metabolism of nicotine to less toxic compounds.

"However," he continued, "For this process to occur at significant levels that allow survival of aphids that feed on tobacco plants the gene producing this enzyme needs to be present in many more copies than the normal two copies, up to 100 copies in the most resistant aphids.

"We have also been able to show that changes in the part of the gene that gives information as to when and where the enzyme should be made (i.e. the regulatory region of CYP6CY3) contribute to the overexpression of the gene. Together these two mechanisms work in concert to produce high levels of the enzyme which breakdowns nicotine and has also pre-adapted tobacco-adapted races to resist man-made insecticides".

Professor Lin Field, also at Rothamsted Research, added, "The findings of this study are very exciting because they provide novel insights into the fundamental evolutionary processes that have driven adaptation in an aphid and similar mechanisms may be employed by other insect species. Additionally, we now have further understanding of the molecular mechanisms that can drive insecticide resistance and this can be utilised when developing pest management strategies."