The map matters: crop-dominated landscapes have higher vector-borne plant virus prevalence

In this post Suzi Claflin discusses her paper ‘Crop-dominated landscapes have higher vector-borne plant virus prevalence

It’s been clear for some time that landscape composition—that is, land-use types and the connections between them—strongly affects the community of creatures living in a given area. When it comes to insects, the landscape has been shown to shape the communities of both the ones we like, such as the beneficial predators, the ladybirds, and the ones we dislike, such as the infamous farm pest, the aphid. But what about the effect on what those insects might be carrying? What about insect-borne disease?

That’s the crux of what we explored in our study: how landscape composition impacts the spread of an aphid-borne plant virus (specifically, potato virus Y or PVY). And what we found is both intuitive and exciting. Areas with greater amounts of cropland had greater prevalence of PVY, likely because of the effect of landscape composition on the aphid vector (disease carrier) community.

Potato fields. Photo credit: Flickr.

That result— while novel— makes sense, when you consider the natural history of aphids. Although they are often fairly specific in their taste in host plants, they are lousy at host selection. They identify hosts haphazardly from the air based on colour contrast (if it’s green, it’s good), then descend and probe the plant to test for tastiness, often only to find they have made a terrible mistake. They then fly off to the next plant. This is the perfect mechanism for virus spread.

On the face of it, that would mean the more aphids there are, the more the disease will spread. And that’s generally true. But, it’s not quite that simple. Some aphid species are better at spreading PVY than others (some can’t spread it at all), so it’s really the more good PVY carriers there are, the more PVY will be spread. Considering that, it makes sense that the aphid community (both abundance and species richness) should have an effect on PVY prevalence.

That brings us back to the landscape’s role. As a network of resources that are more or less palatable to specific species, the landscape composition should have a profound effect on the aphid community. And that’s what we found: the amount of cropland in an area impacts aphid species richness.

What’s most exciting about these results is their scale. We found effects of landscape composition on virus prevalence from 500 to 1500 metres around our research sites, which were on working farms. That means that the risk of disease spread could potentially be controlled on even small-scale farms.

Here’s how:

  1. Isolate your potatoes. Keep them away from your other crops by at least 500 m.
  2. Insulate your potatoes with non-crop plants. Surrounding potatoes with forested and other unmanaged land areas appears particularly effective.
  3. Do not save potato seed tubers. Although a common practice on small-scale farms, saving potato seed tubers to plant in the next year greatly increases your risk of introducing the virus into your crop at planting. When you plant saved seed, you are often literally sowing the seeds for later disease spread in your crop. Instead, plant potato seed produced by a certified grower who tests for PVY.

Plant disease can seem unstoppable, something that blows through with no warning and leaves you with no recourse. Our work suggests that the problem might be perspective. This study demonstrates that, at least in the PVY system, a wider view can offer clues for how to contain the disease. It shows that even for insect-borne plant viruses, the map matters.

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