The wrong fruit fly, in the wrong place, at the wrong time: A tale of rot and ruin

I’ve written before about the problems caused by invasive species. I don’t know how they compare to other forms of habitat destruction for which we are responsible, but I think it’s safe to say that they’re often underrated as a problem. It’s true that the damage caused by the earthworm invasion of North America doesn’t seem as big of a deal as clear-cutting or climate change, but as I’ve said before, wearing away the diversity of an ecosystem wears away at its resilience. It’s like pulling blocks out of a Jenga tower – the ecosystem may be able to retain its basic structure for a long time, but after a certain point, it will be unstable enough that it can collapse at the smallest touch.

And keep in mind that the undermining of our biosphere is happening from multiple directions at once.

It seems that an invasive species of fruit fly is working away at one of those Jenga blocks:

The invasive spotted wing drosophila (SWD), introduced from South-East Asia, is a well-known fruit crop pest. It lays its eggs by destroying the mechanical protection of the fruit’s skin, providing an entry point for further infestation. Egg deposition and inoculated microbes then accelerate decay, and as a result the fruit rots and becomes inedible. While this small fly is known to cause massive economic damage in agriculture, little is known about its ecological impact on more natural ecosystems such as forests.

A recent study by Swiss scientists from the Swiss Federal Institute for Forest, Snow and Landscape Research WSL and the Ökobüro Biotopia, published in the scientific journal NeoBiota, concluded that the SWD competes strongly with other fruit-eating species and that its presence could have far-reaching consequences for ecosystems.

The research team assessed the use of potential host plants at 64 sites in forests from mid-June to mid-October 2020 by checking a total of 12,000 fruits for SWD egg deposits. To determine if SWD attacks trigger fruit decay, they also recorded symptoms of fruit decay after egg deposition. In addition, they monitored the fruit fly (drosophilid) fauna in the area, assuming that the SWD would outnumber and possibly outcompete other fruit-eating insects.

The authors found egg deposits on the fruits of 31 of the 39 fruit-bearing forest plant species they studied, with 18 species showing an attack rate of more than 50%. Furthermore, more than 50% of the affected plant species showed severe symptoms of decay after egg deposition. The egg depositions may alter the attractiveness of fruits, because they change their chemical composition and visual cues, such as colour, shape and reflective patterns, which in turn might lead seed dispersers such as birds to consume less fruits.

Given the large number of infested fruits, significant ecological impacts can be expected. “Rapid decay of fruits attacked by the spotted wing drosophila results in a loss of fruit available for other species competing for this resource, and may disrupt seed-dispersal mutualisms due to reduced consumption of fruit by dispersers such as birds,” says Prof. Martin M. Gossner, entomologist at the WSL. “If the fly reproduces in large numbers, both seed dispersers and plants could suffer.”

This, of course, is on top of the fact that the invasive species is outcompeting, and crowding out those fruit fly species that aren’t able to get an early start on a fruit by penetrating its outer skin. On a long enough timeline, the crises caused by invasive species would probably resolve themselves, if that was the only problem facing those ecosystems.

If only…

When I say that we need to make ecosystem management a priority, this is part of what I’m talking about. We don’t need to return to some imagined pre-industrial “perfect state”, but we do need to have an understanding of what is happening in our ecosystems. In case it hasn’t been clear, this could create many millions of jobs all over the world, and unlike so many jobs that exist today, these ones would be focused on actively making the world a better place for those that come after us. Fewer hedge fund managers, and more hedgerow managers!

This is also another reason why we should move as much of our food production indoors as possible. It may be that with a population as large as ours, there’s still value in monoculture farming for the sake of efficiency. The problem is that that’s an ecologically devastating practice. Completely aside from the damage done to the soil, pollution and runoff, and the land cleared, big farms that grow one crop pull in pests and diseases that can then spread to other crops. They act as an endlessly renewing fountain of pestilence that surrounding ecosystems are forced to absorb. Doing that in a closed system, indoors, would not just protect the crops from the weather and from pests, it would also protect the ecosystem around the farm from being inundated with fertilizers, pesticides, pests, and pathogens.

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