Insects and That
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Insects and That

SWD: the tiny fly casting a large shadow on our fruit supply

Fruit fly on damaged raspberry
Photo: Hannah Burrack, North Carolina State University, Bugwood.org

When a pest gets an abbreviation, you know it’s serious. Such is the case with Drosophila suzukii, spotted-wing drosophila, or more commonly-known, simply SWD.

It’s a name extremely familiar to growers of soft fruit, but no so much by the general population — aside from the time it enjoyed a tangential route into semi-public consciousness via a series of TikTok videos. These viral favourites saw people dunking strawberries into salt water, expressing surprise and disgust at the discovery that plants might have close relationships with insects.

From a human perspective, such closeness can have both positive and negative effects. Pollination, pest control: good; eating all the fruit: bad. SWD sits firmly in the latter camp.

Too fruity by far

This invasive species of fruit fly, originally from Asia, has widened its range at phenomenal speed in recent years, and is now well established in Europe and the Americas. It won’t stop there; it has recently been confirmed in sub-Saharan Africa.

While there have always been fruit flies in these territories, this one is different: its ovipositor is serrated, so whereas other, similar flies can only lay eggs in rotting or damaged fruit, SWD can do so in the freshest produce going. It is considered in some quarters to be the most severe threat to UK fruit growing. SWD can render as much as 80% of a crop worthless, while there are anecdotal reports of soft fruit growers so affected by this ferocious fly that they choose to switch to different produce.

SWD’s close relative Drosophila melanogaster is famous for more positive reasons — as a model species used in research into the genetic origins of, and potential treatments for, numerous serious human diseases. Outside labs, though, D. melanogaster isn’t always such a help. Its young are raised on damaged fruit and it is associated with sour rot, a serious disease affecting berry bushes and vines. What’s more, it appears that SWD, the bad influence relative, allows for D. melanogaster infestation, and double trouble for growers.

Spotted wing drosophila ovipositor
The ovipositor geared to puncture your raspberries. Photo: Martin Hauser

Damage limitation or bust

All in all, the global movement of SWD has proved a total success story from the fly’s point of view, and a mounting disaster from that of the soft fruit industry.

In the UK, we love our soft fruit, from pick-your-own strawberries to grabbed-from-the-supermarket blueberries. There’s outside demand for what we grow here, too (or at least there was pre-Brexit) — exports of soft fruit are big business. It’s easy to assume that the abundance of familiar foods is going to last in perpetuity, but the reality is that we don’t tend to factor in threats out in the field. SWD has put soft fruit production, if not quite at crisis point, certainly at a crossroads.

The fight is on to outgun SWD before its upper hand becomes unsurmountable. This struggle is about far more than dark skies over our summer snacks and desserts, though. The ecological knock-on effects of taking on this well-adapted pest could be felt for decades to come.

Trapping and spraying holds back the tide

Currently, there are only a couple of options to keep SWD in check where it is found.

One is lots and lots of trapping — basically, the art of killing using what your enemy loves. At the moment, the most commonly-used trap attractants are liquids derived from the process of wine-making, though various species of yeast also show potential for fatal irresistibility. Pheromone attractants have also been introduced as a new part of the trap-and-kill arsenal, mainly as a means of catching SWD during the off-season, when they tend to retreat to hedgerows and wooded areas.

At present, mass trapping is proving its worth in holding back the tide — but it’s not likely to prove enough over the longer-term.

The other option is pesticides — a bit of a sore issue. Perhaps the biggest problem in dealing with SWD is its renowned knack of becoming resistant to killer chemical compounds. This means that this line of warfare against SWD will eventually prove a fairly futile affair. New and stronger pesticides will only work so long, before the fly’s fast turnaround of generations will lead to the mutations that give it resistance. All the while, this approach may well kill off the only native natural enemies they have, as well as causing wider ecosystem damage that could be very hard to reverse.

All this means that more creative solutions are required — and with time of the essence, researchers are frantically working to come up with new options to control this tiny, highly-adapted pest.

Fruit fly caught in spider web
Photo: Katja Schulz https://www.flickr.com/photos/treegrow/28329504289

Knowing your enemy’s enemy

The search for effective enemies for SWD is one big part of this, and a number of candidates are being considered as methods of biological control. Importing natural enemies from their native territories — an approach known as classical biocontrol — is an option, but far from an immediate one, considering legislative hoops that need jumping through. Such checks and assurances have some justification, of course: nobody wants a control option that is not only a natural enemy of SWD, but plenty of native species besides. That way ecosystem carnage lies.

Potential hurdles have not stopped a search for the perfect enemy — and tiny parasitoid wasps appear a promising avenue for exploration. Following a survey in China, it appears that that the parasitoid Ganaspis brasiliensis could well be an option due to its narrow range of potential hosts, while a similar wasp, Leptopilina japonica, has been observed in Europe.

Not all potential enemies come in the form of wasps looking to lay their eggs in larvae. There are a pretty large array of other contenders for the role of fruit fly controller, including predators such as earwigs and rove beetles, spiders and predatory bugs. To date, there’s a lack of definitive evidence that any one species is the outstanding candidate — but the combination of a few could add up to a lot. Yet another argument in favour of the preservation of suitable habitat for a broad range of arthropod life to thrive.

UK native species of Nematodes have also been tested in the lab, with promising results — although the potential issue there might be making sure the microscopic roundworms suspended in water are applied in such a way that they make it to the places where they can do damage to developing SWD.

To push, pull, sterilise or outcompete?

Slightly more conceptual ideas than finding something that kills SWD may prove useful in reigning it in. A technique known as push-pull is currently being honed by researchers, involving the use of repellents and attractants in tandem, ‘pushing’ SWD away from crops and ‘pulling’ them to traps and their doom. Studies to date have demonstrated its promise, but it remains some way away from perfected. The discovery of new, effective deterrents will surely help.

Faced with the present obstacles to putting the brakes on the marauding progress of SWD, there are hopes that a new application of an approach to insect population control that’s been around for some time might do the trick.

Sterile insect technique (SIT) does what it suggests it might, in the sense that it hinges on insects with no ability to reproduce. The target species is reared in a lab and sterilised by exposure to radiation, then released into affected habitats to mate in a way that produces no young. The idea is that this brings about a population collapse, and so it has proved on a number of occasions, including against two problematic pest fruit flies, Ceratitis capitata and Anastrepha ludens.

Now, a large trial of SIT against SWD is ongoing in Argentina — with presumably a lot of eyes on progress. It is set to move on to France, and round up in 2023.

The financial incentive to find a solution means what’s detailed here is just scratching the surface of the research that’s being funded with the aim of outmanoeuvring SWD. It’s even possible, for example, that our old friend D. melanogaster could be pitted in competition against SWD, to reduce the emergence of larvae from eggs.

A broad truth of pest control is there is rarely one outright ‘winner’, more it will be a case of doing what works best where you are, at a reasonable cost. It’s unlikely that SWD will be entirely eradicated from the territories it has invaded — though this perhaps is immaterial.

From a grower perspective, all that matters is that there are ways and means of keeping damage below what’s known as the ‘economic threshold’. For the layman, that simply means that it’s worth their while to keep supplying the fruit we love to our tables. Few would like to see a seismic ripple in the raspberry market, but insect invasions wait for no-one. Tick tock, tick tock.

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Gary Hartley

Gary Hartley

Writer of different things. Come for the insects, stay for the odd literary works, or vice versa. @garyfromleeds https://medium.com/insectsandthat