International research summarised

Spotted wing drosophila invaded Europe and North America more than 15 years ago. What have researchers since learned? By Anna Mouton.

The global spread of spotted wing drosophila has been alarming but impressive. These tiny insects are reported to fly up to 100 metres in a day — roughly equivalent to a human walking 55 km. But whereas it took Homo sapiens tens of thousands of years to overrun the planet, Drosophila suzukii has reached nearly every continent in less than a century.

Although modern transport helped D. suzukii reach new shores, it doesn’t explain the flies’ success once they arrived. This question is the topic of a recent genomic study published by Louise Camus of the French National Institute for Agriculture, Food, and Environment, and co-authors.

They examined genotype-environment associations, concluding that spotted wing drosophila’s genetic make-up was already close to the optimal genetic composition for the new environments. This means that the flies were already well-adapted to the areas they invaded.

The scientists cautioned that spotted wing drosophila could likely establish rapidly in areas where it’s still absent, including the Amazon basin in South America and the Great Lakes region in East Africa (it’s currently present in Kenya).

In South Africa, the flies’ genes seem to be a good match for coastal regions and the eastern part of the country. The hotter, drier interior is less suitable, which may explain why D. suzukii has not yet been trapped in the Northern Cape.

Researchers in several countries, including Brazil, Portugal, and the United States, have looked at population dynamics, as understanding these is critical for successful integrated pest management. In general, spotted wing drosophila seems to prefer non-crop habitats such as forests and wild blackberries, and the risk to crops increases in complex landscapes.

Female D. suzukii are attracted to ripening fruits such as berries and grapes. They enter cultivated areas to lay eggs but shelter in more humid, non-crop habitats. Non-crop plants are also important alternative hosts for the flies year-round.

Family feuds

Despite the relative surge in interest in spotted wing drosophila, Drosophila suzukii remains in the shade of its cousin, Drosophila melanogaster, which has been the darling of biologists for over a century and was the first animal sent to space.

Several studies have shown that D. melanogaster also outcompetes D. suzukii in other ways, raising interesting questions about how spotted wing drosophila will interact with the 70 or more other South African vinegar fly species.

A group of Italian researchers investigated the courtship behaviour of D. melanogaster and D. suzukii males toward D. suzukii females. When Flavia Cerasti and colleagues introduced males of either species to D. suzukii females, they found that males of both species spent about 30% of their time courting the females.

However, when they placed both species of males with D. suzukii females simultaneously, the D. melanogaster males spent about 20% of their time courting, and the D. suzukii males spent only 10% of their time courting.

Yongzhou Chen and co-workers in China found that rearing D. melanogaster and D. suzukii together significantly reduced the successful mating of D. suzukii. In this case, it was because 95% of the D. suzukii males courted D. melanogaster females.

In these experiments, the D. melanogaster males didn’t court D. suzukii females when there were D. melanogaster females present. For the most part, D. melanogaster females also didn’t mate with D. suzukii males.

The rivalry between the species doesn’t end with courting. Egg-laying by D. suzukii is reduced in the presence of D. melanogaster. Charles Kwadha and colleagues in Sweden inhibited D. suzukii egg-laying by treating blueberries with synthetic pheromones from D. melanogaster females. This suggests that D. melanogaster pheromones could have a place in D. suzukii control.

Furthermore, East Malling researcher Trisna Tungadi and co-authors report that the presence of other flies reduces the survival of D. suzukii larvae, and D. suzukii avoids laying eggs in fruit containing larvae of different flies. The scientists tried but failed to identify the chemical signal deterring the D. suzukii females.

Unfortunately, not all flies share D. melanogaster’s ability to subdue D. suzukii. Brazilian entomologists assessed the interaction between D. suzukii and the African fig fly, Zaprionus indianus, both invasive in Brazil.

Larine de Paiva Mendonça and co-workers concluded that Z. indianus didn’t significantly affect the reproduction of D. suzukii, but Z. indianus seemed to produce more offspring in the presence of D. suzukii.

Expanding the arsenal

The control of spotted wing drosophila, especially in North America, has leaned heavily on synthetic chemicals, so the emergence of insecticide resistance is not unexpected. A team of United States researchers recently described the molecular mechanisms for pyrethroid and spinosad resistance in D. suzukii.

In some cases, resistance occurred due to increased expression of metabolic genes that allow the flies to detoxify the insecticides. Christine Tabuloc and colleagues suggest that adding metabolic enzyme inhibitors to insecticides can help overcome this type of resistance.

In other cases, resistance was caused by cuticular changes that reduced insecticide penetration, which could potentially lead to cross-resistance.

The researchers argue that adopting integrated pest management would decrease the selection pressure for insecticide resistance. Their data suggest that insecticide resistance is costly to D. suzukii, so resistance in a population tends to diminish rapidly when insecticides are alternated with non-chemical control.

One avenue of research has been erythritol, a non-caloric artificial sweetener popular with food manufacturers and dieters. Erythritol and some of its derivatives can kill up to 100% of all D. suzukii life stages and reduced blueberry infestation by up to 93% in one field study. Unfortunately, as in dieting, the effectiveness of erythritol is diminished by the presence of other sugars, so more work is needed.

Many researchers have explored natural compounds to kill or repel D. suzukii, and several promising options exist. However, most of these natural compounds must be applied at much higher — about 1 000 x — concentrations than synthetic insecticides.

Formulation can also be challenging. For example, some essential oils repel D. suzukii, but oils are difficult to incorporate into water-based spray applications.

Natural compounds, like synthetic compounds, can have off-target effects, including phytotoxicity. A Hungarian study on salicylic acid in vineyards found that it reduced D. suzukii damage. The researchers, Erzsébet Németh and colleagues, speculated that this could have been due to increased berry skin thickness and looser bunches rather than directly influencing the flies.

Attract-and-kill

Both mass trapping and attract-and-kill of D. suzukii are under investigation, and both are only as effective as the lure.

Several researchers have reported on mass trapping using food-based baits. Female flies seeking to lay eggs are more attracted to volatile compounds released by ripening fruit than fermented substrates typically used in baits. This is thought to be one reason trap catches of female spotted wing drosophila tend to decline as the fruit ripens in vineyards.

While mass-trapping results have been mixed, attract-and-kill may be a more viable control option. United States researchers Arun Babu and co-workers compared different commercial attract-and-kill formulations based on spinosad and a proprietary lure consisting of a blend of attractants and sugar. The lure is also available without spinosad.

The team found a 2–8 times reduction in blueberry infestations and a 2–5 times reduction in raspberry infestations on some sites when the attract-and-kill product was combined with a standard insecticide spray application. However, the product was less effective on smaller sites near alternative habitats.

D. suzukii is known to be attracted to specific yeasts, including Metschnikowia pulcherrima and Hanseniaspora uvarum, although the flies’ enthusiasm for these yeasts is affected by season and other factors. Nonetheless, experiments have shown that yeast-based lures combined with insecticides can be effective in attract-and-kill formulations, and commercial products are under development.

Recent work by Gabrielle Tate of Oregon State University and colleagues showed that D. suzukii females like laying eggs in food-grade gum, even though their larvae cannot develop in this substrate. In some of their field trials, gum dispensers reduced damage by up to 78%.

Results were better when gum dispensers were combined with conventional spray programmes. However, the gum was less effective on windy sites, probably because the gum volatiles were blown away. Wind will likely affect other lures similarly.

This study did not test gum dispensers incorporating an insecticide, which could be another option for attract-and-kill.

Biologicals

There has been and continues to be much interest in biocontrol for spotted wing drosophila, not least because of insecticide resistance from both flies and human fruit consumers, albeit through different mechanisms. Although none of the existing biologicals offer stand-alone control, they can be a valuable component of integrated pest-management programmes.

Philip Fanning and colleagues from Michigan State University demonstrated that Chromobacterium subtsugae in rotation with spinosad could significantly reduce spotted wing drosophila in raspberries. Other bacteria have also been shown to kill larvae and adults.

Ricardo Toledo-Hernández and co-workers in Spain and Mexico evaluated 15 formulations of entomopathogenic fungi in a laboratory setting. The results demonstrated that Beauveria bassiana, Cordyceps javanica, and Metarhizium anisopliae were effective, killing 75%–95% of exposed D. suzukii adults.

However, various field trials have had mixed results, probably because environmental conditions were unfavourable for the fungi. Some researchers have suggested a fungal version of attract-and-kill, called lure-and-infect, that would maintain fungal viability until contact with a host.

Entomopathogenic nematodes, including Heterorhabditis bacteriophora, Steinernema carpocapsae, S. feltiae, and S. kraussei, can kill D. suzukii larvae in the laboratory but are less effective against pupae and in the field where larvae are protected inside fruit. Nematodes may work better if applied to soil to kill larvae that drop from fruit to pupate.

Parasitoids have been extensively studied, and an East Asian wasp, Ganaspis brasiliensis, has been widely released in the United States since 2022. Time will tell how successful this strategy is, but considering parasitism rates in Asia range from 18% to 40%, parasitoids will have to be deployed in combination with other pest-management strategies.

More management tools

As mentioned above, several studies have monitored spotted wing drosophila to understand population dynamics better. The findings highlighted some problems with using trap catches to monitor this pest, notably that catches don’t always correspond with population numbers or fruit infestation, partly because the flies’ response to lures changes seasonally in response to fruit availability and physiological shifts. Temperature and humidity can also affect trap catches.

Barriers are highly effective at preventing damage, provided they are correctly implemented. For example, Mary Rogers and colleagues from the University of Minnesota found that exclusion nets reduced raspberry infestation to 35%, compared with 60% in insecticide-treated open plots and 81% in untreated, open controls. Plastic tunnels reduced fruit infestation even further, to 2%.

Exclusion nets with mesh smaller than 1 mm are widely used for crop protection in some regions, such as cherry production in Northern Italy.

Even though D. suzukii females lay eggs in undamaged fruit, they feed on overripe and decayed fruit, and their larvae can develop in fallen fruit. Therefore, orchard sanitation is indispensable for controlling this pest (as it is for many others). Sealing berries in plastic bags for 32 hours has been shown to kill 99% of the larvae, provided bags were left outside in a sunny spot.

Hanna McIntosh from the University of Wisconsin and colleagues evaluated plastic mulches in raspberries. Black, white, and metallic plastic reduced fruit infestation by 40%–72% and killed 80%–100% of larvae and pupae on the mulch’s surface.

High temperatures explain larval and pupal deaths on the mulch. The mulch created hotter, drier, lower-canopy conditions, deterring adult flies. The mulch also increased the ultraviolet light in the lower canopy, reducing egg-laying. This aligns with other research showing that D. suzukii females prefer dark conditions for laying eggs.

In addition, some research suggests that D. suzukii larvae struggle to reach the soil when a weed mat is present. Unable to pupate, the larvae die on the surface.

Lastly, research on a sterile insect technique for spotted wing drosophila is ongoing. Technologies exist to produce sterile males, and field trials are underway, most recently in Oregon.

For a comprehensive review of D. suzukii integrated pest management, refer to Gabriella Tate and colleagues’ 2021 open-access paper.