Monitoring and mitigation

How can growers best protect their fruit against spotted wing drosophila? By Anna Mouton.

The establishment of spotted wing drosophila in fruit-growing areas worldwide is associated with significant damage to susceptible crops. Berries and cherries have been especially hard hit, but losses aren’t limited to these fruits — most stone fruits are vulnerable to attack.

“The outcome in South Africa is uncertain,” says Dr Minette Karsten, crop-protection programme manager at Hortgro Science. “We have better information for berries than stone fruit.”

Waiting for damage to escalate is not an option. Instead, Karsten and colleagues have compiled several information resources, including a fact sheet and best-practice guidelines, to prepare growers for spotted wing drosophila.

Karsten and Dr Gulu Bekker, applied entomology researcher at Stellenbosch University and technical consultant at BerriesZA, spoke to Fresh Quarterly about monitoring, mitigation, and why integrated pest management remains growers’ best strategy.

Keeping track of the enemy

Traps

Winning against spotted wing drosophila requires knowing where it is and what it’s up to because populations can explode under favourable conditions. Flies can complete their life cycles in 10 days, and each female lays about 400 eggs. Growers can’t afford to delay in detecting this pest.

Trapping is the best way to monitor adult flies. Options include yellow McPhail traps with the Insect Science SWD Pherolure, Scentry traps with the Scentry SWD lure, or yellow McPhail or homemade traps with a homemade lure. For detailed information on spotted wing drosophila traps, visit the Hortgro YouTube channel.

Place one trap every 30 metres (the attractive range of the lures is only about 30 metres) around the orchard perimeter to provide an early warning of flies invading the crop. Once the pest is detected, traps can be put 10 metres inside the orchard, again 30 metres apart.

Spotted wing drosophila likes shaded, humid spots, so hang traps inside tree canopies, hedges or other dense vegetation.

The females lay their eggs in ripening fruit. Start trapping as the fruit begins changing colour and before sugar levels rise. According to extension pamphlets from Oregon State University, sugar levels below 10 °Brix usually indicate that fruit is too firm for egg-laying.

Service traps every 7–14 days, keeping in mind that current spotted wing drosophila lures are non-specific, and traps rapidly accumulate bycatch.

Fruit assessments

Adult trap catches aren’t strongly correlated with population numbers or fruit infestation. One reason is that ripening fruit competes with traps for female flies’ attention. Therefore, fruit assessments are critical.

“I visited a large blueberry grower in the United States,” reports Bekker. “They sample berries to check for eggs. They use egg-laying as a metric of when to initiate management and whether what they’re doing is working.”

Many drosophilids will lay eggs in damaged or decaying fruit but only spotted wing drosophila lays eggs in undamaged fruit. So, sample undamaged fruit from the tree, not the ground, for assessment.

Sample weekly from ripening (start at the same time as hanging traps) until harvest. As the flies launch their attacks from alternative hosts surrounding the orchard, fruit should be sampled from trees about 10 metres into the block.

Besides inspecting the fruit for eggs, which are difficult to spot, growers can look for larvae by using the flotation method demonstrated on the Hortgro YouTube channel. The flotation method can also be performed with plain water instead of a sugar or salt solution.

Monitoring isn’t only to inform spraying decisions. “For a commercial farmer, you do damage assessments before harvest to determine the risk for that block,” says Bekker. By monitoring, growers can proactively market their fruit appropriately rather than reactively dealing with unexpected quality problems.

The grower’s arsenal

Pesticides

“Spray programmes will likely always be a part of control,” says Karsten. “But crop protection can’t come exclusively out of a bottle. We need to do more than spray.”

Two products have received emergency registrations for spray applications in fruit, of which only one, containing spinosad, is registered for stone fruit. Bait sprays currently used for Medflies and other tephritids are not effective for spotted wing drosophila. The details of product registrations are available in the Hortgro best-practice guidelines.

Results from Oregon show that the proportion of adult flies in the population is highest in winter and early spring, but adults only represent 5%–15% of the population during the growing season. Research has demonstrated that spinosad is effective against all life stages of spotted wing drosophila, even those inside the fruit.

The flies’ population dynamics have yet to be determined for South Africa (read about a new project on spotted wing drosophila population dynamics here). For now, growers should spray as soon as they detect spotted wing drosophila.

Note that spotted wing drosophila likes to lurk inside canopies. Open canopies aid control by discouraging the flies and facilitating spray penetration.

Ideally, spray programmes must incorporate more than one active ingredient to slow the development of resistance. As more products obtain registration, growers should rotate products with different modes of action.

Besides efficacy and resistance prevention, spray programmes are shaped by withholding periods, maximum residue levels, and market requirements.

“To sell fruit in Europe, our growers must meet strict regulations,” says Karsten. “So, we can’t follow the same spray programmes as, for example, the United States. We have to pay attention to all the other management aspects.”

Trapping

Instead of (or in addition to) full-cover sprays, growers can also lure pests to their death using mass trapping or attract-and-kill. Mass trapping involves traps similar to those deployed for monitoring, with flies entering the traps dying through drowning or poisoning.

“Mass trapping is done in other parts of the world,” says Karsten, “but there isn’t data supporting its effectiveness locally. It will contribute to control — it’s not a stand-alone strategy.”

She adds that mass trapping is labour-intensive, requiring high trap densities (200–300 per hectare) and weekly trap servicing.

“It’s only as effective as the work you put in,” she cautions. “And, in the case of spotted wing drosophila, the lures are non-specific and have a small attractive radius, also limiting effectiveness.”

One of Bekker’s clients used mass trapping as part of their integrated pest management programme for berries in Portugal. The bait was with a mixture of apple cider vinegar, normal vinegar, sugar, yeast, and water. Traps were placed every 10 metres in every row. However, adopting attract-and-kill has enabled the farm to reduce mass trapping.

Attract-and-kill products incorporate a lure and a pesticide. Insects visiting the dispenser are exposed through contact or ingestion and die a little later. Compared to mass trapping, attract-and-kill products are more user-friendly for growers.

One attract-and-kill product has received emergency registration and is expected to be commercially available in South Africa by March 2025. The Hortgro best-practice guidelines provide details about the product.

As for spray applications, mass trapping or attract-and-kill must be initiated as soon as spotted wing drosophila is detected.

Sanitation

Orchard sanitation is the crop-protection equivalent of eating veggies and exercising more. While universally recognised as important, it is far from universally adopted. Yet, orchard sanitation is an effective, chemical-free intervention to reduce pests and diseases.

“From a commercial farming perspective, stripping the fruit and not leaving any fruit behind, whether on the plants or the ground, is absolutely crucial for controlling spotted wing drosophila in berries,” says Bekker. “It will be the same for stone fruit.”

Although spotted wing drosophila females lay eggs in undamaged fruit, the larvae continue developing in the damaged fruit when it decays or drops. Damaged and decayed fruit is also a food source for adult flies.

Orchard sanitation must be implemented whenever fruit falls, including after storms or thinning, as well as during and after harvest.

While wine grapes are not a preferred host, extension pamphlets from Oregon State University assert that egg-laying increases in cracked, diseased, or damaged berries, including those affected by rain or hail. This is probably true for other fruits and should be considered when managing alternative hosts.

Sanitation doesn’t end when fruit leaves the orchard. Exposed fruit shouldn’t be left anywhere on the farm or at pack houses.. To dispose of fruit on site, either bury it at least 30 cm deep or bag it and leave it in the sun for two weeks.

Alternative hosts

Flexibility with regards to hosts is one of spotted wing drosophila’s superpowers. Studies from all over the world have demonstrated the importance of non-crop hosts to the flies’ survival, so the management of alternative hosts is indispensable for control.

Female spotted wing drosophila prefer sweet, thin-skinned fruit, but they will lay eggs in almost any fruit — there are even instances of the flies completing their life cycle on flowers. Fruiting plants near crops support flies year-round, boost their populations during the crops’ growing season, and increase infestation pressure.

Besides acting as a food source, non-crop plants offer shelter. Fly numbers are frequently greater in non-crop vegetation than in orchards or vineyards. This may be partly driven by the flies’ sensitivity to dry heat.

Where possible, remove alternative hosts. If not, remove and destroy their fruit. However, spotted wing drosophila may still exploit dense vegetation as shelter, even if there are no fruits.

Common alternative hosts

There hasn’t been much local research on spotted wing drosophila hosts, but extensive host lists from other parts of the world offer guidance. The following list contains plants that are known hosts elsewhere and common in South Africa. Many are also invasive here.

• Nearly any commercially cultivated fruit type (including tomatoes) is a potential host, but not all are equally attractive. Apples and pears are only attacked if damaged.
• Pineapple guava (Acca sellowiana or Feijoa sellowiana) and cherry guava (Psidium cattleianum)
• Many plants in the rose family are hosts, including roses, loquats (Eriobotrya japonica), cotoneasters, pyracanthas, and brambles (raspberries and blackberries).
• Mulberries and domestic figs are known hosts in the fig family, but the many indigenous South African figs could also be hosts.
• Oleaster (Elaeagnus x ebbingei)
• Honeysuckle (Lonicera japonica)
• Virginia creeper (Parthenocissus quinquefolia)
• European elder (Sambucus nigra)
• Black or common nightshade (Solanum nigrum)
• The North American hackberry (Celtis occidentalis) is a host. The exotic Chinese hackberry ( sinensis) and the native white stinkwood (C. africa) are widely grown in South Africa and may also be hosts.
• Some pokeweeds (Phytolacca species) are known hosts. The invasive belhambra ( dioica) fruits profusely in South Africa and may also be a host.

Growers can use the fruit flotation method described for larval monitoring to check whether a plant is a host. Collect fruit from the plant, not the ground, and contact an entomologist to confirm larval identifications.

Other cultural methods

“Keep your trees as open as possible,” says Bekker. “You don’t want excessive vegetative growth that creates a protected, humid environment for the flies.” He has seen reductions in berry damage by combining canopy management and orchard sanitation.

Spotted wing drosophila females seek dark places to lay eggs. Increased light penetration due to reflective mulch and canopy management discourages egg-laying.

Anything that makes the orchard hotter and drier, such as drip irrigation, will help deter spotted wing drosophila and reduce pupal survival. Pupae are quickly killed by dehydration and heat. Research has shown that up to 90% of the larvae pupate in the soil, presenting an intervention opportunity.

Barriers, including plastic mulch, weed fabric, or even matted plants, can prevent larvae from entering the soil, so they die.

Exclusion nets are effective and widely applied in some parts of the world, such as cherry production in Northern Italy. However, the microclimate created by the fine (less than 1 mm) mesh can be problematic, and the nets are expensive to install and maintain.

“We want to explore whether applying insecticides to the sides of existing net structures could prevent the influx of flies,” says Bekker. “We plan to test it this year.”

Both Bekker and Karsten emphasise that an integrated management approach is the only hope growers have when dealing with spotted wing drosophila. While recognising the significant challenges posed by this pest, they remain optimistic.

“We can draw on all the research that’s been done in Europe and the USA since 2008,” says Karsten. “We have the opportunity to build on that and hopefully avoid following the same path with respect to pesticide use.”

Bekker is confident our industry will break new ground with spotted wing drosophila control. “South Africans are tenacious,” he says. “I have confidence in our fruit industries.”