Pest and disease control under shade netting

Over the years, South Africa has established a reputation for supplying fruit to local and international markets. Farmers have adjusted their practices to comply with strict global phytosanitary regulations amid challenges like adverse weather conditions, water scarcity, and load-shedding, with an ongoing emphasis on year-round pest and disease control.

To reduce the risks associated with weather conditions like hail, sunburn and wind damage in certain production areas, the area under nets in South Africa is expanding rapidly.

Integrated pest management (IPM) programmes, which use a mix of chemical, biological, and cultural controls alongside habitat manipulation, have been widely adopted in the fruit industry for long-term pest management and damage prevention.

By installing nets, farmers are creating a different microclimate for their orchards, which may require an adaptation of their pest and disease control management practices, says André Labuschagne, campaign lead for potatoes and subtropical fruit at Syngenta.

Failure to adapt, he warns, can result in a loss of all the benefits associated with crops under protection.

Open orchards versus netted areas

Production in open fields allows for full sunlight exposure that can help reduce numbers of certain pests, as some insects are less active or unable to survive in intense sunlight. Open fields also allow natural predators like birds, ladybugs, spiders, and parasitoids, such as wasps, to control pests. These beneficial organisms thrive in open environments.

However, there are also some challenges associated with production in open orchards and fields. Some crops are more susceptible to pests in open fields due to lack of protection. Although weeds and other plants in the vicinity can harbour beneficial insects, they also serve as hosts for pests and compete with crops for resources.

Applying pesticides evenly across large open areas can also be challenging.
One of the immediate advantages of production under netting is the barrier it creates against pests and birds, Labuschagne points out.

“Shade nets can act as physical barriers that prevent pests from accessing plants, and therefore the moths of certain Lepidoptera species that feed on the crops can be kept away from the plants. Insect pressure should be lower, depending on the type of netting used, as it would be more difficult for them to enter the area, especially in a closed netting facility.”

Diseases such as leaf spot and citrus canker may also be prevented with the use of shade netting. Labuschagne says that pest management is usually more effective under cover, as products are applied in a controlled, windless area.

Although shade nets reduce temperature and thereby create a less favourable environment for certain pests, the altered microclimate may favour the development of certain weeds, bacteria and fungi.

When it comes to pest control, the choice between open fields and shade netting is thus not that easy and depends on various factors. Open fields rely more on natural processes, while shade netting provides targeted protection.

“It’s important to consider your crop type, local climate, and pest pressure when deciding which method to use,” says Andries Duvenhage, marketing campaign manager at Syngenta South Africa.

Research insights

In a research paper by Marsberg et al, the authors state that protective netting increases the levels of certain key pests, most notably Aonidiella aurantii (red scale) and Planococcus citri (citrus mealybug), and possibly also Thaumatotibia leucotreta (false codling moth, or FCM), but decreases the levels of other pests, such as Scirtothrips aurantii (South African citrus thrips) and Ceratitis capitata (Mediterranean fruit fly).

The paper states: “Although levels of both red scale and citrus mealybug were significantly higher under nets than in open orchards during two successive seasons, infestation levels were nonetheless still low and could not be considered as problematic.”

According to Dr Tamryn Marsberg, a research entomologist specialising in IPM at Citrus Research International in Gqeberha, Eastern Cape, further studies are required with higher levels of these pests to observe the trends and efficacy of their biocontrol complexes under nets, relative to open orchards.

In a particular study focused on the effect that nets have on key citrus pests in the Eastern Cape, orchards under nets and open orchards of similar cultivars, ages and management practices were monitored at several sites over two seasons for pest infestation and damage.

Weekly monitoring was conducted for FCM infestation, while other pests were monitored either monthly or once a season.

According to Marsberg, FCM infestation was higher in orchards under nets during the first season. She notes that no FCM infestation was recorded in either of the orchard types during the second season (2019).

“This was mainly due to generally lower than usual pest prevalence as well as the dramatically improved area-wide management of FCM. We used pheromone traps to monitor FCM males, including sterile moths used in a sterile insect technique programme.

“Although higher catches of wild FCM moths were recorded in orchards under nets, so too were there higher numbers of sterile moths and a higher ratio of sterile-to-wild moths, indicating the potential for better pest suppression for orchards under nets,” says Marsberg.

Variable results for each species

The research team monitored various other key pests and recorded variable results for each species. The main focus was to determine whether nets could reduce FCM levels and whether temporary eradication of this pest was possible.

“Pests that were elevated under nets include citrus mealybug and red scale. Pests that seemed to be lower under nets were fruit fly and South African citrus thrips. Nets had no effect on Empoasca distinguenda (leaf hopper), Penthimiola bella (citrus leaf hopper) and Eriophyes sheldoni (citrus bud mite).

These differences in pest levels between netted and open orchards trigger an important debate on whether nets are beneficial for or detrimental to the successful implementation of an IPM programme,” she adds.

“Contrary to our expectation, FCM infestation was recorded to be higher in orchards under nets than in open orchards, particularly in the first season of monitoring. The higher FCM infestation under nets during that particular season was likely due to the nets having been erected over mature orchards that had experienced high FCM population levels during the previous season, before netting, as reported to us by the grower.

“Another possibility is that the nets created a more favourable environment, preventing moths from dispersing and protecting them from predators. Even though there was no significant difference in temperature and humidity data, thus small abiotic differences, the effects on FCM could have been biotic,” explains Marsberg.

“Various studies have reported an increase in population numbers of small insects under nets for apple and pear crops. These insects include rosy apple aphid, woolly apple aphid, summer fruit tortrix, and apple aphid. We can only speculate that the increase in the population numbers of these species could be due to the increased temperature and humidity under nets, albeit a statistically insignificant increase, and the exclusion of natural enemies.”

Marsberg says the research team also found that nets seemed to effectively reduce Mediterranean fruit fly levels, as no flies were captured under nets. This is in accordance with other studies where netting was found to be highly effective against Asian fruit fly for stone fruit (Lloyd et al, 2005).

Marsberg cites another study, Candian et al, 2020, which found that nets successfully excluded spotted wing drosophila.

“Candian noted that this may not necessarily be due to the physical barrier created by the net, but because of photo selection, causing optical disruption and preventing the flies from finding the host crop,” she points out.

According to Marsberg, in their research they found that nets also seemed to have a positive effect on South African citrus thrips management.

“In two successive seasons, fruit damage caused by citrus thrips was significantly lower in orchards under nets. This could have been due to the higher incidence of the predatory mite Euseius addoensis under nets, and possibly also the greater abundance of spiders under nets.

It would, however, be necessary to monitor both pest and natural enemy levels at frequent intervals throughout the season in order to establish whether this relationship was indeed causing reduced damage under nets.”

According to the findings of the study, other natural enemies that were shown to occur in higher numbers under nets were the two spider species, Badumna longinqua (grey house spider), and Chresiona invalida (chresiona mesh-web spider).

Research done by Dr Ansie Dippenaar, previous Agricultural Writers SA national agriculturist of the year, showed that spiders could be incorporated into the natural enemy complex as general predators.

Although spiders may not control major outbreaks of pests, they can regulate low-density pest populations, potentially keeping them below economic threshold levels (Dippenaar et al, 2013).

Marsberg says while it might seem that temperature and humidity do not differ significantly under nets, relative to open orchards, they could still be biologically significant as the differences may lead to more generation cycles and a rapid build-up of insect species under nets.

Nadine Botha, technical field service representative at Insectec in Limpopo, points out that they found a higher prevalence of mealybug on citrus produced under netting.

“We generally release a higher dose of control insects under netting. We found that the mealybug is reproducing quicker and therefore we not only advise a higher doses, but also shorter intervals of releases,” says Botha.

Importance of timing and scouting

“The timeous release of control agents is essential. Different control agents and methods, as well as seasons and crops, require different timing. You want to give the control agents the best chance of accomplishing their goal, and therefore it is essential to determine the best time of release. The only way to do that is by regular scouting,” Botha points out.

Timing is also critical when using chemical products, Labuschagne emphasises.

“Irrespective of applying chemical products on crops in open field conditions or under shade netting, timing of application will be crucial. This is not only for best control of pests or diseases, but also with withholding periods in mind. Timing of application is especially important for farmers who produce for the export market and have to adhere to certain export protocols,” says Labuschagne.

The Role of pruning

Pruning trees, especially under shade netting, plays a vital role in the overall well-being and longevity of an orchard, says Jan van Vuuren, agronomic solution manager – horticulture, at Bayer Africa Cluster. Pruning also facilitates better penetration of sprayed chemical products.

“With production under shade netting, pruning is particularly important. By selectively removing specific branches and foliage, pruning can also serve as a preventive measure against disease and pest infestation. The removal of dead or diseased branches, potential breeding areas for pests and disease, are eliminated.

“Additionally, pruning promotes better light penetration and airflow, creating an environment less favourable for the development and spread of fungal diseases,” Van Vuuren explains.

Source: Marsberg T, et al (2024). ‘The effect of permanent protective netting on insect pest prevalence in citrus orchards in South Africa’. African Entomology.