Data-driven crop protection

Better decisions from diverse data

The late Dr Ken Pringle was the father of integrated pest management and monitoring in South African pome and stone fruit. He classified pests into sporadic, perennial, and chronic pests.

Sporadic pests are only problematic in some seasons. Perennial pests are problematic every year, but at different times during the season. Chronic pests are naturally always present in sufficient numbers to cause losses and require on-going control.

By factoring in different data sources, growers can choose the most appropriate management interventions for sporadic and perennial pests and assess whether their control strategies for chronic pests are working.

The six main data sources available to growers are orchard history, scouting results, trap catches, weather forecasts, and pre- and postharvest damage assessments. Although the rest of this article will focus on scouting, the value of the others shouldn’t be underestimated.

For example, Ferreira describes considering the orchard’s history and packhouse feedback from one season to highlight problems needing special attention in the following season. He also takes preharvest damage assessments into account.

Dr Minette Karsten, Crop Protection Programme Manager at Hortgro Science, agrees that growers should draw on multiple data sources when making crop-protection decisions. “We know that trap catches and damage assessments aren’t the same – damage assessments give you much more information,” she says. “Damage assessments and scouting are labour-intensive. However, you have to measure something to manage it.”

Scouting basics

When

Scouting involves going into the orchard in search of pests. Ferreira recommends weekly scouting during the growing season, starting at green tip and continuing until harvest. According to Van Son, irregular scouting is one of the most common sources of errors that lead to inappropriate or ineffective control. “It’s important to understand the pest dynamics so you can time your sprays correctly or decide whether a spray is even necessary,” he says.

For example, a pest might be found during scouting in one week, and its population may be higher when the scout returns three weeks later, prompting the grower to spray. Meanwhile, the pest population peaked in the second week, and natural predators have been reducing pest numbers steadily ever since. Spraying at this point will likely kill those predators, allowing the pests to rebound.

If growers are uncertain whether pest numbers are increasing or decreasing, Van Son suggests follow-up monitoring after three to four days rather than reaching for pesticides. Regular scouting is also essential for selecting the appropriate biocontrol or chemical application. For example, certain spider mite predators are only effective when spider mite populations are high, and some pesticides only affect actively growing and moulting spider mites.

How

The system developed by Pringle remains the foundation of present-day scouting. Pringle discussed monitoring in a 2019 interview with Fresh Quarterly, the Hortgro Science technical magazine – available online.

In addition to monitoring a representative sample of trees, Ferreira recommends including trees in the outermost rows. “Insects come from the outside,” he explains. “So, our preharvest assessment is a perimeter assessment where we visually check for damage.”

The outermost trees are also usually dustier and may therefore be the first to suffer spider mite infestation.

For sporadic sucking insects, such as antestia bugs, Ferreira advises scouting during flowering to detect nymphs.

Besides checking leaves for woolly apple aphids, Ferreira examines the soil surface for crusts that form in heavy infestations.

If weevils are a problem, he suggests placing cardboard bands around trunks. The weevils like to congregate under the cardboard where scouts can easily find them.

Scouts must be able to identify both predators and pests. Firstly, because spraying some pests is unnecessary when sufficient predators are present. Secondly, because mistaking predators for pests could lead to growers spraying the predators, thereby simultaneously shooting themselves in the foot.

Scouts must be able to identify both predators and pests.

Common errors

“Failing to monitor is the biggest mistake,” says Ferreira. “The second is the wrong identification of insects and damage. It’s difficult to identify damage when the insect isn’t present – it requires experience.”

Training in pest identification is vital. Several service providers, including the Koue Bokkeveld Opleidingsentrum, offer this service. “It’s not only the monitors that require regular training and support,” comments Matthew Addison, previous Crop Protection Programme Manager at Hortgro Science. “The farm management does as well, to allow for better decision-making.”

When in doubt, scouts should collect samples or take pictures to obtain an accurate identification. If their managers or technical advisers can’t assist, samples can be submitted to a diagnostic service such as the Stellenbosch University Plant Disease Clinic, which offers both disease and pest identification services.

“We’ve seen instances where farmers see thrips and they think these are citrus thrips that are going to cause damage,” says Van Son, “but then it’s something that isn’t a problem.” He has also encountered growers panicking when they spot heavy spider mite infestations on cover crops or native plants near fruit trees. “The mites are destructive to those plants, and it looks terrifying to have webbing all over the plants among your trees,” he says, “but those mites won’t necessarily move onto your trees.”

As mentioned, spraying for a pest that isn’t present can precipitate out-breaks of other pests by killing their predators or altering their biology. For example, sub-lethal doses of several pesticides can stimulate increased egg laying in spider mites.

Van Son reckons that not knowing where to scout is another common error. “Certain pests occupy certain parts of the plant. For example, mealy bugs like to hide, so we check the calyx of the fruit,” he says. “A targeted approach can narrow down how much time you’re spending and how effectively you find pests.”

“A targeted approach can narrow down how much time you’re spending and how effectively you find pests.”

Multipurpose monitoring

Addison is excited about the potential for technology to enable better monitoring. “I seriously believe we need higher resolution, area-wide monitoring,” he says. “With electronic data capture and computing, collecting representative data is no longer a hindrance.”

Addison would like all monitoring data to be gathered in a single repository, where it can be analysed with machine learning to recognise population shifts, predict hotspots, and facilitate area-wide management of phytosanitary and invasive pests.

Hugh Campbell, previous General Manager of Hortgro Science, makes a similar argument in a 2019 Fresh Quarterly article.

“Our markets are another reason to monitor,” says Ferreira. “Monitoring is becoming a GlobalG.A.P. requirement, and I think our markets will eventually want to see our monitoring data for phytosanitary reasons as well.”

Returning to the orchard level, Karsten shares Addison’s optimism about new technology, but she stresses the importance of implementing the basics of monitoring correctly.

“You have to ensure that you’re using the right methods and looking for the right things,” she says. “It’s not enough to merely put out a few fruit-fly traps. Drive-by monitoring – someone on a quad bike checking a trap – is easy. But it’s not the same as monitoring 25 trees in each block.”

“It’s not enough to merely put out a few fruit-fly traps.”

Basic scouting system for pome fruit Adapted from Brown L. and Pringle K.L. 2006. Monitoring system for pests on pome fruit. Pamphlet published by the Department of Conservation Ecology and Entomology at Stellenbosch University.

Method

1. Divide the orchard into blocks of two hectares.
2. Select and mark 25 evenly spaced trees per block.
3. Scout these trees every week.
4. Specifically check five shoot tips, five leaf axils, five fruit clusters, and 10 leaves from the outside and the inside of the tree.
5. All pests and diseases should be recorded, not only those for which a threshold is listed in Table 1.
6. Scouting is supplemented by trapping, and fruit damage assessments before thinning and harvest.