There is life after chlorpyrifos

Growers can achieve effective mealy-bug control with the correct use of existing tools. By Anna Mouton.

Chlorpyrifos was a mainstay for controlling mealy bugs and other sucking insects for nearly 50 years until safety concerns spelled its doom. “The first nail in its coffin was hit in 2010 when the European Union started imposing regulations,” said Dr Gideon van Zyl, crop-protection specialist at agricultural consultancy ProCrop.

In South Africa, the Department of Health initiated the phasing out of chlorpyrifos in 2020 and gazetted the deletion of chlorpyrifos maximum residue limits earlier this year. DALRRD recently confirmed that they will issue the appropriate bans soon.

“Rest in peace, chlorpyrifos,” quipped Van Zyl. “But the show must go on, so we need to plan what to do next.”

Know the enemy

The main mealy-bug species requiring control on pome and stone fruit are the obscure mealy bug (Pseudococcus viburni) and the long-tailed mealy bug (Pseudococcus longispinus). Obscure mealy bugs are the most common species in pome fruit.

Van Zyl explained that all life stages of the obscure mealy bug are present year-round on trees. “There will be adult females, eggs, first- to third-instar female crawlers, and flying adult males,” he said. “Depending on the climate and temperatures throughout the season, they can produce up to eight overlapping generations annually.”

Mealy bugs overwinter under bark or in crevices, where they continue breeding slowly. The mobile first- to third-instar crawlers start migrating from their overwintering sites when temperatures rise in spring.

“They move to newer parts of the plants, including into the fruit calyces,” said Van Zyl. “We need to hit them before they reach the fruit.”

Once the fourth-instar females settle down to feed, they develop a water-repellent waxy coating that protects them from spray applications. Adult females are also cryptic — they hide where pesticides can’t reach them.

“You must target control where you have the best chance of contact with the necessary pesticides or biologicals,” emphasised Van Zyl. “You should aim at the mobile stages.”

Monitoring and detection

“When we talk about monitoring and detection, the best thing you can do is know your orchard status,” said Van Zyl. “Make sure you’re monitoring the whole orchard throughout the season.”

He uses a straightforward system to rate orchards from zero to two after harvest. A zero rating means no mealy bugs were detected; one means mealy bugs were detected but didn’t cause losses or affect exports; and two means mealy bugs were widespread or affected exports.

“You can do this once per season,” he said. “You’ll manage each of those orchards differently — this is important because it’s going to get expensive without chlorpyrifos.”

During the season, monitoring focuses on spotting crawlers. “Make sure you start in at least week 39,” specified Van Zyl. “The best positioning of seasonal sprays is when those crawlers start moving.”

Monitoring entails the weekly visual inspection of 25 trees per two hectares. The trees must be spaced evenly throughout the orchard because mealy-bug infestations tend to be patchy. Scouts must check five fruit clusters, five shoots, two leaves, and preferred overwintering sites — old stems and crotches — on each tree.

Van Zyl suggests putting 15-centimetre-wide corrugated cardboard strips around potential overwintering sites. Female mealy bugs like to settle under these, making it easier to spot migrating crawlers. The bands are also convenient for assessing parasitoid activity.

Pheromone monitoring has great potential, but specific pheromones for the relevant mealy bugs are not currently available in South Africa.

Choose your weapons

“Chemical control is the basis of mealy-bug control in South Africa,” said Van Zyl. “Mealy bugs are difficult to control chemically because of their small size, cryptic nature, patchy distribution, and possible resistance to organophosphates.”

He highlighted prothiophos and methidathion as alternatives to chlorpyrifos for dormant applications. Unfortunately, both are more expensive, and availability may become limited as demand grows. Depending on the infection pressure and orchard status, one to three sprays are indicated.

“Another issue, specifically with methidathion, is that we don’t know what the risk of residue carryover is with late dormant applications,” cautioned Van Zyl.

For the growing season, acetamiprid and sulfoxaflor both work well if correctly implemented. According to Van Zyl, the key is hitting the vulnerable crawlers and not expecting effective chemical control of adult females.

“Application is critical,” he stressed. “You must adjust the forward speed and air volume to apply the active ingredient to the canopy only.”

He warned against over- and under-application, especially during the dormant season in high-density orchards. “Use the correct spray volume. Watch out for using the tree-row volume as a reference — check whether you’re putting a film on the tree.”

In low-risk scenarios, one acetamiprid application during weeks 47–50 may be sufficient, while in high-risk scenarios, two acetamiprid applications during weeks 47–50 can be followed by two sulfoxaflor applications from week 51 to harvest.

Expanding the arsenal

Van Zyl mentioned four actives currently registered in other crops, such as citrus that are likely to be registered for pome fruit in the near future.

“And the world of crop protection isn’t shrinking,” he added. “There are exciting new alternatives on the way, around five years into the future.”

Regarding biological control, three agents are registered in South Africa: Beauveria bassiana, Metarhizium anisopliae, and natural pyrethrum. “Unfortunately, we have little to no published data showing that these are in any way effective when used throughout the season,” said Van Zyl.

Biologicals have low residual activity, require multiple applications, are not always compatible with fungicides, and are relatively expensive when added to spray programmes without replacing existing sprays. However, growers wishing to employ biologicals should again target crawlers with regular applications, generally from week 47 onward.

Although parasitoids theoretically augment mealy-bug control, Van Zyl thinks the jury is still out on the efficacy of commercially available parasitoids.

In conclusion, chlorpyrifos’ fate was inevitable. The challenge now is to conduct the impact studies necessary to protect the remaining organophosphates.

“We still have good tools to control mealy bugs effectively,” concluded Van Zyl. “And there are more tools on the way. But good stewardship of the active ingredients we still have left is critical.”

Watch Van Zyl’s presentation on the Hortgro YouTube channel.