The macadamia industry is plagued by numerous insects and mites, with one of the biggest pest problems for farmers being the notorious stinkbug.
Infestation of the bugs results in young nuts dropping off the trees and older nuts developing lesions, which can add up to several millions of rands in annual crop losses.
This means most farmers have to spray large quantities of insecticides to keep the stinkbugs under control, and in a season of excessive rainfall, spraying is a challenge.
“It is not the many millimetres of rain that hampered spraying in the first quarter of this year, it was the frequency of the rain restricting access to orchards” said Green Farms Nut Company’s Technical Manager Barry Christie.
Weather data showed that Nelspruit received 495mm of rain in February compared with 62mm in the same month in 2022. Levubu received 56mm in February 2022 and 241mm during February this year.
Christie said at their factory at Shaka’s Kraal on the KwaZulu-Natal North Coast, 207mm of rain was measured in February this year and 63mm during the same month last year.
But it was the fact that it rained almost every day in February in some regions that made it impossible for most farmers to get into the wet orchards with their sprayers, he added.
“Some farmers from the Nelspruit region reported having only four sunny days with favourable conditions for spraying in February. Wet soil kept us from entering the orchards, even on those precious warm days.”
Mieg Botha, who farms in the same region, said his inability to spray in the rainy weather did not have the same detrimental effect on his macadamia harvest as it did on some of the other farmers in the Nelspruit/White River region.
Botha is passionate about restoring the eco-balance in his orchards and only sprays chemicals when it is necessary.
“The days of indiscriminately spraying all of the blocks multiple times a year are over. Such actions do not promote sustainability in the orchards and have a negative impact on a farmer’s profitability,” he said.
About two years ago, Botha decided to partner with nature by using parasitic wasps to control the stinkbugs thriving in his orchards. Green Shield (Trissolcus basalis), also known as Samurai wasps, are tiny insects – the size of a sesame seed – but very strong flyers, which distribute effectively throughout an orchard.
One female Samurai wasp can parasitise up to 300 stinkbug eggs and they are a hardy species, able to survive for up to two weeks without food. The Samurai feeds on pollen and nectar and uses the stinkbug eggs to reproduce.
Botha identified African Wild Basil as a preferred feeding ground shared by the bees and these tiny wasps, and planted it wherever he could.
When the time for reproduction comes, the parasitic wasp targets the eggs of various stinkbug species. The adults lay their eggs inside the stinkbug eggs, leaving larvae that eat the developing bugs before they pupate. The stinkbug eggs change from a cream colour to black after being parasitised. The newly hatched Samurai wasp will then emerge from the stinkbug egg, to soon parasitise more stinkbug eggs.
Botha waits until the macadamias are in full bloom before releasing the wasps into his orchards. He gives the little Samurais the whole of September, October, and November to stabilise and parasitise stinkbug eggs. After the premature nut drop in November, he starts to scout intensively – 15 trees in a hectare.
The scouts will lay down shade nets beneath the trees, spray knock-down, and wait for 15 minutes before collecting the dead insects on the shade cloth to establish the severity of the stinkbug infestation in relation to the numbers of other harmless and beneficial insects.
Only when the number of stinkbugs crosses Botha’s economical threshold will he revert to chemical spraying – and then only on the blocks identified by the scouts.
“I believe this was why the higher than usual rainfall in February and March did not affect me as much. I only needed to spray infested blocks, and the time that was available on those scarce sunny days was ample for me to get done what was needed,” he said.
He believes in an integrated pest control programme, where insect pathogens should form a bigger part of the crop protection set-up, and chemical control should be used only if pest levels warrant control determined by scouting, not as a form of insurance.
Alternative control measures that were investigated on the farm include trap crops, planting of pest resistant cultivars, manipulation of tree size to improve the effectiveness of chemical spraying, and other mechanical and physical practices, such as the collection and manual destruction of stinkbug eggs.
“It is highly unlikely macadamias will ever be produced in South Africa without pesticides, but farmers must make more effort to reduce chemical use. It is not only the right thing to do from an environmental perspective, but it would also result in healthier and better yielding orchards.,” Botha said.