Thrips in macadamias

Oct 7, 2020

Thrips feed by rasping the outer layers of the epidermal cells of fruit, flowers and tender leaf flushes. The ensuing damage typically manifests as shallow scarification of the fruit, leaf and flower drop as well as malformed leaves.
In macadamias, damage manifests as dry flowers, “bronzed’ nuts as well as smaller leaves and leaf drop. Trials conducted in the Nelspruit region proved that the bronzing of the nuts was not detrimental to the overall quality of the nut. In fact, 100% bronzed nuts had thinner husks and shells which may even lead to higher kernel recovery percentages. What is not clear however, is the effect of thrips induced leaf drop and leaf malformation on crop volume. In California, citrus trees appear to be particularly robust and can tolerate a fair amount of leaf injury before economic losses ensue. Macadamias appear to be more susceptible to this kind of damage but studies quantifying the damage potential of the thrip complex are urgently needed. This is important, because the amount of money and effort spent on controlling thrips should be a function of the damage caused by these insects.
During a recent IRAC (Insecticide Resistance Action Committee) meeting, the thrip question was discussed in detail. Many crops in South Africa experienced similar problems during the previous season and the consensus was that the hot and dry conditions experienced during the last decade or so was probably to blame. From scientific literature, it is well known that these insects thrive under hot and dry conditions. In fact, scientific literature indicate that it may become near impossible controlling these tiny creatures. For this reason, it was also thought that general resistance or at least a loss in sensitivity to registered insecticides has not happened yet. It must be taken into consideration that due to the short generation time of these insects many generations could develop in macadamias each season and if exposure to pesticides is constant, then pesticide resistance could indeed develop in future.
What do we know about thrips that can be used against them?
1) Thrips like to feed on the flowers. Many of the thrips species noticed on macadamia during this time are not pests and actually feed on the pollen and are therefore regarded as saprophytes. Never spray the flowers as it will harm pollinators (bees) and will compromise nut set.
2) Thrips prefer to feed on tender leaf flushes. If you have pruned your orchard during the winter, it will in most cases be advisable to protect the new growth provided that thrip damage becomes evident. This spray (if necessary) should reduce the numbers of phytophagous thrips during the flowering season.
3) Thrips are attracted to bright yellow and deep blue colors. Commercially available sticky traps can be used to monitor these insects although a microscope and some entomological knowledge are required for accurate species identification.
4) Trips are key pests in many crops because the cosmetic damage they cause affects the marketability of the fruit. This is not the case in macadamias but broad-spectrum products should be avoided because they will considerably exacerbate the problem. Organophosphates and synthetic pyrethroids are specifically products that should be avoided.
Although the entire region reported trouble with these tiny insects, a very wet season is forecasted. If this is indeed true, we can expect lower pest pressure and growers are advised to monitor carefully and to spray only when high numbers or damage are reported.