Key biotic components of the indigenous Tortricidae and Heteroptera complexes occuring on macadamia in South Africa
Schoeman, Pieter Schalk
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In South Africa macadamia nuts are attacked by a variety of mostly indigenous pests which can be divided into two basic complexes, namely a nut borer complex (consisting of 3 tortricid moths.) and a stink bug (Heteroptera) complex consisting of approximately 35 insect species. The Heteroptera complex causes approximately 60% damage in unsprayed orchards and the estimated annual heteropteran induced crop loss could be as high as R24 million. Gravid female tortricid moths could discriminate between various cultivars and significant differences regarding oviposition on the 17 macadamia cultivars that were evaluated became apparent. Incidence of larval damage early in the season was negligible and this complex may therefore largely be regarded as pests of large (mature) nuts. Kernel distance (combined husk and shell thickness) and early maturing cultivars were probably not the primary determinants of resistance and the presence of toxic cyanogenic secondary metabolic compounds in the nuts should be investigated as a future research priority. Hybrid cultivars as well as cultivar 800 appeared to be more prone to tortricid damage while cultivars 788, 294 and 741 were significantly less damaged. There were also significant differences amongst immature nuts regarding susceptibility of various cultivars towards the Heteroptera complex. It was speculated that kernel distance is not the primary determinant of resistance because the kernel distance in immature nuts is not large enough to offer protection for any cultivar. Stink bug induced damage to immature nuts was the highest for the cultivars 800 and 863 while cultivars 741, 816 and 788 suffered the lowest incidence of damage. When the adult nuts were evaluated, cultivar 600 and 741 suffered the lowest Heteroptera induced kernel damage. The resistance mechanism is unclear at the moment but involve more than one parameter. When the effects of various control strategies (fixed interval spraying and IPM) were compared, IPM compliant farms (farms that monitored and sprayed according to threshold levels) did considerably better. These results were hardly surprising as fixed interval spraying entails spraying trees irrespective of the economic threshold level. Because damaged nuts generally do not drop early and cannot be distinguished from undamaged nuts, any damage resulting from a mistimed spray (spray after economic threshold level has been reached) must therefore be regarded as additive and will be reflected in the unsound kernel reports of processors. Increasing tree density had a important positive effect on tortricid (r2 = 0.821) and Heteroptera (r2 = 0.922) damage. An effective pruning programme is therefore an important prerequisite for high density macadamia orchards. Populations of both pest complexes were heterogeneously distributed throughout the orchard and will adversely affect the accuracy of scouting procedures based on knockdown sprays if the quantity of data trees during weekly sampling is insufficient. Two theories for the formation of hot spots exist and are briefly discussed. Tortricids cause an estimated crop loss of approximately R3 million annually in South Africa alone. Although a number of larvae can be found inside the nuts relatively soon after anthesis, the majority of early larval damage occurs from ± 14 weeks post anthesis onwards. Eggs and recently eclosed first instar larvae should therefore be more numerous during the ninth week post anthesis. This would also be the most important period when an insecticide with a contact action has to be applied (late November). Oviposition occurred when nuts reached a mean medial diameter of ± 20mm but this relationship is coincidental and is more related to the phenology of macadamia trees (end of premature nut drop). Control strategy (IPM vs. fixed interval spraying) had inconclusive results as the IPM compliant farms suffered severe infestations when compared to fixed interval sprayed farms, as well as the organic and unsprayed farms. The success of tortricid control probably pivots around the November insecticide application. Tortricid larvae feeding on the insides of the pericarp may contribute significantly towards immaturity because feeding damage invariably severs the vascular tissue connecting the developing nut to the plant. The relative seasonal occurrence of heteropteran damage indicates that levels gradually increase in spring and taper off during mid January. Exclusion trials in an unsprayed orchard confirmed this observation and the apparent reduction in damage during January could probably be ascribed to the hardening of the shell at the same time. The damage profile of Bathycoelia natalicola was calculated and indicated that mouthpart lengths of fourth and fifth instar nymphs are probably sufficient to penetrate kernels of the Beaumont cultivar up to harvest. Compensation for early crop damage was studied and where Heteroptera damage was artificially simulated by flower removal, the trees were able to compensate for early crop damage. Compensation for Heteroptera damage was confirmed when early sprays were withheld on a semi commercial field trial. Withholding early sprays had no effect on tortricids as the initial spray of this pest complex has to be applied during late November which coincided with spray applications on all three spraying regimes that were tested. Due to asynchronous flowering the first Heteroptera spray should probably be applied before the end of October each year.