Relative abundance and damage by species in the Lepidoptera borer complex of macadamia nuts in the Barberton area, South Africa

Abstract

Larvae of the nut borer complex, which include Thaumatotibia leucotreta, T. batrachopa, Cryptophlebia peltastica and Ectomyelois ceratoniae, cause extensive economic damage to macadamias in South Africa. Monitoring the changing species composition in this complex over a growing season provides valuable information for managing populations. In this study, pheromone traps were placed in two orchards planted with two different cultivars and used to monitor moth numbers over a 28-week period, while larval numbers and nut damage were monitored weekly over a 21-week period from the onset of flowering. Larvae were identified using a combination of morphology and DNA sequences of ~658 bp of the cytochrome oxidase I (COI) gene. Both larval and moth numbers were dominated by T. leucotreta, followed by T. batrachopa, with respective trap catches of 69% and 27%. Spearman correlations indicated strong linear relationships between nut diameter and numbers of eggs per nut for both the Beaumont and 816 cultivars. Poor correlations were found between weekly moth numbers and the number of eggs per nut. A two- to three-week lag was observed between weekly T. leucotreta trap catches and egg numbers. In contrast, egg numbers correlated strongly with larval infestation levels and the incidence of damaged nuts. Although the numbers of eggs and larvae were higher in Beaumont than cv. 816 nuts, higher incidences of nuts with husk and kernel damage were recorded for cv. 816. This study highlights the importance of monitoring of moth and larval numbers in macadamia orchards and showed that such data could be used to predict pest incidence during the season. This study questions the norm in South Africa, which is that macadamia nut borer (MNB) is usually the most abundant species in the nut borer complex.