Ecology and management of Bathycoelia distincta (two-spotted stink bug) in Macadamia orchards in South Africa

Abstract

The aim of this PhD was to investigate the ecology and management of B. distincta (Pentatomidae) associated with macadamia orchards in South Africa.

In chapter 1, the available literature of the life history traits of the Pentatomidae and their potential for use in management programmes was reviewed. The Pentatomidae are hemimetabolous with egg, nymphal and adult life stages, which differ in morphological, ecological, and behavioural traits. Some of these traits, such as the pheromone mediated aggregation and the disruption of symbiont acquisition are important targets for Pentatomidae control. Other traits such as the alarm pheromone and substrate-borne vibration also deserve further consideration. Finally, colour vision and flight ability are still poorly studied in Pentatomidae despite their potential importance for stink bug management.

In chapter 2, a population genetic analysis was conducted to determine the genetic diversity of B. distincta in South Africa. Individuals were sampled over three consecutive seasons from the three main macadamia regions of South Africa. Two gene regions, namely Cytochrome c Oxidase Subunit 1 (COI) and Cytochrome b (Cytb) were sequenced and subjected to various population genetic analysis. The analyses revealed a high haplotype diversity, an absence of cryptic species and a low genetic differentiation, despite the important geographic distances separating the various growing regions. These results suggest that this species is not currently structured among the various regions in South Africa which might be explained by its high dispersal capacity, and the rapid expansion of macadamia orchards in South Africa over the past decade.

In chapter 3, the composition of B. distincta alarm pheromone, antennal sensitivity and behavioural responses were studied. Gland extractions and dynamic headspace sampling revealed the presence of fourteen compounds, with only six of them that are released when stink bug adults are stressed. The quantification and antennal sensitivity of these six major compounds were determined and showed differences in doses and compound tested, but no differences were observed between sexes. Finally, the behavioural bioassays revealed that the alarm pheromone response observed in B. distincta is induced by a blend of at least (E)-2-hexenal, (E)-2-decenal, and (E)-4-oxohex-2-enal.

In chapter 4, the lethal and sublethal effects of insecticides used in the management of B. distincta in macadamia orchards were investigated. Firstly, the toxicity of four commercial products (i.e., acephate, lambda-cyhalothrin, beta-cyfluthrin and tau-fluvalinate) was assessed on adult stink bugs and behavioural tests were conducted to determine the effects of these products on the mobility of this pest. Thereafter, the acute toxicity of the lambda-cyhalothrin was determined and sublethal effects of this insecticides at LC10 and LC30 were analysed to determine their effect on the biological parameters of parent B. distincta and offspring generations. The results showed that pyrethroids lambda-cyhalothrin, beta-cyfluthrin and tau-fluvalinate are less toxic for B. distincta than the organophosphate acephate, which was also visible in the mobility bioassays. Sublethal effects were observed when B. distincta adults were exposed to sublethal concentrations of lambda-cyhalothrin, including a reduction in male longevity and fecundity with LC30, and a shorter developmental time of the offspring. These results suggest that B. distincta is not susceptible to pyrethroids and could explain why the macadamia industry is struggling to control this pest.

In chapter 5, various commercial lures and traps were evaluated in order to assess their effectiveness to capture B. distincta and other stink bugs in macadamia orchards. The commercial pheromone products were tested since cross-attraction with these commercial lures has been observed within Pentatomidae and may occur between stink bugs present in macadamia orchards in South Africa as well. Two trials in Limpopo and Mpumalanga were conducted over two periods, corresponding to phenological stages of macadamia nut development (from nut set to harvest). Overall, low numbers of stink bug species were captured during both the Limpopo and Mpumalanga trials, especially on clear sticky traps. However, considering the low density of bugs present in both orchards during the trials, the effectiveness of these pheromone-baited traps is not clear. More research, including the identification of pheromones emitted by the species present in orchards, needs to be conducted before pheromone baited traps can be implemented as part of the IPM program of the macadamia industry.

To summarize, this thesis showed that the ecology and management of B. distincta is complex and specific to South Africa. Since its discovery in the 80’s in the Limpopo province, this pest has spread extensively over South Africa as the genetic analyses showed. Its management mainly relies on the chemical utilisation of insecticides, especially pyrethroids and organophosphates. We showed there are some indications that this species might be less susceptible to pyrethroids than organophosphate. The pheromone-based management program is still in development, but the characterization of its alarm pheromone might help the industry to develop it.