Monitoring of an extreme case of host shift: toward an understanding of the biology and ecology of an emerging crop pest and potential invader.

Postdoctoral fellowship - Feb 2018-March 2020

Hosts: Prof. John Terblanche & Prof. Desmond Conlong

Stellenbosch University, South Africa

Host shifts are defined as the colonization of a new host by a population of insects or parasites while other populations of the same species remain on their original host. In most cases, it consists of a succession of steps that includes facing a novel environment, and successfully sensing and rapidly responding to a suite of novel cues, plastic changes at an individual level, followed by a suite of genetic adaptations occurring across generations over a relatively short period of time. Therefore, host shifts are generally regarded as a complex phenomenon and complementary research approaches are needed to understand their specific mechanistic basis as well as any generalities that may exist.
Cacosceles (Zelogenes) newmannii (Thomson) is a longhorn beetle species native to Mozambique, eSwatini and South Africa. The species host is not known, however, a population outbreak has recently (2016) been detected in a sugarcane plantation in Ntumeni (KZN) where it continues to cause severe crop damage, resulting in ongoing significant economic loss for the growers.
This emerging agricultural pest could potentially spread and invade sugarcane plantations across KZN given the extent of the cane growing area, but also across Africa, as various cane growing areas typically have similar climates and other soil characteristics. It is therefore critical to understand why some populations of this species have changed host, but also its biological characteristics that might facilitate population persistence in novel environments, as well as the factors influencing key behaviours (e.g. feeding and damage rates) and population abundance. This will lead to an assessment of the further risk of invasion into adjacent growing areas, in order to control its potential range expansion onto sugarcane in South Africa through containment and biocontrol.

This project is based on collaborations with national and international institutions, both academic and non-academic.
We plan to develop a long-term sustainable integrated management plan of C. newmannii, based on a comprehensive knowledge of its underlying biology, while simultaneously generating world class academic publications on the critical themes of host shifts, sustainable agriculture and food security using multi-disciplinary and holistic research approaches.

Biology of the species

Thermal tolerance and impact of oxygen limitation: The variability of upper thermal tolerance (heating) and the relative impact of oxygen limitation on larvae and adults’ metabolic activity was monitored to test which developmental stage was the most impacted by oxygen limitation. Carbon dioxide production and thermal tolerance (measured as the critical thermal maximum, CTmax) of individuals was measured as a proxy of metabolic activity under several experimental controlled oxygen treatments. We complemented these data with high quality three-dimensional micro-computer tomography (CT) scans of the larvae and adults morphology and respiratory (tracheal) system. This allowed us to test if the scaling of respiratory supply and demand changes between life-stages and explains among-stage variation in thermal tolerance. Indeed, understanding how tracheal morphology can affect gas transport to the tissue can have physiological and ecological implication and could help developing efficient pest control strategies (e.g. fumigation). The computer tomography scans allow additional detailed anatomical information on other aspects of their biology (e.g. reproductive organs) without altering the internal structures.

Read the publications : Javal et al., 2018; Javal et al., 2019; Lehmann et al., 2021

Metabolomics and RNA sequencing: We would like to contrast responses to chronic and acute stress (hypoxic and thermal stresses), using potential genetic markers of cell stress responses. Metabolomic and transcriptomic (RNA sequencing) analyses are planned.

Sexual dimorphism in mandible sizes: Sexual dimorphism is frequent for coleopteran species, but little is known about the physiological correlates of variations in sexually dimorphic traits in animal. In Cacosceles newmannii, males are characterized by very long, non-symmetrical mandibles, whereas females mandibles are much shorter but thicker than males’. The aim is to to verify whether the sexual dimorphism of C. newmannii is the result of sexual selection, and if mandible size is a proxy of the ability of a male to fight over resources, which may be critical for females mates choice and male-male competition.

Read the publications : Javal et al., 2021

Ecology

Barcoding of the Prioninae sub-family: Barcoding Prionini from the Afrotropical region will allow i) to identify C. newmannii with the closest and vicariant species, ii) to identify potential cryptic species, and iii) to highlight a potential evolution of the host plant association within the tribe of Prionini.

Read the publication : Javal et al., 2021

Variations in microbiome communities: Cacosceles newmannii shift onto sugarcane can possibly be attributed to several potential mechanisms, including abiotic and biotic factors. We tested the hypothesis that the possible polyphagia of this species could result from changes and modulations of the microbiome composition depending on the host.

Read the publication : Javal et al., 2019

Biocontrol: We assessed the virulence of three nematodes species and one fungus species on larvae of C. newmannii in order to examine their potential as biocontrol agents.

Read the publication : Javal et al., 2019

Read other publications on this topic : Smit et al., 2021a; Smit et al., 2021b; Lehmann et al., 2021

Collaborations

Anton du Plessis (CT Scanner Facility, Central Analytical Facilities, Stellenbosch University, South Africa)
Investigating the tracheal structure of a longhorned beetle.
Philipp Lehmann (Department of Zoology, Stockholm University, Stockholm, Sweden)
Tracheal development in oxygen limited environment
Antoinette Malan (Department of Conservation Ecology and Entomology, Faculty of AgriSciences, Stellenbosch University, South Africa)
Assessment of the virulence of biocontrol agents again C. newmannii larvae
Olivia Le Moëne (Department of Psychology, University of Tromsø, Huginbakken 32, 9037, Tromsø, Norway)
Behavioural assessments of C. newmannii fight propensity
Heiko Vogel (Max Planck Institute for Chemical Ecology)

Students

Chantelle Smit (Master student, Stellenbosch University, South Africa)