Clarke van Steenderen

Clarke van Steenderen

Postdoctoral researcher in the Department of Zoology and Entomology, Centre for Biological Control, Rhodes University

Talks and presentations

South Africa is a hotspot for previously unknown stem-boring wasps of grasses (Tetramesa; Eurytomidae)

July 26, 2021

Conference proceedings talk, Grassland Society of Southern Africa, 56th Annual Congress, Online event,

The stem-boring wasp genus Tetramesa (Hymenoptera: Eurytomidae) comprises 203 species that feed exclusively on grasses. The wasps are highly host-specific, typically feeding on a single or a few closely-related grasses, and can cause significant damage to their host grass (e.g. reducing seed production, increasing tiller mortality). These attributes often result in Tetramesa being serious grain pests, but it also makes them ideal biological control agent candidates for controlling invasive grasses. Very little is known about the Afrotropical Hymenoptera in general, and to date, almost all the sampling effort in collecting and describing Tetramesa species has taken place in the northern hemisphere. Only four African species have been described; none of which are from South Africa. The Centre for Biological Control (CBC) at Rhodes University has been investigating biological control options for several African grasses that have become invasive in Australia and the Americas, and have been collecting Tetramesa specimens across South Africa since 2017. The insect communities associated with more than 60 different native grasses have been surveyed over this period. The uniform morphology of adult and larval Tetramesa has, however, made it impossible to determine whether these wasps are a single polyphagous species, or multiple oligophagous and/or monophagous species. We are currently using genetic barcoding tools (mitochondrial COI and nuclear ITS2 regions) and species delimitation methods to solve this problem. Our preliminary results have identified at least six potentially undescribed Tetramesa species from South Africa. Each novel Tetramesa species was highly specific, with five of the six potential species feeding and completing their development on a single host grass species. This work will facilitate using biological control techniques to manage invasive alien grass species and highlights a previously unknown diversity of Tetramesa species associated with South African grasses. It is likely that we will uncover many more undescribed Tetramesa species in the region as our sampling effort escalates.

A genetic investigation of the native stem-galling Tetramesa Walker (Hymenoptera: Eurytomidae) in South Africa, and their potential use as biological control agents

June 28, 2021

Conference proceedings talk, 22nd Hybrid Congress of the Entomological Society of Southern Africa (ESSA), Tshipise Forever Resort, Limpopo, South Africa

The Tetramesa genus (Hymenoptera: Eurytomidae) comprises at least 200 species that feed exclusively on grasses. The highly host-specific behaviour of these wasps, and the damage that they can cause to their host plants, makes them ideal biological control agent candidates for invasive grasses. Very little is known about the Afrotropical Hymenoptera in general, and to date, almost all of the sampling effort in collecting and describing Tetramesa species has taken place in the Northern Hemisphere. Only four African species have been described; none of which are from South Africa. The Centre for Biological Control (CBC) at Rhodes University has been investigating biological control options for several African grasses that have become invasive in Australia and the Americas, and have been collecting Tetramesa specimens across South Africa since 2017. The insect communities associated with more than 55 different native grasses have been surveyed over this period. The uniform morphology of adult and larval Tetramesa has, however, made it impossible to determine whether these wasps are a single polyphagous species, or multiple oligophagous and/or monophagous species. We are currently using genetic barcoding tools (mitochondrial COI and nuclear ITS2 regions) and species delimitation methods to solve this problem. Our preliminary results have identified at least four putative species (or rather ‘molecular operational taxonomic units’ (MOTUs)). These were collected from single host plants, confirming their host-specificity and potential as biological control agents. It is likely that we will uncover many more undescribed species in the region as our sampling effort escalates.

Cochineal identification: how molecular techniques can distinguish between biological control agents and agricultural pests.

April 26, 2021

Conference proceedings talk, 2nd International Congress of Biological Control (ICBC2), Davos, Switzerland

Invasive Cactaceae cause considerable damage to ecosystem function and agricultural practices around the world but some cacti are also important and valued crop species. The most successful biological control agents used to combat cactus weeds belong to the genus Dactylopius (Hemiptera: Dactylopiidae), commonly known as ‘cochineal’, but the worst pests of cactus crops are also members of this genus. Cochineal lineages used for biocontrol of cactus weeds are host specific and only certain species and lineages will feed on cactus crops, so cactus biocontrol can be safely implemented without harm to cactus agriculture. Many of the Dactylopius species are so morphologically similar, and in the case of intraspecific lineages, identical, that numerous misidentifications have been made in the past. These errors may result in cactus farmers incorrectly assuming that the biocontrol agent is damaging their crop. This study aimed to generate a multi-locus genetic database to enable the accurate identification of dactylopiids. This was achieved through the nucleotide sequencing of three gene regions (12S rRNA, 18S rRNA, and COI) and two inter-simple sequence repeats (ISSR). Nucleotide sequences were very effective for species-level and D. tomentosus lineage-level identification, but could not distinguish between the two lineages within D. opuntiae commonly used for biological control of various Opuntia spp. Fragment analysis through the use of ISSRs successfully addressed this issue. This is the first time that a method has been developed that can distinguish between these two D. opuntiae lineages. Using the methods developed here, one can distinguish between what is a potential pest, and what is a beneficial biological control agent.

The genetic barcoding of the species and lineages of Dactylopius Costa (Hemiptera: Dactylopiidae)

May 17, 2019

Conference proceedings talk, The National Symposium on Biological Invasions (15-17 May 2019), Waterval Country Lodge, Tulbagh, Western Cape, South Africa

The ability to use genetic barcoding tools to distinguish between the species and intraspecific ‘biotypes’ within the Dactylopius genus (Hemiptera: Dactylopiidae) is highly beneficial to the biological control of cactaceous weeds in South Africa. The present study used DNA sequencing and ISSR fragment analysis methods to create a database of genetic barcodes for Dactylopius species found in the country, as well as from the native range, with a particular focus on the biotypes found within Dactylopius opuntiae. This has important applications in the mass rearing of pure insect cultures and the inoculation of the most effective biotype on target Cactaceae.