​As a child, a book about weevils borrowed from the Benoni Library gave Dr Steffan Hansen his first glimpse into the world of these beetles. Over the years his interest in insects grew to such an extent that it became his chosen study field. Earlier this year, at the age of 25, he received his doctorate in Entomology from Stellenbosch University (SU), in part for his role in setting up the first "genetic library" for banded fruit weevil species in South Africa.

This "genetic library" or barcoding database already contains the mitochondrial gene sequences or DNA code of 41 South African species. The information is also included in two international databases, GenBank and BOLD. Being collated as such is making it much easier for researchers and agricultural consultants to on a molecular level identify the weevil species to which the eggs, larvae or pupae they collect in soil or from crops belong.

Information about the initiative appeared in the Biodiversity Data Journal.

Dr Hansen counts himself lucky to be doing research on weevils, as they are the largest groups of animals in the world.

"More than 51 000 species have already been identified worldwide, and many more still need to be described or discovered. It will probably take a room full of taxonomists years to be able to classify every one of them," he notes.

South African species

He describes many South African species as so-called "cryptic species", meaning that they are extremely difficult to distinguish from one another on face value, even when scrutinized by a trained eye under the best of microscopes. It's often only through the study of their genetic material that differences between species come to light.

Among the best known species in South Africa are probably the different types of grain weevils that irritatingly might appear in flour being kept in a cupboard.

South Africa's Cape Floristic Region is home to many weevil species. Because of agricultural expansion, between 15 and 20 of these fynbos species are now also found in orchards and vineyards growing adjacent to fynbos veld.

"While most species do not do much damage other than leave small holes in fruit trees'  leaves, there are species such as the banded fruit weevil (Phlyctinus callosus) that cause direct damage to fruit. The invasive Fuller's Rose weevil (Naupactus godmanni) from South America causes farmers' headaches when they lay eggs in irrigation systems and so block water flow."

Weevils living in fruit trees can sometimes accidentally also end up in the packaging of export fruit being sent overseas - an issue that sometimes sees consignments rejected. Weevils are therefore considered to be a phytosanitary pest. Local research is currently being conducted to minimize the risks to fruit exporters associated with them.

Weevil finds

Dr Hansen is thankful that his research has practical value to the agricultural sector.

"I was drawn to studying plant pathology and entomology at SU's Faculty of AgriSciences as a first year student, because of the way that it goes

 about working on the pros and cons of insects' effect on farming," he remembers.

By his fourth year, lecturers in the Department of Conservation Ecology and Entomology already knew full well about his keen interest in weevils. He was subsequently invited along on two collection trips that stretched from Knysna to Stellenbosch, in the company of more experienced entomologists. These excursions saw the discovery of five species of weevils new to science in the greater Phlyctinus callosus species complex, most of which live on indigenous Asteraceae species. The 2020 paper in the European Journal of Taxonomy, with the French weevil expert Dr Julien Haran as lead author, was the first academic publication to which Dr Hansen contributed.

Dr Hansen is quick to point out that the ongoing discovery of new species does not mean that South Africa is being invaded by innumerable types of invasive species.

"It's thanks to better technology that we are these days able to on a genetic level distinguish between the different types of weevils that one for instance finds in the Cape fynbos and its associated fynbos ecosystems," he explains.

He feels honoured to contribute his bit to an ongoing project within the SU Department of Conservation Ecology and Entomology on weevils.

"With every publication one is able to contribute one more 'brick' to our knowledge about them," he explains.

Banded fruit weevils

As part of his doctoral research, he specifically focused on the banded fruit weevil species considered to be vineyard and orchard pests. These were among the five species described in the 2020 paper in European Journal of Taxonomy to which Dr Hansen contributed.

He investigated whether there is value in using two nematode species found naturally in South African soil, Steinernema yirgalemense and Heterorhabditis indica, or the fungus Beauveria, to naturally control them. This part of his studies formed part of another ongoing project within the SU Department of Conservation Ecology and Entomology, through which researchers consider the use of natural control agents (such as nematodes) as part of integrated pest control programmes.

Dr Hansen's field trials among others showed that nematodes work quite well to control weevil numbers when they are released into the soil and subsequently infiltrate weevil larvae.

He is currently working as a postdoctoral researcher at SU as part of a research team looking into ways to curb the spread of the shothole borer beetle and its associated fungus. This highly invasive combination is increasingly having an impact on trees of aesthetic and agricultural importance in South Africa.

* The study leaders for Dr Hansen's PhD research was Prof Pia Addison and Prof Antoinette Malan of the SU Department of Conservation Ecology and Entomology, and DrJulien Haran of CIRAD in Montpellier in France. 

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