About 3500 years ago the ancient Egyptians revered the dung beetle (Scarabaeus sacer) as a symbol of the Egyptian god Ra, who rolled the sun across the sky each day, “transforming bodies and souls”. The belief came to life in nature as Egyptians associated the dung beetle with Ra rolling his “sun ball” day-in-day-out. Today, millennia later, at the southern tip of Africa, our relationship with the dung beetle’s cousins could take a parallel course.
Eastern Cape and Kwa-Zulu Natal farmers growing non-native sugarcane, pineapple, and wattle crops are in a battle against scarab beetle larvae, commonly known as ‘white grubs’. In an article for SA Forestry Online Dr Benice Sivparsad details how white grub wreaks havoc among crops: fine roots of newly planted seedlings are systematically devoured, and in severe cases, the root plug is completely eaten away, with the underground portion of the stem ring-barked up to ground level – the plant equivalent of death by hanging.
For many, our first acquaintance with white grubs is in the garden as children, being alarmed by a creamish white worm that looks like something from an alien movie. Larvae are C-shaped, with a distinctive brown head and three pairs of well-developed legs. Some species roll on their backs and move off like caterpillars when dug up.
Currently, white grubs are controlled using non-specific insecticides, fungal diseases, and cultivation practices like burning and tilling, and late planting.
Unfortunately, like the aftermath of a shotgun approach, these methods have a spectrum of collateral effects on other organisms including killing white grubs’ natural predators (like ants, tiger beetles and spiders) and parasites (like nematodes and fungi), and harming beneficial insects like springtails and microbes that benefit the crops by naturally processing compost that enriches the soil and retains water.
These unwanted collateral effects can be reduced by using a more nuanced approach using tailored control methods, more like using a sniper rifle, to target weak points in each white grub species’ biology, for example, targeting their breeding season or their preferred depth in the soil profile.
Importantly, to target a particular species, agricultural growers need to be able to identify which species they are facing.
And this is the challenge…
If one finds a white grub in South Africa, it could be any one of 60 species depending on where you are. In addition, it could be a representative of one of four families such as the rhinoceros beetles, leaf chafers, or Christmas beetles.
Although adult scarab beetles are fairly easy to identify, little work has been done on the identification of their larvae in South Africa.
Prof Martin Villet, an expert entomologist from Rhodes University, received a small grant from the SANBI Foundational Biodiversity Information Programme (FBIP) in 2016 to develop means to identify the white grub larvae of individual species.
80 morphological characters
Three publications detail unidentified species of white grub attacking sugarcane crops, black wattle (13 species), and pineapple plantations (6 species) in the Eastern Cape respectively. Unidentified species are also pests in KwaZulu-Natal.
According to Prof Villet published data to identify South African white grubs is scarce, so his team aimed to provide morphological and molecular tools for growers to identify white grubs on farms (via morphological analysis) and in the laboratory (via morphological and DNA analysis).
Corrina Bazzelet and Carmen Wormald (an MSc student on the project), examined 80 morphological characters for about 63 species through a microscope, took photographs, and described the characters with standardized terminology.
The morphological data have been captured in specialised taxonomic software packages named DELTA and Lucid, which allow interactive identification by public end-users. The Lucid package is available at the dedicated Lucid internet site, and the DELTA package data will be placed on the dedicated DELTA internet site for public access.
DELTA will also be used to generate text descriptions and dichotomous identification keys suitable for publication in professional journals.
The other contribution to South African foundational biodiversity information science is 165 DNA sequences (>1000 base pairs long) from the cytochrome oxidase I gene. These have been generated for about 27 species.
A professional publication is being prepared to announce and summarise the sequence data for broader public dissemination through the internet, after which the sequences themselves will be archived on two international public databases, BOLD and GenBank.
According to Prof Villet, global change in insect populations has been associated with drivers that include pesticide prevalence. Formulating alternatives to pesticides would potentially open up technologies that produce employment opportunities in the bioeconomy.
“Specifically, these include industrial jobs culturing, packaging and marketing fungal and bacterial pathogens, consulting jobs advising farmers and foresters on bespoke cultural practices, and academic jobs studying the biology of individual species of white grub for new strategic points for their control,” he says.
Tailored control methods that target weak points in each species’ particular biology would allow a more nuanced approach with fewer collateral effects, and this requires a means to identify the white grub larvae of individual species.
Even with the identification tools that Villet’s team is providing, there will be an increased demand for expertise to use them in white grub identification and bionomics.
It is hoped that this work also takes us in the direction of viable biological control where the white grub pest can be controlled through predation, parasitism, or other natural mechanisms, but typically also involves an active human management role.
Ultimately, a less adversarial scenario should be possible between humans and white grubs, one which brings us closer to the reverence that the ancient Egyptians had for the white grub’s cousins, the scarab beetle.