The weevil that will assist in eradicating the current salvinia weed on the Hartbeespoort Dam has arrived in South Africa.

Salvinia on Hartbeespoort Dam

On Friday. 27 July 2021, a healthy population of the ravenous salvinia weevil, Cyrtobagous salviniae, arrived at the Centre for Biological Control (CBC) Quarantine Facility at Rhodes University. The weevil was imported from the warm waters of Gainesville, Florida, from the Florida Department of Agriculture and Consumer Services (FDACS). There, it is mass-reared and used exclusively to combat the noxious Salvinia minima or common salvinia in the southern states of the US.

“Currently, common salvinia is running rampant on Hartbeespoort Dam, and has spread to several other water bodies in the North West and Gauteng, and the newly imported weevil will be used to combat this invasion,” said CBC researcher, Matthew Paper.

Researchers at the CBC have started the initial phases of host-specificity testing, whereby the potential biological control agent is introduced onto various closely related plant species to determine if the insect will feed and, more importantly, reproduce on plants native to southern Africa. One of the most significant species being tested is the elusive Salvinia hastata, an African species of Salvinia whose closest population is found in the Lower Zambezi and Madagascar.

“The host-specificity testing of the Salvinia weevil will be relatively simple, as previous research in the US has shown that the common salvinia weevil is genetically identical to the salvinia weevil used to control Salvinia molesta or Kariba weed in South Africa. Kariba weed was widespread throughout South Africa, until the release of the weevil.”

The weevil, Cyrtobagous salviniae

Why then must a new agent be imported? It all simply comes down to size. The common salvinia that has invaded the southern states of the US and now the North West and Gauteng is far smaller than Kariba weed. Because of this difference in size, the salvinia weevil used to control Kariba weed across South Africa cannot successfully control common salvinia because these plants are not big enough. The newly imported salvinia weevil is slightly smaller in size as it has evolved alongside common salvinia, creating what is known as an ecotype. An ecotype is an organism that is genetically similar to another, but it has slight differences in its physical appearance because of the environment it developed in. “We, as biological scientists, can use this size difference to control this potentially devastating weed safely and for the long term,” he said.

Once host specificity testing is finished, applications to the South African Government can begin for the release of common salvinia weevil from the CBC quarantine facility.

“Once permission to release has been granted, the CBC and partners around Hartbeespoort Dam will begin mass-rearing the weevil. The mass-rearing process involves monitoring temperature, water chemistry, plant health and insect populations to provide the optimal growth conditions for the weevil to flourish. When the population has grown sufficiently, shipments of the weevil to impacted areas can begin, and the next phase of the program can occur. Some time is needed to test, mass-rear and safely release the agent onto invaded sites, but the CBC is fully ready for the task ahead.”

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