Insect-killing fungi: An option to manage sweet potato weevils

Damage done by sweet potato weevils has contributed to a decline in the crop’s production over the last two decades. More studies and research need to be conducted to understand how to efficiently use biological agents as a viable pest management option in our farming system, WILFRED WAU and MELANIE PITIKI write

SWEET potato production in Papua New Guinea has declined over the last two decades.
One of the reasons for this was the damage done by sweet potato weevils.
Both the cylas sweet potato weevil (cylas formicarius) and the West Indian variant (euscepes postfasciatus) are found worldwide but the latter is a more recent introduction into the country.
It has now spread into 14 provinces across the country and is being closely monitored.
Weevil attacks leave tubers rotten and unfit for household consumption and commercial use.
In order to address this issue, the National Agricultural Research Institute (Nari) has been working with the PNG University of Technology (Unitech) to develop improved crop protection options, supporting the intensification of sweet potato production in PNG with funding support from the Australian Centre for International Agriculture Research.
The project investigates the potential of using biological methods to manage both the cylas sweet potato weevils and West Indian weevils.
Researchers from Nari and Unitech have set up trials to test the effectiveness of a native, naturally occurring insect-killing fungus (metarhizium) to control the two sweet potato weevil species.
Trials were done over a four-month period at two sweet potato farming sites in Morobe from last October to February.
Preliminary findings indicate great potential for the use of the fungus in lowlands sweet potato farming systems.
Sweet potato weevils enter through cracks in sweet potato mounds and lay eggs at the roots and main stems of the crop.
Serious damage is caused when their larvae begin to feed on the storage roots and main stems of sweet potatoes.
Careful analysis is required to correctly discern which weevil species was active at any given time and place as their symptoms are very similar.

Metarhizium, the insect-killing fungus, is widely present in the soil and kills harmful sweet potato weevils.
It is effective against almost 200 species of pests, and, therefore, is widely used as a biological pest control option around the world.
For the trials, the fungus was extracted from soil samples collected from various commercial sweet potato production sites in the high and low altitude ecological zones and multiplied on rice grains at the Unitech biotechnology laboratory.
One trial was set up at a farmer’s field at Poahom village of Situm, in the Labuta local level government area.
The other was at Unitech’s Taraka campus farm.
Both sites consisted of 120 square meters of raised mounds.
Each mound was planted with two cuttings of the pathogen tested (PT) beauregard cultivar.

About 250 grams of the metarhizium fungal samples were multiplied on rice grains and applied monthly around the sweet potato mounds. Laboratory experiments at the Unitech biotechnology centre showed that the insect-killing fungus can kill the two devastating sweet potato weevils within 10 to 15 days.
Once this fungus has infected the insect host and killed it, the fungus grows out of the dead insect, multiplies and continues to attack potential sweet potato pests.
Preliminary results from field trials revealed that insect-killing fungus was seen actively growing not only on the rice grains, but also on the soil, indicating that it is able to persist for a long time in the soil.
Therefore, it may not be necessary to reapply the fungi for another season as the fungus has already adapted to the environment and can keep on multiplying and attacking weevils and other pests.
There is potential for PNG to use these fungi and other useful non-chemical control options such as crop rotation, sanitation, as well as the use of PT cultivars and pheromone lures in our sweet potato farming systems.
This will help to reduce our reliance on chemical-based pest control agents to manage weevils and improve sweet potato yields.
Furthermore, biological agents are environmentally friendly and pose no threat to the health of humans and livestock.
More studies and research need to be conducted to further understand how to efficiently use metarhizium fungus as a viable pest management option in our farming systems.

More information about the subject of this article can be sourced from author, Wilfred Wau on email http://[email protected].
Interested persons or groups can also visit Nari’s Highlands Regional Centre at Aiyura in Eastern Highlands.

• Wilfred Wau is a crop protection scientist at the National Agricultural Research Institute’s Highlands Research Centre at Aiyura, Eastern Highlands.
• Melanie Pitiki is a crop protection scientist based at the PNG University of Technology, Lae.