Two Insects Lead the Fight Against Destructive Invasive Weed—But There’s Still More to Be Done
By Sara Hendery
Where the invasive weed Parthenium hysterophorus once thrived, the release of two host-specific insects—Zygogramma bicolorata and Listronotus setosipennis—is significantly increasing farmer access to fertile lands.
Parthenium is an incredibly destructive weed native to Central and South America that has accidentally been introduced to many regions of the world including Australia, Asia, Africa, and the Pacific Islands. It dramatically reduces crop yields, causes human health issues such as respiratory problems and rashes, and taints valuable livestock milk. Found scouring roadways and invading farmland, Parthenium creates difficult conditions for farmers to grow food.
Beginning in 2005, Virginia Tech’s Feed the Future Innovation Lab for Integrated Pest Management (IPM Innovation Lab) and Virginia State University initiated a classical biocontrol program to manage the weed in East Africa, where the spread of invasive species is especially detrimental to biodiversity and the livelihoods of rural people who rely on natural resources.
Recently, Zygogramma bicolorata—a leaf-feeding beetle—demonstrated consistent establishment in Ethiopia and Uganda, and Listronotus setosipennis—a stem-boring weevil—did so in Ethiopia. Since the two natural enemies of Parthenium were released in Ethiopia five years ago, and more recently in Western Uganda, life stages of the insects survived the dry seasons and emerged from the soil in rainy seasons to feed on the weed. After defoliating the weed in those locations, the insects have now spread from the initial release sites to adjacent fields.
“Biocontol of Parthenium is an especially important initiative to apply right now to reduce burdens for rural farmers,” says Muni Muniappan, Ph.D., director of the IPM Innovation Lab. “On top of food insecurity, the COVID-19 pandemic has further limited farmer access to food. Mitigating the spread of this destructive weed will generate a chain reaction. It will help clear land for animal grazing, increasing milk yields. It will reduce weeding time, which will be particularly meaningful for women, who often hand-weed. It will open land for possible crop production without the use of pesticides, which will generally improve human and environmental health.”
The insects Z. bicolorata and L. setosipennis were selected for release in East Africa due to their specificity to the weed, their suitability to the climate, and the ease in which they can be reared in the laboratory at low cost. Thirty years ago, in Australia and India, and more recently in South Africa, Z. bicolorata and L. setosipennis were among numerous natural enemies introduced to assist in management of Parthenium. In just one year, the spread of the weed had caused $22 million worth of yield losses in Australia. While Z. bicolorata demonstrated immense capability in defoliating Parthenium, the additional presence of L. setosipennis provided supplementary control of the weed, especially in areas of low-rainfall and extended dry periods.
In Ethiopia, the IPM Innovation Lab’s classical biocontrol program of Parthenium is the first formal one of its kind. Since Ethiopia has several native species closely related to Parthenium, vital to the biocontrol program was testing Z. bicolorata and L. setosipennis in conditions beyond Australia and India. The specificity of Z. bicolorata and L. setosipennis was assessed against 29 and 31 non-target plant species, respectively, in Ethiopia. Although oviposition by Z. bicolorata occurred on six non-target species, it was significantly lower than on Parthenium, and no larvae developed. Additionally, L. setosipennis did not oviposit on any of the non-target species assessed. Based on these and other host range tests, permission was obtained to field release Z. bicolorata and L. setosipennis in Ethiopia.
“We have waited a long time for this establishment to occur, and we are grateful to have success with it,” says Wondi Mersie, Ph.D., associate dean and director of research at Virginia State University and lead scientist on the project. “A number of factors, including identifying suitable release sites and acquiring necessary permits for release, inhibited the early establishment of Z. bicolorata and L. setosipennis in East Africa, so there is much to be done to prevent challenges like that in the future.”
Mersie says that there is one major aspect of introducing IPM technologies against Parthenium that could significantly improve the efficiency of its control, which is evaluation of suitable sites for multiple natural enemies in areas where the weed’s spread is apparent or expected.
An example of this kind of work is already being done in a different IPM Innovation Lab project in Nepal that evaluates the spread of invasive weeds along the elevation gradient over the last 30 years. One of the project’s recent studies showed that among 81 locations where Parthenium was present in the Kathmandu Valley, winter rust (Puccinia abrupta var. partheniicola) infestation was observed at 98 percent of locations. At many of those locations, the impacts of the rust on Parthenium were medium-to-high, with aboveground biomass and seed output of Parthenium reduced by 47 percent and 73 percent, respectively.
“Studies like these indicate that winter rust incidence is widespread in some areas of Nepal and therefore has the potential to reduce the growth of Parthenium or be used as a component of IPM applied to the weed,” says Pramod Jha, Ph.D., professor emeritus at Tribhuvan University and lead scientist of the Nepal project. “Modeling has immense potential to help predict scenarios that will ramp up strategic pest management possibilities.”
The study also showed that if the same climatic conditions persist in the central Nepal area, the suitable habitat of Parthenium will expand by 10 percent in the future, causing potential threats to native vegetation. These findings reiterate the value of early detection and eradication plans for Parthenium and other destructive invasive species.
In the meantime, Z. bicolorata and L. setosipennis continue to defoliate Parthenium, with thousands of insects released to date in Ethiopia and Uganda. The IPM Innovation Lab recently helped Kenya acquire permits for release of the natural enemies as well, where they will be implemented as soon as COVID-19 restrictions are lifted.
The IPM Innovation Lab partners with a number of institutions around the world to combat the spread of Parthenium, including the Agricultural Research Council-Plant Health and Protection in South Africa, the National Agricultural Research Organization in Uganda, and Ambo University in Ethiopia, among others.
Sara Hendery is the communications coordinator for the Feed the Future Innovation Lab for Integrated Pest Management at Virginia Tech. She holds an MFA in nonfiction from Columbia College Chicago. Email: email@example.com.