Invasive plants can be extremely harmful to crops. “Famine weed” or parthenium is one such plant. Native to tropical America, it was accidentally introduced to several countries. In Pakistan, the weed now covers thousands of hectares of agricultural and rangeland. And it poses an economic burden to farmers.

Parthenium can reduce crop yields by between 46% to 90%. This harms not only farmers’ livelihoods, but also food security. Furthermore, the weed has severe health effects on humans and livestock. It’s poisonous, causing allergic reactions. And it hosts malaria-carrying mosquitoes. With no natural enemies to control it in its invasive range, it grows in vast monocultures. This displaces native plant species causing biodiversity loss.

Chemical pesticides can partially control parthenium. But they’re harmful to human health and the environment. However, natural alternatives are proving helpful in the control of parthenium. Biological control (biocontrol) is turning the tide on parthenium. Beneficial insects are bringing about a more sustainable approach to managing this dangerous weed in Pakistan.

To control parthenium naturally, PlantwisePlus has been collaborating with partners in Pakistan. The main project partner is the National Agricultural Research Centre (NARC) in Islamabad. PlantwisePlus is also working closely with universities in Faisalabad, Mardan, Swabi, Tandojam, Multan, Bahawalpur and Rawalpindi.

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Invasive species like parthenium invade new locations without their natural enemies. And this is a big problem. Without these beneficial plant diseases and insects, the weed spreads out of control. Biocontrol re-introduces these beneficial enemies. The aim of biocontrol isn’t to eliminate the weed; it’s to control it.

To control Parthenium hysterophorus, the PlantwisePlus project team has been looking for a suitable biocontrol agent. ‘Biocontrol agent’ is the term that describes natural enemies of the weed, insects, for example. In the wild, these organisms naturally control the weed’s spread. The scientists have been looking for an agent that won’t impact other native species.

Scientists need to select biocontrol agents that specifically target the pest or weed. The agent must not pose a threat to other species. Parthenium hysterophorus has a number of natural enemies. And it’s these enemies that the project team has been researching.

The benefits of biocontrol for controlling parthenium weed

For several years, the team has been studying the effectiveness of Listronotus setosipennis in controlling parthenium. This is a stem boring weevil native to the Americas. The team has rigorously tested the weevil for its host specificity in quarantine laboratory. They’ve been able to confirm the weevil has a narrow host range. This means it only develops and feeds on parthenium and no other plants.

The weevils work by laying eggs in the plant’s flowers. The hatched larvae then tunnel into the stem and feed on the plant. By feeding on the plant, they control its ability to grow and spread. In Australia, the weevils have successfully controlled parthenium, particularly in drought-prone areas.

Local authorities in Pakistan have approved the weevils’ release into the wild. Ongoing monitoring has been essential to ensure that the weevils survive the winter. The weevil has shown great promise. Dr Phil Weyl leads the project under the PlantwisePlus programme. In the spring of 2023, he reviewed the weevils at the release sites. And he found that they had successfully overwintered.

Upscaling the release of the weevil

PlantwisePlus has trained people in many institutions in Pakistan to manage more releases in their local areas. The goal is to release 10,000 adult weevils by the end of 2023. So far, they have released about 10,150.

In June 2023, the project team released 500 Listronotus setosipennis weevils at multiple sites in Pakistan. The release ceremony at NARC was attended by Dr. Ghulam Muhammad Ali, Chairman of the Pakistan Agricultural Research Council (PARC). Dr. Babar Bajwa, Senior Regional Director, Asia, represented CABI at the event.

This PlantwisePlus collaboration has brought about a breakthrough in combating parthenium weed. The introduction of the stem boring weevil shows signs of great success. It offers a more sustainable way to manage the spread of parthenium without relying on harmful chemicals or machinery.

CABI aims to publish the project results in 2023, contributing valuable data to further control parthenium weed in Pakistan.

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