Landscape configuration drives fall armyworm dominance over native stemborers in smallholder maize systems in the Congo Basin
This study demonstrates that in the Congo Basin's smallholder maize systems, complex and aggregated landscape configurations promote the dominance of the invasive fall armyworm over native stemborers, whereas increased forest cover and edge density help suppress fall armyworm infestations.
Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer
In the vast, humid expanse of the Congo Basin, the maize fields that feed smallholder farmers are under siege. For decades, these fields have hosted a community of native pests, primarily moths whose larvae bore into the stalks of the corn, known as stemborers. These insects are part of a local ecosystem that has evolved alongside the crops. However, a new, invasive enemy arrived from the Americas: the fall armyworm. This pest is a generalist, capable of eating a wide variety of plants and moving quickly across great distances. Its arrival has not just added to the problem; it has fundamentally altered the balance of power in the fields, often pushing the native pests aside.
Scientists have long known that the shape and arrangement of the land around a crop field—the landscape—can influence how pests spread. A patchwork of forests, fields, and edges creates a complex environment that can either help or hinder an insect's journey. But it remained unclear how this physical arrangement of the land specifically affects the battle between a fast-moving invader like the fall armyworm and the slower, more specialized native stemborers. Does a complex landscape help the invader take over, or does it protect the crops? Understanding this is critical for farmers who need to manage their fields without relying solely on chemicals, especially as climate change and agricultural intensification reshape the environment.
A team of researchers from universities across the Democratic Republic of the Congo set out to answer this question by looking at the land itself. They traveled to six different locations near Kisangani, a city in the heart of the rainforest, to survey maize fields managed by local farmers. They focused on the pre-tasseling stage of the crop, a critical time when the plants are most vulnerable. In each field, they carefully counted how many plants were damaged by the fall armyworm, how many were hit by the native stemborers, and how many suffered from both. To understand the context of these fields, they did not just look at the crops; they looked at the entire landscape surrounding them. Using satellite imagery, they mapped the forests, the fields, and the edges where different habitats meet within a one-kilometer radius of each farm. They measured specific features of this landscape, such as how jagged and irregular the shapes of the forest patches were, how much forest cover existed, and how clumped or scattered the different habitats were.
The results revealed a stark reality: the fall armyworm has become the dominant pest in these systems. In some of the surveyed fields, nearly all the plants were infested with the invasive worm, reaching levels as high as 93 percent. In contrast, the native stemborers were found in very low numbers, and it was rare to see both pests attacking the same plant at the same time. The data showed that the fall armyworm was not just present; it was effectively replacing the native species in these environments.
When the researchers connected these infestation numbers to the landscape maps, a clear pattern emerged. The fall armyworm thrived in landscapes that were complex and aggregated. Specifically, fields surrounded by land with highly irregular, jagged patch shapes and where habitats were clumped together saw the highest rates of infestation. These complex landscapes seemed to act as a highway for the mobile invader, allowing it to spread and establish itself easily. Conversely, the native stemborers struggled in these same complex environments. Their numbers dropped significantly as the landscape became more irregular and aggregated.
The study also highlighted the protective role of forests. Areas with a higher proportion of forest cover and those with more edge density—the boundaries where forests meet fields—actually saw fewer fall armyworm infestations. This suggests that while a complex, clumped landscape might help the invader, a landscape that retains significant forest cover and creates distinct edges can disrupt the pest's ability to colonize the crops. The native stemborers, which are less mobile and rely on stable grassy habitats, appeared to be hindered by the very complexity that helped the fall armyworm.
These findings suggest that the physical arrangement of the land acts as a filter, favoring the invasive, highly mobile fall armyworm while constraining the native, specialized stemborers. The researchers found that the shape and connectivity of the landscape were more important drivers of this shift than the simple amount of forest present. This means that the way fields and forests are arranged in space can inadvertently create conditions that allow an invasive pest to dominate. For farmers and land managers in the Congo Basin, this implies that managing pests requires looking beyond the individual field. Strategies that increase habitat diversity or preserve forests must be carefully planned to ensure they do not accidentally create the specific type of complex, aggregated landscape that the fall armyworm uses to its advantage. The study points toward a need for pest management that considers the entire landscape, aiming to find a spatial arrangement that limits the invader while supporting the local ecosystem and the livelihoods of the farmers who depend on it.
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