The invasive pest fall armyworm (FAW) has posed significant threats to Chinese agriculture since first reported in 2019, but it is still unclear exactly how the magnitude and distribution of threats vary between Chinese provinces. Here, we calculated the total potential cost of FAW to each of 31 Chinese provinces and identified the provinces posing the greatest threat to the rest of China. We found that (1) only 16.1% and 25.8% of provinces remained at upper and lower levels of potential threats (PTp), respectively. (2) The total potential costs (TPCp) of four provinces including Yunnan (830.51 M$), Guangxi (346.09 M$), Sichuan (116.87 M$), and Shandong (116.43 M$) were more than 100 M$. Six of the top 10 provinces exhibiting the highest TPCp were also among the top 10 agricultural producers. (3) Nine of the top 10 provinces with the largest TPCp values relative to regional GDP were underdeveloped provinces ranking bottom 15 in regional GDP. (4) Based on the total invasion cost (TICs), the underdeveloped provinces Yunnan (233.31 M$) and Guangxi (218.44 M$) ranked first and second as potential source provinces. Overall, larger agricultural producers might be more at risk and could suffer a bigger absolute loss from the further FAW threat. Underdeveloped provinces might be more vulnerable to FAW in relative terms and more likely to be a major FAW source. Our study suggests considerable scope for ongoing FAW redistribution by summarizing its threat to Chinese agriculture on a province-by-province basis and thus highlights the need for cross-provincial cooperation and national coordination to slow its spread.
Interspecific interactions (e.g., competition, predation) are core determinants of insect population evolution, geographical distribution, trophic dynamics and ecosystem functioning. Following its recent invasion of eastern Asia, the fall armyworm, Spodoptera frugiperda (Lepidoptera: Noctuidae) co-inhabits maize fields with native lepidopteran herbivores, such as the common cutworm, Spodoptera litura. Here, drawing upon laboratory and field studies, we demonstrate how late-instar S. frugiperda larvae directly prey upon S. litura immature stages and competitively displace them from the maize whorl. Individual maize plants did not mediate the above interactions, with herbivore-damaged leaves not affecting the development rate of either species. In the field, ecological niches of S. frugiperda and S. litura overlapped, with species-specific occupancy rates of whorl-stage leaves (vegetative phase) and ears or tassels (reproductive phase) exceeding 70% and 90%, respectively. Field cage trials showed that S. frugiperda larvae either preyed upon or repelled S. litura larvae from maize plants, routinely resulting in more than 90% mortality of the latter. Field visits and cage studies in Yunnan (SW China) also revealed how—within an approx. 1-year time period—S. frugiperda has become a dominant species in local maize fields and co-infestation of both herbivores on the same plant is rare. Overall, our work shows how the invasive S. frugiperda exhibits a clear competitive advantage over native lepidopterans and could replace certain species within local agroecosystems. This study not only unveils the mechanistic causes of rapid ecological shifts within S. frugiperda-invaded cropping systems, but may also guide subsequent monitoring and management interventions.
