The rice leaf roller (RLR), a small weak-flying moth, is one of the world’s most destructive rice pests causing significant economic losses in rice ecosystems in Asia, Australia, and other regions. This migratory pest relies on seasonal winds for long-distance travel to suitable habitats. In East China, the moths ride the East Asian summer monsoon winds northward as far as northeast China in spring and south-westward to south China and southeast Asia in autumn.
A new study co-authored by NRI’s Dr Don Reynolds shows that climate change is disrupting the monsoon wind patterns, critically affecting the rice leaf roller's migratory behaviour. The analysis indicates that late-season winds remain northward longer than before, preventing moths produced in the northern breeding areas from migrating southward. The monsoon’s delayed retreat southward denies the moths the favourable winds essential for their autumn migrations. This disruption creates an ecological trap, reducing the likelihood of the moths returning to their winter breeding areas in South China and Southeast Asia. But it is not just the leaf roller that is affected.
Benefits and downsides
At peak abundance in late summer, RLR can reduce rice yields by over 500,000 tons in China alone. To survive, the moths must retreat to Southeast Asia and extreme south China where rice is grown year-round, and temperatures are suitable for larvae development through winter. The Lower Yangtze River Valley (LYRV) in China, where this research was conducted, faces RLR infestations
annually. The study reveals that although late season winds still carry moths into the LYRV, the rice is no longer suitable for their development resulting in few or no surviving offspring. This disruption in RLR migration is therefore good news for rice farmers in the valley battling this devastating pest.
While farmers celebrate reduced pest pressure from the leaf roller, the authors caution that such changes in insect migration patterns severely impact windborne migrants, ecosystem functioning, and pest management strategies. Besides RLR, the monsoon facilitates the mass transport of over 200 species, including many beneficial biocontrol agents and pollinators, north-eastwards from Southeast Asia each spring. Although reduced RLR populations benefit agricultural productivity and food security, similar disruptions in non-pest species could adversely affect ecosystem functions, such as the provision of beneficial services and essential food sources for higher levels of the food chain.
Filling a key research gap
Climate change impacts on migratory insect pests in East Asia are well documented. These studies however have been focused primarily on the effect of warming temperatures including potentially favouring population growth and winter survival as well as expanding migratory ranges.
By focusing on wind patterns, this study addresses a key gap in the research with interesting results. It demonstrates that changes to the East Asian summer monsoon have led to decreased abundance and pest severity of two major regional crop pests: the brown planthopper and RLR.
Dr Reynolds said, ‘Splitting the 24-year study period into two, field surveys showed a significant reduction in RLR densities in rice paddies in the recent 12-year period. There were still plenty of moths appearing in light-traps in the Lower Yangtze area, but these immigrants were arriving too late in the season and were doomed to have few surviving offspring.’
Building on past research
NRI has a long history of research on migratory pests. The institute’s entomological radars deployed in China from 1988 to 1991 were used to observe and document the spectacular southward movements of RLR during that period. Building on this foundational work, Dr Reynolds and his collaborators are able now to demonstrate a dramatic change in RLR fortunes over the years.
The authors stress that climate change will have intricate effects not only on migratory insects but also on the broader ecosystems they connect with. They advocate comprehensive studies on these impacts across other regions to better understand and mitigate the consequences.
Dr Reynolds commented: ‘Apart from the seasonal insect migrations which utilise the East Asian Monsoon, annual migration cycles of this type have been documented in other parts of the world, for example, in North America, Europe and the Middle East. Research is needed to understand how changes in intensity and timing of these seasonal wind systems will affect insect migrations, with consequent harmful or beneficial effects for people in these regions.’
Read the full paper here: https://www.pnas.org/doi/10.1073/pnas.2422595122