Gone With the Wind: Changing Climate Conditions Lead to Migratory Insect Decline in Eastern China
Featured Image Caption: The rice leaf-roller moth is a common rice pest in East Asia. Although small, these incessant insects can cause serious damage to rice fields, reducing agricultural yield and stealing major profit from farmers. Image source: “Photo 119774459” by MSOne via iNaturalist, CC BY-NC-ND
Source Article: Zhang, S.-Y., Zhang, Y.-Y., Yang, F., Zhou, C., Shen, H.-M., Wang, B.-B., Zeng, J., Reynolds, D. R., Chapman, J. W., & Hu, G. (2025). Climate change is leading to an ecological trap in a migratory insect. Proceedings of the National Academy of Sciences, 122(9), e2422595122. https://doi.org/10.1073/pnas.2422595122
There’s This Question That’s Been Bugging Me…
Where do insects go in the winter? In places where winter is accompanied by a drastic drop in temperature, insects face a critical choice: either brave the cold or temporarily migrate to a more favorable habitat. Migratory insects, such as butterflies, moths, and locusts, are aerial insects that can fly to warmer latitudes in the wintertime. However, many of these insects aren’t powerful fliers and cannot make these journeys unassisted. Migrations are energetically expensive and span long distances, so migrating insects exploit insect flyways, atmospheric channels made of directionally favorable winds, to make these routine flights.
Mass aerial migration is often associated with birds, but insects do so as well! Locusts, like the ones pictured above, are just one example of a migratory insect. Image Source: “Саранча” by Петр Бобровничий via Wikimedia Commons, CC BY-SA 3.0.
Unfortunately, these once stable insect highways are now in jeopardy. The world’s climate has been changing for centuries, with temperatures rising and seasonal weather patterns becoming more variable and intense. If changes in wind patterns are extreme enough, insects that rely on the wind for migration may become trapped in environments for which they are not adapted, leading to reduced reproductive success and increased adult mortality. The continued survival of many of the planet’s migratory insects depends on the wind, but little is known about how the winds in insect flyways have been changing over time, and if migratory insect populations have changed in turn.
Winds of Change: Are Rice Leaf-Roller Moths Affected by a Changing Climate?
Recent research has focused on changing insect populations within the East Asian Insect Flyway, the planet’s largest insect flyway. Over 200 species of insects in the region use these regular seasonal winds for migration, relying on the annual progression and retreat of the East Asian monsoon to transport them southward in fall and return them northward during springtime. The rice leaf-roller moth (Cnaphalocrosis medinalis) is one of such species. This moth is a major rice pest across Asia, capable of reducing rice production by over sixty percent. Due to its relevance to agriculture, researchers investigated the population dynamics and migration patterns of RLR moths over the last twenty-four years, comparing any population shifts with changes in regional wind patterns.
To do so, investigators used an existing 24-year dataset on rice leaf-roller (RLR) abundance at 11 unique field stations in the Lower Yangtze River Valley, China. At these stations, moth populations in rice fields were estimated using field surveys and ultraviolet light traps. For field surveys, researchers walked through the rice field and counted how many adult RLR moths flushed from the crop and the number of RLR eggs and caterpillars within a standardized section of land. This measure provided a good estimate of immature moth populations and local breeding density, which is something that the ultraviolet light traps could not capture. However, the light traps were useful for illustrating recent emergence of adult moths from the rice fields, along with any immigration of adult moths from different regions. By combining these two methods, researchers could discern how many immature and adult moths were within a given area, as well as make inferences on the source of these moths (i.e., whether they were from the rice fields or had immigrated from other areas).
Blown Away – Literally! Moth Migration Interrupted by Shifting Winds
The data showed a stark decrease in RLR populations over time. Between 2000 and 2011 (Period 1), RLR populations in rice paddies averaged 16,690 adult moths per hectare, or about 1-2 moths per square meter. Adult moth density peaked in mid- to late August, having a significant impact on rice yields in the infested fields. As time went on, however, fewer adult moths, caterpillars, and eggs were found in the fields. Between 2012 and 2023 (Period 2), adult moth density peaked at merely 7,785 individuals per hectare, less than half of the average of the previous years. Peak adult density had also shifted to be later in the year, moving from August to early September. At this time of year, rice crops are less susceptible to damage from RLR, and the crop is less ideal for RLR growth, further reducing RLR outbreaks from the fields.
At first, it was unclear why moth populations had changed over time. However, the answer soon became clear when comparing data from the field surveys to data from the light traps. Field surveys detected significantly more moths in Period 1 than Period 2, but this trend did not hold for the light traps, as more moths were captured in Period 2 than Period 1. Additionally, both field surveys and light traps detected an increasing number of moths over time during Period 1. Moth abundance continued to increase in the light traps in Period 2, but populations appeared to decrease in the fields during Period 2.
These contrasting results between the field surveys and light traps suggested that delayed adult moth mass migration was to blame for the changing population structure. Fewer moths in the fields meant reduced hatching and local survival, while more moths in the light traps meant an increase in immigration. But why were moths immigrating later in the year? The answer likely lies with the winds of the East Asian summer monsoon, which has become much more variable over time. In years with a delayed retreat of the summer monsoon, late-season migration was at its highest. The prevailing winds continued to blow north or northwestward in late August and September, promoting increased immigration from the south into the study area and preventing adult moths from returning south for the winter.
The winds of the East Asian summer monsoon have been shifting over the last two decades, spelling trouble for the continued survival of the RLR moth. Image Source: “A Storm Warning” by Dave Hoefler via Unsplash.
An Invisible Prison: Wind-Related Ecological Traps Lead to Dwindling Insect Populations
These northern winds are trapping late-emerging local moths, as well as late-season migrants, at higher latitudes as temperatures start to cool. This change in habitat spells trouble for the continued success of this population of moths, as local conditions are not favorable for the established lifestyle of the RLR moth. Larvae that hatch in September rice are not likely to survive, as the crop is no longer ideal for their growth and development. The few caterpillars that do make it to adulthood will likely fail to produce a new generation, as there are few other suitable host plants in the region, and falling temperatures will further stunt their success. Escape to lower latitudes is also not an option, as these adult moths are short-lived and can only engage in one to three days of migratory flight. Therefore, the change in local wind patterns has created an ecological trap for RLR moths, leading to an overall decrease in population over time due to a shift in migration timing.
Cases like that of the RLR moth demonstrate just how severe the effects of climate change can be on sensitive species. Something as simple as a week or two’s delay in wind can cause an entire population to deteriorate over a relatively short period of time. Climate change, as a whole, is sometimes poorly understood by the public, as it has become synonymous with global warming. However, it is critical to realize that there is more to the climate than temperature. Factors like winds, precipitation, ocean currents, sea levels, ice cover, and soil quality are all part of the climate, and are all changing due to human activity. As these factors are so diverse and critical in shaping the life of our planet, it is important to continue monitoring these changes and to understand how they affect the larger world around us.
Reviewed by: Cassie Welander
