Six-Year Light Trap Study Reveals How Weather Drives Rice Pest Outbreaks in Bangladesh

In the rice fields of northern Bangladesh, the timing of a pest invasion is written in the weather. A six-year monitoring study conducted at the Bangladesh Rice Research Institute’s regional station in Rangpur has now mapped, month by month and season by season, how the region’s most damaging rice insects and their natural enemies rise and fall in response to temperature, rainfall and humidity. The findings, published in the open-access journal Heliyon, offer one of the longest continuous portraits of rice agroecosystem dynamics in the country, and they arrive at a moment when climate change is reshaping pest pressure across South Asia’s most important staple crop.

Rice is the backbone of food security in Bangladesh, which ranks third in global production behind China and India, with an output of roughly 36 million tons. Farmers there grow rice year-round across three distinct seasons: Aus, a short-duration pre-monsoon crop; Aman, the dominant rain-fed monsoon season; and Boro, the fully irrigated dry season. Together these cycles covered about 11.69 million hectares and produced 38.15 million tons of grain in the 2021-22 season. Such continuous cultivation creates an essentially permanent habitat for arthropods, including the 267 harmful insect species and 375 beneficial arthropod species recorded in Bangladeshi rice ecosystems, of which 20 to 33 pest species are considered economically significant.

The research team, led by Tapon Kumar Roy and colleagues, ran a permanent penicillium-type light trap fitted with a 100-watt tungsten bulb beside the institute’s 15-hectare research fields from January 2019 through December 2024. The trap operated every evening from dusk to dawn, for 10 to 12 hours depending on day length, with no competing light sources within at least 200 meters. Each morning, researchers collected the trapped insects, killed them rapidly in a deep freezer at minus 20 degrees Celsius, and identified them using taxonomic keys, magnifying glasses and a stereo microscope. Because light traps attract only nocturnal, light-seeking insects, minute parasitoids were excluded from the analysis, but the method reliably captures the major moths, planthoppers, leafhoppers and bugs that dominate rice pest pressure, as well as many of their predators.

The diversity analysis told a striking story of dominance. The Shannon-Wiener diversity index for the community came out at 1.040, well below the maximum potential value of 2.565, and Pielou’s evenness index was just 0.406. In plain terms, a small number of species accounted for the overwhelming majority of individuals. The brown planthopper, Nilaparvata lugens, was the undisputed king, making up nearly 74 percent of all trapped insects. This sap-sucking bug is one of the most destructive rice pests in South and Southeast Asia, capable of causing hopperburn, the sudden wilting and death of entire field patches, and its dominance was amplified by intensive cultivation and heavy nitrogen fertilizer use, both of which favor rapid reproduction. Green leafhopper and stem borers ranked next, though at far lower relative abundances of roughly 7 and 5 percent respectively.

Monthly patterns revealed a clear seasonal rhythm. Stem borer abundance climbed through the monsoon and peaked dramatically in October, when trap catches reached an average of 374 individuals per month, significantly higher than any other month. The brown planthopper followed an even more explosive trajectory: numbers stayed low through the spring and early summer, then surged from August onward, peaking in November at an average of nearly 17,000 individuals, before declining through December. Green leafhopper and zigzag leafhopper also peaked in November, while white-backed planthopper peaked across October and November. Rice bugs showed a bimodal pattern, with high catches in May and June and again in November. The rice leaf folder was present year-round but most abundant from September through December. Taken together, April through July and September through November emerged as the high-risk windows for most pests, coinciding with warm temperatures between roughly 29 and 33 degrees Celsius maximum and 17 to 23 degrees Celsius minimum, and elevated relative humidity.

The seasonal breakdown sharpened the picture further. The Aman monsoon season proved to be the most pest-prone period for nearly every key insect, including stem borer, rice leaf folder, brown planthopper, white-backed planthopper, green leafhopper and zigzag leafhopper. The Aus pre-monsoon season ranked second, and the cooler, drier Boro season carried the lowest pest load, with the notable exception of hopper activity during its transition into the Aus season. The researchers attribute the Aman season’s vulnerability to a convergence of factors: it covers the largest cultivated area, coincides with the monsoon’s high humidity and favorable temperatures, and supports vigorous vegetative growth that provides abundant food and oviposition sites. Heavy rainfall in the Aus season, by contrast, appears to suppress some insect populations directly through mortality and egg wash-off, offsetting that season’s warm temperatures.

Natural enemies followed their own calendar. Carabid beetles were the most consistent predators, present throughout the year with peaks in May and again in October and November, suggesting they establish early and could contribute to preventive biological control. Dragonflies and damselflies peaked from July to September, tracking the humid monsoon months that support the aquatic larval stages of these odonates. The green mirid bug, Cyrtorhinus lividipennis, an important predator of planthopper eggs and nymphs, showed the opposite pattern, peaking in the cooler months from December through February, consistent with its preference for temperatures between 10 and 30 degrees Celsius. Ladybird beetles remained scarce overall but appeared in greatest numbers in October and November, apparently responding to the surge of planthoppers and leafhoppers, a classic density-dependent predator-prey signal.

Correlation analysis against weather variables added mechanistic texture to these patterns. Maximum, minimum and mean temperatures were positively and significantly associated with rice bug, white-backed planthopper and damselfly abundance, and mean temperature correlated positively with stem borer, zigzag leafhopper, rice leaf folder, carabid beetle and ladybird beetle. Rainfall told a more divided story: it favored damselflies, dragonflies, white-backed planthopper and rice bug, but was negatively associated with brown planthopper, green leafhopper, green mirid bug and several beetle predators, reflecting how intense monsoon downpours can kill small insects or disrupt egg-laying. Relative humidity significantly boosted damselfly and dragonfly numbers. Among the predators, ladybird beetles and carabid beetles were positively and significantly associated with most key pests, confirming their value as biological control agents, while staphylinid beetles and green mirid bugs correlated negatively with the pest community, hinting at suppression effects or responses to different microclimatic conditions.

The ecological implications reach well beyond northern Bangladesh. Because insects are poikilothermic, their body temperatures and metabolic rates track the ambient environment, and warming of just 2 degrees Celsius can add one to five extra generations per year for some species, while a 10-degree rise roughly doubles metabolic rate, accelerating feeding, movement and fecundity. Climate change is therefore expected to alter pest phenology, extend activity periods and increase outbreak frequency, potentially elevating minor pests to major status. The study’s identification of specific temperature and humidity windows associated with peak pest abundance gives forecasters concrete thresholds to work with, and the finding that transitional periods between seasons, such as the Boro-to-Aus shift, are sensitive windows for pest buildup adds a new dimension to timing-based management.

The authors are candid about the limitations. The monitoring relied on a single light trap at one location, so the abundance estimates reflect local conditions and may not generalize to other rice-growing regions with different cropping intensities, varieties or landscape structure. Light traps also miss day-active and weakly phototactic species, and no yield or economic injury measurements were collected alongside the insect counts. Still, previous work cited in the study suggests light traps capture roughly 94 percent of the target pest population in a field, making them a strong proxy for monitoring. The team calls for multi-location networks using standardized trapping, combined with direct field sampling and yield assessment, to strengthen regional forecasting models.

For farmers, the practical message is that pest management in Bangladesh’s rice fields should be climate-responsive rather than calendar-driven. Scouting and control efforts should concentrate on the Aman season and the shoulder months when temperatures sit in the 29-to-33-degree range and humidity climbs, while conserving the natural enemies, green mirid bugs, ladybird beetles, carabid beetles, dragonflies and damselflies, that the study shows track and sometimes suppress pest populations. Combined with light traps for early warning, economic-threshold-based insecticide use, and the broader toolkit of integrated pest management, this six-year dataset provides the temporal scaffolding on which a more sustainable, less pesticide-dependent rice protection strategy can be built, at a time when both food security and environmental health depend on getting the timing right.

Subject of Research: Seasonal dynamics of rice insect pests and natural enemies in relation to weather variables in northern Bangladesh

Article Title: Temporal dynamics of rice insect pests and natural enemies in relation to weather variables in northern Bangladesh

Article References: Roy, T. K., Sannal, A., Rana, M. M., Akter, S., Nayeem, A., Tamanna, S., Shultana, R., Hossain, M. M., & Hasan, M. R. (2026). Temporal dynamics of rice insect pests and natural enemies in relation to weather variables in northern Bangladesh. Heliyon, 12(15), Article e45396. https://doi.org/10.1016/j.heliyon.2026.e45396

Image Credits: AI Generated

DOI: 10.1016/j.heliyon.2026.e45396

Keywords: rice, brown planthopper, light trap, integrated pest management, climate, Bangladesh, stem borer, natural enemies, seasonal abundance, temperature, rainfall, agroecosystem