Different hatching combinations of egg masses of yellow stem borer [Scirpophaga incertulas (Walker)] in rice ecosystem with special reference to emergence of egg parasitoids

Introduction

Rice is the third most produced agricultural product in the world. About 90% of the world's rice is produced and consumed in Asia (Khush et al., 2002) [2] . Stem borers are among the primary pests that cause significant damage to rice crops worldwide (Shu et al., 2000) [3] . Rice stem borers can cause economic losses of up to 60% (Rahman and Stout, 2019) [4] . Each unit increase in white ear damage has a greater negative impact on rice yield (Jiang et al., 2005) [5] . Scirpophaga incertulas (Walker) can reduce rice yield by 38% to 50% (Prasad et al., 2007) [6] . Controlling this pest from the early stages is crucial, as it is a monophagous pest that attacks the crop from seedling to maturity. Rice ecosystems also host a variety of beneficial insects in addition to pests. Egg parasitoids (Trichogramma sp., Telenomus sp., and Tetrastichus sp.) and predators (beetles, mirid bugs, earwigs, and spiders) are widely used as biological control agents against stem borers (Chakraborty, 2012) [7] . Reports indicate that parasitoids such as Trichogramma sp., Telenomus sp., and Tetrastichus sp. can naturally parasitize up to 95% of yellow stem borer eggs (Lakshmi et al., 2010 [8] ; Rahman and Stout, 2019 [4] ). In the Terai region of West Bengal, Tetrastichus sp. and Telenomus sp. were found to be naturally occurring egg parasitoids of rice yellow stem borer (Sankeerthana, 2023) [9] . These were later confirmed up to species level as Tetrastichus schoenobii and Telenomus dignus (Sankeerthana et al., 2025) [10] .

Materials and Methods

Egg masses of the rice yellow stem borer, Scirpophaga incertulas, were collected from different paddy fields (kharif rice 2024 and boro rice 2025) within the campus of Uttar Banga Krishi Viswavidyalaya. Egg masses were collected based on as many egg masses available at the time of sampling from randomly selected fields (Manjunath, 1990) [11] . While collecting, a portion of the leaf containing the egg mass was also taken and stored separately in plastic pouches.

These pouches were carried to the laboratory of the department of Agril Entomology for further examination. Each egg mass was observed under a stereozoom microscope (Olympus SZX7) and transferred into a plastic container covered with a white cloth secured by a rubber band. To maintain humidity, a moist cotton swab was placed inside each container. The samples were kept undisturbed until the emergence of parasitoids or pest larvae or both. Emerged larvae or parasitoids were killed and preserved in 70% alcohol after 24 hours of emergence. The containers were properly labeled and stored for further identification and counting. Observations were recorded on the hatching pattern of each egg mass. All emerging individuals were counted and recorded separately.

Calculating percent parasitism of YSB egg masses

The data was used to generate the percentage parasitism of egg masses. To determine the percent parasitism, the total number of egg masses collected and the number of parasitized egg masses were noted. The degree of parasitism in yellow stem borer egg masses was determined (Vennila et al., 2018) [12] .

Parasitism (%) = (Number of parasitized egg masses / Number of sampled egg masses) × 100

Results and Discussion Percent hatching and percent parasitism

During the entire study period, a total of 282 egg masses were collected from kharif rice (2024) and 59 egg masses were collected from the boro rice (2025). In the present study the percent parasitism of egg masses ranged from 0% -88.64% in kharif 2024 with highest parasitization occurring during 39 th to 40 th SMW. In case of boro paddy, the parasitoids were found to become active for a very short span and percent parasitization ranged from 0% to 76.67% with peak parasitization occurring during 20 th to 21 st SMW. The overall parasitization of egg masses was quite high for both the seasons. Considering both the seasons together, the average hatching percentage was 65.18% and percent patrasitism was recorded as 62.36%. The reason behind this seasonal variation may be due to difference in the availability of host eggs during both the seasons. During Kharif, as there were abundant paddy crops growing inside the campus, availability of YSB egg masses were also abundant. For this, the parasitoids got many host egg masses and the value of percent parasitization was moderate (53.54%). In contrary, boro rice was grown in limited amount of land restricting the availability of both rice crop as well as YSB egg masses for parasitization. The parasitoids targeted the limited amount of available egg masses pushing the percent parasitization rate at higher level in boro rice (71.18%). This study suggests that level of parasitization depends on availability or density of host insect's egg masses present in the field. Rauf (2000) [13] from Indonesia has also documented similar observation reporting a high level of parasitism of egg mass reaching as high as 85% in case of white stem borer on rice. Manjunath (2023) [11] also observed that the parasitoids typically record a high rate of patriotization of egg masses and may often reach 100% but the parasitoids are unable to parasitize all eggs laid in the egg masses leading to partial patriotization of egg masses. This observation also supports the finding of Sharmitha et al. 2020 [14] who documented that host density in the field influenced the extent of parasitism.

Relative abundance

Conclusion

The egg masses of rice yellow stem borers were found to be naturally parasitized by two different species of parasitoids under field condition and the overall rate of parasitization was quite high. But a close observation on different hatching patterns revealed partial parasitization of egg masses either by a single species of parasitoid or by multiple species of parasitoids.

Acknowledgement

AcknowledgementWe are sincerely thankful to the Department of Entomology, Uttar Banga Krishi Viswavidyalaya for providing necessary laboratory facilities.

References

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