Investigating the Effect of Temperature on Abundance of Mythimna loreyi (Lepidoptera: Noctuidae) in Corn

Document Type : Research Article

Authors

• Laleh Ebrahimi ¹
• Masood Amir-Maafi ²
• mohammadreza shiri ³

¹ Iranian Research Institute of Plant Protection, Agricultural Research, Education and Extension Organization (AREEO), Tehran, Iran.

² Sunn Pest Research Department, Iranian Research Institute of Plant Protection, Agricultural Research Education and Extension Organization (AREEO), Tehran, Iran.

³ Maize and Forage Crops Research Department, Seed and Plant Improvement Institute, Agricultural Research Education and Extension Organization (AREEO), Karaj, Iran

10.22067/jpp.2024.85113.1164

Abstract

Introduction
Lepidopteran pests are among the most important limiting factors for corn production (grain and fodder corn), which is one of the most important strategic products in the world and in Iran. In recent years, due to climate changes around the world, some secondary pests have become widespread. One of these pests is the false armyworm Mythimna loreyi. In Iran, until the past years, false armyworm did not cause significant damage. However, in recent years, its population has increased, and as a result, its damage is evident in the corn fields.
Materials and Methods
In the present research, the population of M. loreyi and the larval natural parasitism were assessed during three growing seasons (2021-2023). A one-hectare grain corn field was selected on the research farm of the Research Institute of Plant Breeding and Seed Preparation in Karaj, where no spraying was done during the season. In each weekly sampling, the number of M. loreyi larvae and the number of the parasitized larvae were recorded. Parasitized larvae were also transferred to the laboratory and the parasitoids were identified. At the end of season, 30 plants were infested with the target pest and 30 intact plants were sampled and the cobs weight was recorded for both groups. The percentage of cobs, which were infected with fungi after feeding the pest was recorded. To compare the temperature changes in the three evaluated years, the meteorological data for the sampling months were obtained. The SAS software was used for statistical analysis and mean comparisons.
Results and Discussion
The analysis of pest frequency data showed that frequency of the pest population increased, especially in the third year of the survey (2023). Population of the pest larvae showed a significant increase compared to the previous two years. Comparison of the average number of larvae in the first and second years based on the t test did not show a statistically significant difference (t value = 2.05; P = 0.06). However, it had a significant increase in the third year compared to the first year (t value = 4.34; P<0.01) and the second year (t value=2.63; P=0.019). The proportion of larvae population increased by 2.44 and 5.97 times, respectively, in the years 2022 and 2023 compared to 2021. Increase in the ratio of larvae population in 2023 compared to 2022 was also calculated to be 2.44 times. Comparing the average weight of the infected cobs that were fed by M. loreyi larvae with the control cobs showed a significant decrease in the weight of corn cobs due to the pest feeding (t value=3.94; p<0.01). On average, fungal contamination was observed in 75% of the cobs that were fed by the pest. Comparing the cumulative pest density data with the data related to temperature changes showed a possible relationship between larval density and the temperature. Comparison of the minimum temperature for four months (the months of pest activity on corn) in three years has shown that it was higher on June, August and September in 2023 than the corresponding months in 2021 and 2022. Therefore, considering density of the pest larvae and comparing three years, it seems that the minimum temperature had a role in increasing the pest density rather than the maximum and average temperature. Considering that based on the results reported by Qin et al. (2017) on the population of M. loreyi in China, this pest can grow at temperatures between 18 and 30 ̊C, the very high maximum temperature (such as the temperature experienced in August 2021 in Karaj and the mean maximum temperature reached above 37 ̊C) was not favorable for the pest and did not lead to its optimal growth. In the study of Mediouni Ben Jemâa et al., (2023) also a negative correlation has been reported between the number of captured adult M. loreyi and increasing temperature. Recently, M. loreyi has become one of the most important pests of cereals in Asia and Africa (Qin et al., 2017; Mediouni Ben Jemâa et al., 2023). It is possible that the increase in temperature in recent years in Iran has also provided optimal conditions for the appearance of M. loreyi, and results of the present study support this hypothesis. In this study, the average percentage of larval parasitism for three years was calculated between 11.74 and 14.86 percent. According to a study on M. loreyi parasitoids in Turkey, C. ruficrus was the most common parasitoid of this pest, and it was present in 91% of the fields where M. loreyi was active (Sertkaya and Bayram 2005). In the study of Sertkaya and Bayram (2005), the parasitism rate of C. ruficrus on M. loreyi was 38.5%, which was about 2 times more than compared to the highest average percentage of parasitism in the present study (14.86% at 2023). However, this average is related to the entire sampling period, and in all three years in the first half of September, the percentage of parasitism was recorded above 60% (Table 1). Therefore, this parasitoid has a high potential to control the population level of M. loreyi and the results of Sertkaya and Bayram (2005) confirm this claim based on the natural efficiency of C. ruficrus against M. loreyi by 42.4%.
Conclusion
The damage of M. loreyi is higher in late-season corn or corn that is cultivated later. In Iran, fodder corn is often sown as a second crop after harvesting wheat and barley. Considering the water crisis in the country on the one hand, and the importance of fodder corn in feeding livestock and providing fodder in the country on the other hand, which is very closely related to the country's food security, investigation on different aspects of M. loreyi biology, damage and management is necessary, before this pest emerges as an important and main pest in the country.

Keywords

• False armyworm
• Fodder corn
• Lepidopteran pest
• Parasitism
• Temperature

Main Subjects

• pests

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©2024 The author(s). This is an open access article distributed under Creative Commons Attribution 4.0 International License (CC BY 4.0).

https://doi.org/10.22067/jpp.2024.85113.1164

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