بررسی اثر دما روی تغییرات فراوانی mythimna loreyi (lepidoptera: noctuidae) در ذرت

یکی از مهمترین غلات استراتژیک در ایران و جهان ذرت می‌باشد، که بالپولکداران از مهمترین عوامل خسارت‌زای آن هستند. در سال های اخیر، با توجه به تغییرات اقلیمی در سراسر جهان، برخی آفات ثانویه حالت طغیانی پیدا کرده‌ و برخی آفات مهاجم به مناطق غیر آلوده گسترش یافته‌اند. از جمله این آفات، شب پره برگخوار ذرت mythimna loreyi است. در تحقیق حاضر، جمعیت لاروهای برگخوار ذرت و درصد پارازیتیسم طبیعی آن طی سه فصل زراعی (1402-1400) در کرج پایش گردید. تحلیل داده های فراوانی آفت نشان داد که جمعیت لاروهای آفت افزایش معنی داری داشته است (p<0.01). میانگین تعداد لاروها در سال اول و دوم از نظر آماری تفاوت معنی داری نشان نداد (t value = 2.05; p = 0.06)، اما در سال سوم نسبت به سال اول (t value =4.34; p<0.01) و سال دوم (t value = 2.63; p = 0.019) افزایش معنی داری داشت. نسبت جمعیت لاروها به‌صورت میانگین در سال 1401 و 1402 نسبت به سال 1400 به‌ترتیب 2.44 و 5.97 برابر افزایش یافته بود. نسبت افزایش جمعیت لاروها در سال 1402 نسبت به 1401 نیز 2.44 برابر محاسبه گردید. تحلیل داده های هواشناسی، وجود یک رابطه مثبت بین افزایش جمعیت این آفت و افزایش دمای حداقل در سال‌های مورد بررسی را نشان داد. در این بررسی زنبور پارازیتوئید cotesia ruficrus از لاروهای پارازیته برگخوار ذرت به‌عنوان پارازیتوئید غالب تعیین گردید. میانگین درصد پارازیتیسم طبیعی در سال های 1400 الی 1402 به‌ترتیب 12.74، 11.73 و 14.86 درصد بود. مقایسه میانگین وزن بلال های آلوده به آفت و بلال های سالم کاهش معنی داری را نشان داد (p < 0.01). به‌طور متوسط، 75 درصد از بلال هایی که مورد تغذیه آفت قرار گرفته بودند، درجاتی از آلودگی قارچی را نشان دادند. یافته های این تحقیق نشان دهنده رابطه بین افزایش جمعیت لاروهای m. loreyi با افزایش دمای حداقل در ماه های تابستان می باشد.

investigating the effect of temperature on abundance of mythimna loreyi (lepidoptera: noctuidae) in corn

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 discussionthe 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.