نوع مقاله : علمی - پژوهشی
نویسندگان
1 دانشجوی دکتری مدیریت جنگل، دانشکده منابع طبیعی، دانشگاه ارومیه، ارومیه، ایران
2 استادیار، گروه جنگلداری، دانشکده منابع طبیعی، دانشگاه ارومیه، ارومیه، ایران
3 دانشیار، گروه جنگلداری، دانشکده منابع طبیعی، دانشگاه ارومیه، ارومیه، ایران
چکیده
کلیدواژهها
موضوعات
عنوان مقاله [English]
نویسندگان [English]
Background and objectives: Defoliating forest pests are among the most significant threats to the health and stability of forest ecosystems. Climate change and rising global average temperatures have contributed to increased frequency and geographical expansion of these pests in Iranian forests. One such pest, the oak leaf roller (Tortrix viridana L.), causes extensive defoliation and reduces photosynthetic capacity in coppice oak trees by feeding on their leaves. Effectively controlling this pest, especially in hard-to-access forests like those in the Zagros vegetation zone, remains a key challenge for forest resource management. In this context, biological control methods, due to their environmental compatibility and reduced ecological impact, serve as a suitable alternative to traditional chemical pesticides. This study aimed to investigate the effects of aerial spraying with the bacterium Bacillus thuringiensis (Bt) on the oak leaf roller population and to evaluate the efficacy of the remote sensing NDVI index (Normalized Difference Vegetation Index) from Sentinel-2 imagery in monitoring vegetation changes and pest activity impacts.
Methodology: This study was conducted over approximately 926 hectares of oak forests in the Qabr-e-Hossein area of Piranshahr county, northwestern Iran, a known hotspot for this pest infestation. For pest control, ground-based Bt spraying at a concentration of 1 kg per 100 liters of water was carried out between May 6 and 13, 2023 (May 5–12, 2023). A Before–After Control–Impact (BACI) design was employed for quantitative impact assessment. In both the treatment and control areas, three 300-meter transects were systematically randomized. Within each transect, 30 trees were selected at 30-meter intervals (totaling 90 sample plots per area). Larval counts were performed in two stages: before spraying (May 2–4, 2023) and one month after (June 15–16, 2023). A linear mixed-effects model within the BACI framework was used to analyze the spray's effect on larval density, with treatment, time, and their interaction as fixed effects and tree ID as a random effect. Additionally, the percentage reduction in larval density was calculated using the ΔDxt index as a descriptive measure of Bt efficacy. To monitor the vegetation response to pest population changes, the NDVI index was calculated using Sentinel-2 Level-2A surface reflectance imagery from March 21 to September 22, 2023. Image processing and extraction of 10-meter pixel-scale NDVI time series for selected field plots were performed using the Google Earth Engine platform. Finally, the relationship between NDVI and larval density in control and treatment areas, before and after spraying, was examined using simple linear regression.
Results: Descriptive statistics indicated that the mean density of T. viridana larvae in the treatment area decreased from 53.37 ± 15.74 larvae per tree before spraying to 18.73 ± 7.26 larvae after spraying. In contrast, larval density changes in the control area were minimal, with the mean shifting from 48.74 ± 12.25 to 47.03 ± 10.87 larvae per tree. Results from the linear mixed-effects model revealed a statistically highly significant treatment × time interaction (p < 0.001), confirming the real and independent effect of Bt spraying on larval density reduction, distinct from natural population fluctuations observed in the control area. Post-hoc comparisons confirmed a significant decrease in larval density in the treatment area and no significant change in the control area. Calculation of the ΔDxt index showed an average larval density reduction of approximately 65% in the treatment area, whereas only a negligible decrease within the range of natural fluctuations was observed in the control area. Analysis of the NDVI time series showed that both control and treatment areas followed a similar seasonal pattern during the growing season. However, following Bt application, NDVI values in the treatment area remained consistently higher than those in the control area. While NDVI in the control area experienced a more pronounced decline after early June, the treatment area maintained higher NDVI values, indicating reduced larval feeding activity and preserved vegetation cover due to Bt spraying. Simple linear regression results showed a positive and significant relationship between NDVI and larval density in both areas before spraying. After spraying, this relationship remained positive but weaker in the control area, whereas in the treatment area, the relationship between NDVI and larval density became statistically non-significant.
Conclusion: The results of this study demonstrate that biological spraying with Bt significantly reduced the larval density of the oak leaf roller. This reduction, based on the BACI design and mixed-effects model, was clearly distinguishable from natural pest population fluctuations in the control area. The decrease in larval feeding activity post-spraying was associated with the maintenance of higher NDVI values in the treatment area, indicating limited defoliation and preserved photosynthetic capacity in the oak trees. Furthermore, regression analysis revealed that after Bt application, the correlation between NDVI and larval density in the treatment area weakened and became statistically non-significant, while it persisted in the control area. These findings suggest that integrating field data with the Sentinel-2 NDVI index can provide an effective framework for the quantitative assessment of biological pest control efficacy and for monitoring forest health at large scales, offering a sound scientific basis for the sustainable management of oak forests.
کلیدواژهها [English]
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