Population dynamics of the cereal leaf beetle Oulema melanopus and some natural enemies in wheat fields of Varamin county, Tehran province under spraying and non-spraying conditions

Document Type : Research Paper

Author

• Reihaneh Barati

Iranian Research Institute of Plant Protection, Tehran, Iran

Abstract

Keywords

• Seven-spotted ladybird
• flower fly
• chemical control
• natural enemy conservation
• wheat fields

Extended Abstract

Introduction

      Pest management in wheat fields is largely dependent on the use of chemical insecticides. However, these chemicals may have unintended consequences on non-target insects, including beneficial species. Over recent years, the population and damage caused by the cereal leaf beetle, Oulema melanopus, has increased in some regions of Iran. Therefore, it is essential to investigate the population dynamics of this pest, the effects of insecticides on both the pest and natural enemies in wheat fields, and identify optimal pest management strategies. This study aimed to monitor the population changes of O. melanopus, the natural enemies including the seven-spotted ladybug and flower fly larvae, and assess the impact of common insecticides recommended against wheat pests on the pest and natural enemies.

Materials and Methods

    The study was carried out in wheat fields of Tehran province, where the population of O. melanopus and its natural enemies was monitored during the pest larval stage. The changes in the population of O. melanopus and natural enemies including seven-spotted ladybird and flower fly larvae were recorded. For sampling, all larvae of the cereal leaf beetle, seven-spotted ladybird, and flower fly present on 40 wheat stems were randomly counted in the field. The sampling was carried out during the larval stage of the cereal leaf beetle, which is the optimal time for chemical management of this pest. To investigate the impact of common pesticide applications on the cereal leaf beetle population and the natural enemies, the effects of three insecticides namely deltamethrin (0.3 ml/L), fenitrothion (1 ml/L), and matrine (1.5 and 2 ml/L) were evaluated. The study focused on assessing the impacts of the insecticides on the beetle larvae population and the larvae of the natural enemies. Sampling was conducted before and after spraying and the count of the insects was performed using the same method as for unsprayed areas.

Results and Discussion

    The results showed that during the last week of April to the first week of May, there was an average of 2 larvae of O. melanopus per stem. The peak population occurred on May 1, with 2.4 larvae per stem, followed by a decline, reaching 0.5 larvae per stem by May 8. Application of the insecticides significantly reduced the pest population, with deltamethrin, fenitrothion, matrine 1.5 ml/L, and matrine 2 ml/L treatments resulting in 0.4, 0.85, 0.52, and 0.47 larvae per stem, respectively, three days after spraying compared to 2.12 larvae per stem in the untreated control. The results also highlighted the importance of monitoring the O. melanopus population from mid-April to mid-May in Tehran province. In terms of natural enemies, the ladybug larvae population decreased under pesticide treatments. However, after two weeks, the population in the matrine treatment recovered to levels similar to the untreated control, while in deltamethrin and fenitrothion treatments, the population remained 50% lower than in the untreated control. Flower fly larvae were found to be at very low numbers during pesticide application, with populations not significantly impacted after two weeks.

Conclusion

   The peak larval population of O. melanopus in Tehran province occurred in early May, with the highest infestation observed between May 1 and 8. Therefore, it is recommended to monitor the population of this pest approximately from mid- April to mid-May in Tehran, especially in areas where high populations have been observed in previous years. Insecticides such as deltamethrin and matrine were effective in controlling the pest population, but care should be taken to minimize harm to natural enemies. Among the insecticides tested the botanical insecticide matrine was found to be less harmful to ladybug larvae compared to the synthetic chemicals. The ladybug larvae were able to recover faster from exposure to matrine, demonstrating its potential as an eco-friendlier alternative. So, the use of matrine, particularly at lower concentrations, poses fewer risks to the seven-spotted ladybird compared to the chemical insecticides.  If chemical management against O. melanopus is necessary, spraying should be done using reduced risk insecticides and during the early larval stages of the pest to enhance efficacy against O. melanopus while minimizing damage to ladybirds and flower flies, whose populations have not yet increased in the field. As O. melanopus has a high potential for damage, and its prevalence has been increasing in certain regions, establishing clear thresholds for economic injury levels and understanding the pest's sensitivity to climate changes are critical for managing the pest effectively in Iran. Therefore, integrated pest management strategies that include careful timing of pesticide applications to avoid damaging natural enemies are essential.