Document Type : Research Article
Author
Plant Protection Research Department,West Azarbaijan Agricultural and Natural Resources Research Center, AREEO, Urmia, Iran
10.22067/jpp.2026.94924.1247
Abstract
Introduction
Alfalfa (Medicago sativa L.) is among the most extensively cultivated forage crops worldwide and is widely recognized for its high nutritional value. As a perennial species in the family Leguminosae (Fabaceae), alfalfa possesses a deep taproot system with numerous lateral branches that facilitate efficient uptake of water and nutrients from deeper soil layers. Owing to its high crude protein content, essential minerals and vitamins, alfalfa constitutes a fundamental component of livestock diets. In addition to its forage value, alfalfa plays a significant role in agroecosystem sustainability. Its deep root system improves soil structure and aeration while its inclusion in crop rotation systems enhances soil organic matter and increases nitrogen availability through symbiotic nitrogen fixation by Rhizobium spp. Furthermore, alfalfa cultivation contributes to improved soil drainage and long-term soil fertility. The alfalfa weevil, Hypera postica (Gyllenhal) (Coleoptera: Curculionidae), is one of the most destructive pests of alfalfa in many production regions, including West Azerbaijan Province, Iran. Both larvae and adults feed on foliage, often causing severe defoliation and substantial yield losses, particularly in the first harvest. In Iran, infestations of H. postica are estimated to result in annual losses of approximately 1.2 million tons of dry forage from the first cutting, representing nearly 33% of total alfalfa production. These losses impose considerable economic constraints on forage production systems. In response to concerns regarding the environmental and health impacts of conventional insecticides, international organizations such as the Food and Agriculture Organization (FAO) have advocated for reduced reliance on broad-spectrum synthetic pesticides and increased adoption of safer, more selective alternatives. Regulatory restrictions on older-generation chemical pesticides have further stimulated research into environmentally compatible pest management strategies, particularly those aligned with integrated pest management (IPM) principles.
Materials and Methods
A laboratory bioassay was conducted to evaluate the efficacy of six bio-based insecticides Palizin, Azadirachtin, Tondexir, Biobeet, Bassicoura and Naturalis against second-instar larvae of H. postica. An untreated control (distilled water) was included for comparison. The experiment was arranged in a randomized complete block design (RCBD) with six treatments and three replications. For each treatment, seven concentrations (500, 1000, 1500, 2000, 2500, 3000, and 3500 ppm) were prepared. Each concentration was tested in three replicates, with ten second-instar larvae per replicate. Larvae (approximately 3 mm in body length and 0.2 mm in head capsule width) were collected using an aspirator and transferred to plastic containers (8 × 10 × 6 cm). Fresh alfalfa foliage was dipped in the corresponding insecticidal solutions, air-dried and provided as food. Bioassays were conducted under controlled laboratory conditions at 22 ± 3 °C, 60 ± 5% relative humidity and a 8:16 h (light:dark) photoperiod. Larval mortality was recorded at 24, 48, and 72 hours after treatment. Mortality was assessed by gently prodding larvae with a heated needle; individuals that failed to respond were considered dead. Mortality percentages were calculated for each treatment and concentration.
Results and Discussion
Probit analysis of mortality data obtained eight days after exposure to different concentrations of cypermethrin (conventional insecticide) and the six bio-based insecticides revealed significant differences in toxicity against second-instar larvae of H. postica. Naturalis (LC₅₀ = 2973.246 ppm) and Bassicoura (LC₅₀ = 3187.697 ppm) exhibited the highest median lethal concentration (LC₅₀) values, indicating comparatively lower toxicity. In contrast, cypermethrin (LC₅₀ = 1478.279 ppm) and azadirachtin (LC₅₀ = 1859.971 ppm) showed the lowest LC₅₀ values, reflecting greater efficacy. Temporal patterns of mortality indicated that cypermethrin, azadirachtin, Palizin, and Tondexir produced their highest larvicidal effects within 72 hours of application with no substantial increase in mortality thereafter. Conversely, Naturalis and Bassicoura caused no detectable mortality during the first 48 hours with peak effects observed on day 7 (168 hours post-treatment). Biobit similarly showed no noticeable mortality within the first 24 hours but reached maximum efficacy by day 5 (120 hours post-treatment). These findings demonstrate clear differences in both toxicity levels and speed of action among the evaluated insecticides.
Conclusions
The results indicate that cypermethrin and azadirachtin were the most effective treatments against second-instar larvae of H. postica under laboratory conditions. In contrast, the biological insecticides Naturalis (based on Beauveria bassiana) and Bassicoura exhibited comparatively lower toxicity. A pronounced difference was observed in the speed of action: chemical insecticides induced rapid mortality, achieving peak effectiveness within 72 hours, whereas biological insecticides displayed delayed activity, with maximum mortality occurring between five and seven days after application. These differences in efficacy and temporal response are critical considerations for the development and optimization of integrated pest management (IPM) programs targeting alfalfa weevil populations.
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