عنوان مقاله [English]
Introduction: Alfalfa (Medicagosativa L.) is a high-quality forage crop widely grown throughout the world. This plant is attacked by a wide range of pests.Alfalfa weevil, Hyperapostica(Gyllenhal) (Coleoptera:Curculionidae), is a pest of concern in alfalfa on the globe. This introduced pest is most problematic in the early season, causing defoliation and reduced yield and quality. Both adults and larvae feed on alfalfa, but the larvae cause the majority of the damage to terminals, foliage, and new crown shoots. This pest has many natural enemies. Among the beneficial insects, the green lacewing, Chrysoperlacarnea (Stephens) (Neuroptera: Chrysopidae), has received special attention since its larvae are important natural enemies of alfalfa weevil. The control of H. postica was done by chemical pesticides .There is now overwhelming evidence that some of these chemicals do pose a potential risk to humans and other life forms and unwanted side effects to the environment. Palizin (Coconut soap) is a natural compound based coconut extract. In the current research work, the effect of this compound on the adult of H. postica and egg and 1st instars larvae of C. carnea was investigated under laboratory and field conditions.
Rearing of insects
postica: H. postica larvae were collected from alfalfa fields in Miandoab using sweep nets in July 2016 and taken to the laboratory. Larvae were placed in plastic dishes (5 × 10 × 20 cm), fed by alfalfa foliage, and held at 25 ± 2 °C, 75±5% RH to complete their growth and convert to adults. The adults were used for all tests.
carnea: C. carnea adults were collected from alfalfa field at the Miandoab region, taken to the laboratory for identification of species and reared in the laboratory. Clear plastic cylinder containers (with 15 cm diameter × 25 cm height), which were covered with a mesh cloth were used for rearing. A thin layer of food source (mixture of yeast 50%, water 25% and honey 25%) was smeared to a plastic tape which was inserted into the plastic container. Water was provided for the adults through a wet sponge placed on the mesh cloth. To harvest the eggs, the adults were transferred to fresh containers every day. The eggs were laid on a colored paper which was attached to the interior wall of the cage. The eggs hatched after 3 days and the first larvae were used for bioassay.
Bioassay: First, primary experiments were performed to determine the minimum and maximum concentrations. Afterward, seven concentrations and also control were considered. Concentrations used for Palizin were 400, 543, 737, 1000, 1357, 1842 and 2500 ppm and for Fenvalerate were 200, 265, 353, 468, 622, 827 and 1200 ppm. Adult were kept at 4 °C to be inactive. Then, 15 adults were release on the petri plate (with 9cm diameter and 1.5 cm height). Using spray potter tower, 500 microliter of each concentration was sprayed on the petri and the dishes were placed in a growth incubator at 25±2˚C, 65±5% RH. To evaluate the recommended concentrations (2500 ppm Palizin and 1000 ppm Fenvalerate), an experiment was conducted in a randomized complete block with three treatments and five replications under field conditions. The number of H. postica (adults and larvae) and C. carnea (larvae) was counted day before and 3, 7, 14 and 21 days after treatment.
Statistical analyses: The mortality results were analyzed using SPSS data processing software. Significant differences among the groups were determined using the unpaired Duncan-test in 5 % level of significance.The percentage of population reduction in the treatments was corrected in relation to the control (water) by Henderson and Tilton’s formula under field condition.
Results and Discussion: Insecticides may be a good option if other options have not sufficiently controlled alfalfa weevil. Several commercially available products are labeled for control of alfalfa weevil. Although effective, their repeated usagehas disrupted natural biological control systems and led to resurgence of this pest, resulted in the development of resistance and had undesirable effects on no target organisms. In order to prevent resistance, effective chemical control of the pest requires new insecticides with novel modes of action. In laboratory condition, the LC50 values of Palizin and Fenvalerate on H. postica were 601.72 and 321.87 ppm 24 h after treatment, respectively. Toxicity of Fenvalerate for egg and 1st instars larvae of C. carnea was higher than Palizin. The LC50 values of this insecticide were 563.02 and 178.46 ppm and the LC50 values of Palizin were 1664 and 1663 ppm. The results of field trial showed that Palizinhas a longer persistency effect. According to IOBC standards, Palizin and Fenvalerate were categorized as slightly toxic and very toxic to C. carnea, respectively.
Conclusions: The plant pesticide used in this research caused the high mortality on the adults of H. posticawith proper persistency. In addition, its toxicity is less than Fenvalerate to C. Carnea and can be applied as an effective alternative to chemical insecticides in alfalfa fields.