The comparison of toxicity of four wheat herbicides on the wheat aphid, Schizaphis graminum (Rondani), and its predator, Hippodamia variegate (Coleoptera: Coccinellidae), under laboratory conditions

The wheat aphid, Schizaphis graminum Rondani, is one of the most important pests of wheat that damages this crop by feeding on vegetable sap. Among its natural enemies, ladybug Hippodamia variegata (Goeze) is the most active predator of this pest. The application of wheat fields may affect the population of these insects. The present study investigates the effect of common wheat herbicides on wheat aphids and their predator (i.e., ladybug H. variegata).

In the present study, the lethal effects of four herbicides (i.e., Clodinafop-propargyl, Tribenuron-methyl, 2,4-D plus MCPA, and Mesosulfuron-methyl+ Iodosulfuron-methyl-sodium) versus untreated types(control) was examined on the green wheat aphid S. graminum and its predator H. variegata using a completely randomized design under laboratory conditions at 25±1oC, 60±10% RH, and 16:8 hours (L: D). The individuals were transferred from infested wheat to fresh plants every week to keep an appropriate aphid colony. After reproducing several generations on the potted plants, the adults were considered for experiments. Adult specimens of H. variegate were directly collected from the same aphid colony using an aspirator and transported to the laboratory.The leaf-dip method was employed to evaluate the herbicide’s lethal effects. Besides, the mortality rates were recorded 24, 48, and 72 hours after the treatment process. Experiments were replicated for each herbicide four times. In this case, each replication included ten adult insects as subjects.

The results showed that using Mesosulfuron-methyl+ Iodosulfuron-methyl-sodium on aphids killed all insects in the first 24 hours of the experiment. Clodinafop-propargyl plus Tribenuron-methyl caused mortality rates of 55, 70, and 92.5% at 24, 48, and 72 hours post-treatment, respectively. In the case of 2,4-D plus MCPA, the mortality rates were 20, 37.5, and 57.5% after 24, 48, and 72 hours of the experiment. Mesosulfuron-methyl+ Iodosulfuron-methyl-sodium causes lady bug mortality rates of 45, 52.5, and 57.5% after 24, 48, and 72 hours, respectively. These valuesfor Clodinafop-propargyl plus Tribenuron-methyl were 17.5, 37.5 and 45%, while 2,4-D + MCPA caused 12.5, 15, and 27.5% mortality rates on ladybugs. Therefore, the present study recommends Mesosulfuron-methyl+ Iodosulfuron-methyl-sodiumto implement weed control in wheat fields and control S. graminum. Also, it is suggested to utilize Clodinafop-propargyl + Tribenuron-methyl and 2,4-D + MCPA in combination with biocontrol (using H. variegata) to perform the relative control of wheat aphids.

Agricultural herbicides can affect various insects such as pests and natural enemies. This procedure occurs through their direct toxicity or their effects on the population and diversity of their plant hosts. The present study is one of the few studies that simultaneously investigate the influence of herbicides on a pest and its natural enemy. The findings demonstrated that herbicides could directly affect insect populations. The principal purpose of the present study was to determine whether common and widely used herbicides in the cultivation of wheat in Iran can directly cause death in adults of aphids and their main predator (H. variegata). Since both of them have a high population in most wheat fields, their adults were simultaneously established and activated in the wheat farms during herbicides. Based on the recommended concentrations and direct application, four herbicides caused death in adults of green wheat aphids and ladybugs.