Effect of Different Organic and Chemical Compounds on Population Density of Trialeurodes vaporariorum Westwood and Some Secondary Metabolites of Bean Plant

Greenhouse whitefly, Trialeurodes vaporariorum Westwood (Hemiptera: Aleyrodidae), is a serious pest of a wide range of plants, such as common bean, cucumber, tomato, pepper, lettuce, etc, in both field and greenhouse production. It typically feeds on the underside of leaves, sucking phloem sap from the plant, which causes both direct damage, by overall weakening the plant and reducing yield, and indirect damage, by excreting honeydew, on which sooty mold grows, as well as transmission of several plant viruses. The widespread use of insecticides to control T. vaporariorum has resulted in developing resistance to the insecticides and affected human’s health and safety. The utilization of plant extract and elicitors is an environmentally safe method that is used in the control recently. Therefore, alternative control strategies are required to minimize the harmful effects of insecticides. In order to control this pest, environmentally friendly methods, especially of plant origin are recently considered by researchers. For this regard, the effect of some organic compounds including methanolic and n-hexanic extract of walnut’s husk or dill’s seeds and salicylic acid, individually and in combination with spirotetramat on population density of greenhouse whitefly’s nymphs and some secondary compounds of bean plant were investigated.
Seeds of common bean, Phaseolus vulgaris L. (Berloty cultivar) were sown and grown in 15-cm-plastic pots in a greenhouse at 25 ± 3°C, 50 ± 10% RH, and a natural photoperiod. For the experiments, potted bean plants at 6-8 leaf stage, highly infested with T. vaporariorum nymphs, were sprayed with 12 different treatments, including: (1) spirotetramat (SP), (2) methanolic extract of walnut husk (MW), (3) n-hexanic extract of walnut husk (NW), (4) methanolic extract of dill seed (MD), (5) n-hexanic extract of dill seed (ND, (6) salicylic acid (SA), (7) spirotetramat in combination with salicylic acid (SP+SA), (8) spirotetramat in combination with methanolic extract of dill seed (SP+MD), (9) spirotetramat in combination with methanolic extract of walnut husk (SP+MW), (10) spirotetramat in combination with n-hexanic extract of dill seed (SP+ND), (11) spirotetramat in combination with n-hexanic extract of walnut husk (SP+NW), and (12) water as a control (CO). After five days, the population density of T. vaporariorum nymphs on bean plants leaves was recorded. Furthermore, the effects of tested treatments on the total phenol and flavonoid contents of bean plants were evaluated. For this reason, dried leaves (5 g) from each treatment were used for the preparation of extracts. The total phenolic of the extracts were determined using the Folin - Ciocalteu reagent. Sample and standard readings were made using a spectrophotometer (Lambda 45-UV/Visible) at 765 nm against the reagent blank. Furthermore, the aluminum chloride colorimetric method was used for determination of the total flavonoid content of treatments. For total flavonoid determination, quercetin was used to make the standard calibration curve. The absorbance was read using a spectrophotometer (Lambda 45-UV/Visible) in the wavelengths of 415 nm. Data were analyzed by one-way ANOVA using SPSS 23.0 software. Comparison of means for nymph density using SNK (Student-Newman-Keuls) multi-range test and comparison of means for secondary chemical compositions using Tukey's test at probability level five Percentage occurred.
Results showed that the population density of greenhouse whitefly and the secondary metabolites of plant were significantly affected by the tested treatments. The lowest number of whitefly’s nymphs was on spirotetramat and spirotetramat in combination with methanolic extract of dill seed and the highest number was on control. Total phenolic contents ranged from 58.96 to 114.07 mg g-1 dry weight on different treatments, which the highest and lowest amount of it was obtained in spirotetramat in combination with methanolic extract of dill seed and spirotetramat in combination with n-hexanic extract of walnut husk, respectively. The highest amount of total flavonoid content of bean plants was in spirotetramat in combination with salicylic acid. The results of the present study showed that the density of greenhouse whitefly population in spirotetramate treatment was lower than other treatments but there was no significant difference with the combined treatments and methanolic extract of walnut husk. Also, all the treatments compared to the control significantly changed the population density of this pest and increased the amount of total phenolic compound in bean plant. Therefore, these compounds, especially the methanolic extract of walnut husk, can be used in integrated management programs of this pest.
This research could provide valuable information for control of T. vaporariorum in other plants like ornamental plants in the field and greenhouse and thus can be useful for the production of healthy and organic crops. So it can be used in the IPM programs of this pest.