Bioatividade de extratos de meliáceas e timbós no controle da mosca-branca, Bemisia tabaci (Genanadius, 1889) (Hemiptera: Aleyrodidae), em couve, Brassica oleracea L. var. acephala

Abstract

The “whitefly”, Bemisia tabaci is an insect difficult to control, with more than 500 plant species as hosts and that can cause enormous damage to various agricultural crops, in some cases, total loss of production. Among them is the leaf cabbage (Brassica oleraceae), a crop of economic importance to the State of Amazonas. As an alternative to the increasing use of chemical insecticides to control this pest, botanical insecticides appear to be promising and compatible with integrated pest management. The objective of this research was to evaluate the insecticidal potential of meliaceae (aqueous extracts, organic and andiroba oil) and fabaceae (aqueous and organic extracts of two timbó species) common in the Amazon region, “whitefly” controlling in cabbage, at semi-field conditions (laboratory: 24,02±0,20°C;55,13±0,98%UR; Greenhouse: 38,43±0,59°C; 55,08±1,62%UR). First of all, it sought to select insecticidal plants more promising (screening); the best extractor was used within each promising plant. Bioassays and chemical analysis of the promising extracts were performed. The cabbage seedlings in tubes were infested with whitefly nymphs, using a cage covered by voile tissue. After the majority of the nymphs reached the second stage, two leaves per plant were selected, with at least 50 nymphs for the extracts applications. The experimental design was completely randomized. The percentages of mortality were transformed into arcsine [{(x + 0.5) / 100}) 0.5], submitted to ANOVA and the average compared by the Tukey's test (p <0.05). The collected botanical material was dried in a forced air circulating oven (40 ° C) and ground in a lab knife mill to be mixed in water to prepare 5% (w / v) aqueous extracts. The extracts were applied during seven days to assessed mortality of them. The mortalities were chosen to select the most promising extracts. Leaf extracts of Azadiracta indica (21.82%) and Cedrela odorata (24.55%), and D. amazonica root extract (61.67%) were selected. These three species proceeded for the organic extractions in hexane, dichloromethane and methanol. Among the organic extracts, the best results of mortality were D. amazonica in hexane (72.23%), whereas A. indica results (31.42%) and C. odorata results (27.08%) in dichloromethane. Chromatographic fractionation was not able to find azadirachtin between meliaceae fractions or rotenone as main substance among organic extracts of timbó, possibly indicating the action of other metabolites in the effect of these extracts. In the tests with oil C. guiannesis, the concentration of 1.5% achieved a reduction to 88.55% in egg laying, reducing 68.34% of mortality. The aqueous and hexane extract of roots D. amazonica is considered promising in the development of a control for the “whitefly” in cabbages, while the aqueous and organic extracts of the meliaceae obtained inferiors performances. Andiroba oil is also a promising alternative to controlling that pest, making more detailed studies about the properties and scope of these bio-extracts.