Bioatividade de extratos aquosos e orgânicos de diferentes plantas inseticidas sobre a mosca-negra-dos-citros, Aleurocanthus woglumiashby 1915 (Hemiptera: aleyrodidae)

Abstract

Brazil is the world s largest orange producer. As of the 1980s Brazil also became the top orange juice producer. Citrus growing faces a series of agricultural problems, especially in the form of pests and diseases, placing citrus among the crops with the highest losses due to insects, mites and pathogens. Currently, the citrus blackfly (Aleurocanthus woglumi) is one of the main pests, causing a reduction of fruit production as high as 80% and fruit losses of 20-80%. This study evaluates the effectiveness of insecticidal plant extracts (aqueous and organic), both native and exotic, in the control of Aleurocanthus woglumi, in laboratory conditions (29,10±0,15°C; 68,44±0,75% RH). We screened for the most promissing plants, selected the best extractor from those plants, conducted bioassays of translaminar, systemic, and topical action, and conducted chemical analyses of the most promissing extracts. The extracts or fractions were applied using an aerógrafo specific to second stage nymphs or eggs of A. woglumi. After seven or eight days of extract applications, we examined nymph mortality and egg viability. Initially, we determined CL50 with the use of aqueous almond (Azadirachta indica) extracts; subsequently we screened 14 plant species containing possible insecticides with different structures. We selected the three most promissing plant species and from these the best extractors (hexane, dichloromethane and methanol) and fractions (methanol extraction of hexane extract, liquid-liquid extraction and chromatographic fractioning); also bioassays based on mode of action (translaminar, systemic, or topical) and pretests of ovicidal and nymphicidal activity. Extracts were analyzed with joint gas chromatography mass spectrometry (GC-MS) and with high-performance liquid chromatography (HPLC). Bioassay results were analyzed with ANOVA or its non-parametric equivalent, Kruskal-Wallis. Differences were considered significant at p < 0.05. Among the aqueous extracts tested, the most promissing in the control of the citrus blackfly came from roots of ecotype C of the plant timbó (Derris floribunda), followed by almond A. indica extract and leaf extracts from Melia azedarach. In comparisons among solvents of different polarities, greatest bioactivity for the three plant species cited was obtained using the most apolar solvent (hexane). Among timbós (D. floribunda), ecotype C presented elevated toxicity to the blackfly, as much with aqueous extraction as with extraction solvents with successively increasing polarity, especially in the hexane-soluble partition. HPLC indicated that in isolation the rotenoids in timbó extracts do not cause high mortality in A. woglumi, suggesting that the toxic effect may be related to the presence of other secondary metabolites or the synergy among them. GC-MS analyses detected the presence of fatty acids, which are important in bioactivity, but whose mechanism with respect to the blackfly is not clear. Methanol-based extracts of nuts of A. indica, those of roots of D. floribunda and of leaves of M. azedarach, although lacking both translaminar and systemic action, present topical action on A. woglumi.