We studied the effects of crude extracts and fractions of Azadirachta indica, Melia azedarach, Toona ciliata and Trichilia pallida on both egg and nymph mortality and embryonic development of Bemisia tabaci B biotype, using tomato plants grown in a greenhouse. Next, we studied the host selection behavioral effects on the adult whitefly under laboratory conditions. The dichloromethane extracts from all plant species and fractions of the extract from branches of T. pallida (EBTPD) and of the extract from leaves of T. ciliata (ELTCD) in dichloromethane caused mortality of nymphs, but neither affected egg viability. However, the branches of the ethanolic extract of A. indica increased the period of embryonic development of the B. tabaci. In addition, the tomato leaflets treated with the fraction of ELTCD dichloromethane (0.28%) were the least preferred by adults, reducing the number of insects resting on the tomato leaflets. The ELTCD methanol and EBTPD dichloromethane fractions inhibited B. tabaci oviposition. Thus, Meliaceae derivatives can contribute to the reduction of the B. tabaci population. The susceptibility of the B. tabaci to Meliaceae derivatives and the relevant behavioral changes of this pest are discussed.
Secondary metabolites are mediators of interactions between plants and other organisms. The Sapindales are considered to be one of the richer and diverse sources of secondary metabolites in angiosperms (Waterman 1993). Within a broad spectrum of compound classes found in Sapindales, limonoids stand out (Fang et al. 2011). In particular, these compounds characterize members of the family Meliaceae as they are diverse and abundant (Champagne et al. 1992). More than 130 limonoids already were isolated from different parts of the neem tree, Azadirachta indica A. Juss (Sapindales: Meliaceae: Melioideae) (Kanokmedhakul et al. 2005). The azadirachtin is the most investigated and is considered to possess the most potential to be used in the integrated pest management (IPM) programs (Morgan 2009). The identification of limonoids and their subsequent synthesis for IPM has been the subject of research (Heasley 2011). However, the use of a single synthetic limonoid for the IPM of whitefly, Bemisia tabaci (Gennadius) (Hemiptera: Aleyrodidae), is not convenient and would result in the same mistakes experienced with other synthetic insecticides. The genetic variability of the B. tabaci, which is a complex of 11 well-defined high-level groups containing at least 24 morphologically indistinguishable species (De Barro et al. 2011), and changes in agricultural systems have contributed to these species becoming a major pest in modern agricultural practice. The use of pure compounds such as azadirachtin increases the likelihood of developing resistant insect populations (Feng & Isman 1995). However, Meliaceae derivatives often contain a mixture of active substances, which can delay or prevent the development of resistance.
Due to the positive results experienced with neem, other Meliaceae extracts are being investigated with the intent of their application in IPM programs, especially in organic agriculture. In this context, we evaluated the effect of organic extracts from trees belonging to the subfamily Melioideae (A. indica, Melia azedarach L. and Trichilia pallida Swartz) and to the subfamily Swietenioideae (Toona ciliata M. Roemer) on eggs and nymphs of B. tabaci B biotype maintained on tomato plants grown in a greenhouse. Then, to maximize the biological activity and reproducibility of the action of the derivatives, we fractionated the branch extract of T. pallida and the leaf extract of T. ciliata in dichloromethane, and evaluated their effects on nymphs of the whitefly on tomato plants in a greenhouse. We also evaluated the effects of these fractions on host selection of B. tabaci adults under laboratory conditions.
The subfamily Melioideae is characterized by the presence of limonoids with an intact carbon skeleton (Champagne et al. 1992). In this subfamily, the genera Azadirachta and Melia (Melioideae: Meliae) possess similar compounds, such as the highly oxidized C-seco limonoids. However, only the first genus contains species that produce the limonod, azadirachtin (Morgan 2009). Additionally, the genus Trichilia (Melioideae: Trichilieae) has the highest number and highest variability of limonoids, while the trees of the genus Toona (Swietenioideae: Cedreleae) are similar to the Melioideae in that they have limonoids with an intact carbon skeleton (Oiano-Neto et al. 1995).
MATERIALS AND METHODS
Insects and Tomato Plants
Bemisia tabaci adults were acquired from colonies maintained at the entomology sector of the Agronomic Institute of Campinas (IAC), Campinas, São Paulo State, Brazil, previously identified as B. tabaci B-biotype by induction of silvering in pumpkin leaves. The B. tabaci colony was reared in a greenhouse (approximately 2.5 m2) with anti-aphid screens and without automatic environmental controls. Soybean plants [Glycine max (L.) Merrill, cv. IAC-24] were grown in 3L plastic bags and used as hosts for insect rearing. New plants were introduced every 15 d to replace old plants already weakened by the high whitefly population. For the experiments, seeds of tomato plants of cv. ‘Santa Clara’ were planted in plastic trays containing Plantmax Hortaliças HT® substrate (DDL®Agroindústria, Betel Paulínia, SP, Brazil). Fifteen d after sowing, the seedlings were transplanted into 0.5 L plastic bags containing the same substrate for germination. The tomato plants used in this study were 30d old.
Plant Materials for Preparing Meliaceae Extracts
Leaves and branches were collected from A. indica, M. azedarach, T. ciliata and T. pallida trees located at the “Luiz de Queiroz” College of Agriculture campus in Piracicaba (S 22° 42′W 47° 37′), S®o Paulo State, Brazil. These species were selected and tests were conducted with organic extracts (non-aqueous) of their tissues, because promising insecticidal activity had been found with aqueous extracts of these plant materials.
Preparation of Crude Extracts and Different Solvent Fractions of Meliaceae Structures
The leaves and branches were dehydrated in an oven at 40 °C for 48 to 96 h, then ground into powders in a knife mill, and the resulting powders were kept in hermetically closed glass flasks. Two organic extracts from each part of the plant (