Much effort has been focused on plant essential oils as potential sources of insect control agents. Sandalwood (Santalum album L.) is one of the most valuable trees in the world (Fox 2000). It occurs naturally or may be cultivated in India (FAO 1995). The essential oil derived from sandalwood is typically used as a flavor component in many food products, including alcoholic and non-alcoholic beverages (Burdock and Carabin 2008).

The sandalwood oil tested in this study (0.1% solution) caused significant mortality (87.2%) in TSSM adults and significantly decreased the total number of eggs, proportionally according to the number of live versus dead females (Table 3). Lethality of sandalwood oil of the 2nd trial was similar to that of the 1st one. However, the lethality of common thyme oil in the 1st trial was much higher than in the 2nd one, possibly because of high variation among replications in the 1st trial (Tables 2, 3). Even though Miresmailli and Isman (2006) found that rosemary oil was toxic to TSSM with an LC 50 of 13 ml/l (1.3% solution), our rosemary oil was not effective against TSSM in this study. This result suggests that the essential oil of sandalwood trees may be used for effective management of TSSM. There are some other reports that plant-derived essential oils are effective to spider mites. Essential oils extracted from Cuminum cyminum L. and Origanum syriacum var bevanii (Holmes) Ietswaart have been demonstrated to be effective as greenhouse fumigants for the control of the carmine spider mite and the melon/cotton aphid (Tuni and Sahinkaya 1998). Essential oils obtained from some other aromatic plants showed toxic effects on insects and mites (Choi et al. 2004; Tripathi et al. 2000).

GC–MS analysis revealed that sandalwood oil primarily consisted of α-santalol (45.8%) and β-santalol (20.6%) (Table 4). The composition of α- and β-santalol in our sample oil was not different from the analyses of other researchers who reported that α-santalol (~46%) was more abundant than β-santalol (~20%) in sandalwood oil (Anonis 1998). Previous research has also shown that sandalwood oil consists almost exclusively of closely related sesquiterpenoids, in addition to the main constituents of α-santalol (≥43%) and β-santalol (≥18%), supporting the results observed here (Burdock and Carabin 2008).

The santalol solution demonstrated significant toxicity at 0.1% and significantly decreased the number of eggs laid by live TSSM adults (Table 5). Some previous studies reported that the acaricidal and/or insecticidal effects of plant essential oils were related to their chemical compositions (Pascual-Villalobus and Ballesta-Acosta 2003). The major constituents of essential oils have been effective in controlling various pests including TSSM. For example, thymol and carvacrol, the major compounds in thyme (Thymus vulgaris L.) and oregano (Origanum vulgare L.) essential oils, have been identified as potential insect anti-feedants and oviposition deterrents against TSSM (El-Gengaihi et al. 1996) and tobacco cut-worm (Spodoptera litura Fabricius) (Isman et al. 2001). Several laboratory studies have described the acaricidal activity of essential oils and their major constituents (Basta and Spooner-Hart 2002; Cetin et al. 2009). However, no research has been reported on the effect of sandalwood oil and its major constituents against TSSM.

Jiang et al. (2009) indicates that the inactive constituents from Litsea pungens Hemsl. and L. cubeba (Lour.) Pers. (Lauraceae) have some synergistic effect on the active constituents and that, although not active individually, their presence is necessary to achieve full toxicity by contact against third-instar Trichoplusia ni larvae (Lepidoptera: Noctuidae). Also, Miresmailli et al. (2006) reported the acaricidal activity of Rosmarinus officinalis L. essential oil and blends of selected constituents indicated a synergistic effect among the active and inactive constituents against TSSM. In this study, we did not test the synergistic effects on TSSM of minor constituents in sandalwood oil which will possibly be valuable to be studied to get sufficient control efficacy.

Although plant-derived essential oils may be active against certain pests, one concern associated with their use is the possibility of plant injury or phytotoxicity (Arnason et al. 1993). In fact, many plant-derived essential oils are phytotoxic to vegetables and herbaceous and foliar plant material (Isman 1999). However, the extent of plant injury may be dependent on numerous factors, including the concentration of the compound, the rate at which it is applied, plant type, and which plant parts (e.g., leaves or flowers) are exposed during spray applications (Cloyd et al. 2009). In our test, sandalwood oil was not phytotoxic to flowers, foliage, buds, or stems of rose plant at a concentration of 0.1%.

In conclusion, this is the first study demonstrating that sandalwood oil and its constituents have acaricidal activities to TSSM, and decreased the number of eggs on treated leaves. Thus, sandalwood oil and santalol can likely be used for the sustainable management of TSSM on roses and possibly for susceptible greenhouse crops. However, this study only screened the essential oil and its constituents in the laboratory. Further studies are needed to further evaluate the acaricidal effects of the santalol in the field. It would also be valuable to test the efficacy of santalol against small and soft-bodied insect pests such as the western flower thrips, Frankliniella occidentalis Pergande, the cotton aphid, Aphis gossypii Glover, and the green peach aphid, Myzus persicae Sulzer, which are also major pests in many greenhouse crops.