Zanha africana, Radlk

Stevenson, Philip C., Green, Paul W. C., Veitch, Nigel C., Farrell, Iain W., Kusolwa, Paul & Belmain, Steven R., 2016, Nor-hopanes from Zanha africana root bark with toxicity to bruchid beetles, Phytochemistry 123, pp. 25-32 : 26-27

2.2. Biological evaluation of compounds from Z. africana against bruchids

Water, methanol and chloroform extracts of Z. africana root bark (10% w/v) significantly reduced the number of eggs laid per female bruchid when compared to the solvent control both prior to and after the exposure to cowpeas ( Vigna unguiculata L. (Walp)) ( Table 4 and Fig. 2 ). None of the treatments at equivalent concentrations were more toxic than rotenone, the positive control. Water and chloroform extracts assayed as a 10% w/v extract of dry root bark also increased mortality of bruchids over a six day exposure period ( Table 4 ). Prior to the addition of the cowpeas, the females actively probed on vials for suitable oviposition sites and left visible marks in the extract residues on the vial surface indicative of this behaviour; they also deposited eggs. Probing is part of a sequence of behaviours leading to oviposition ( Parr et al., 1996, 1998) and provides a route of absorption of toxins, where present, via ovipositors. Females should have laid eggs on the cowpeas when given the opportunity, but in the presence of all extracts they laid significantly fewer eggs compared with the solvent control ( Table 4 ). Similarly, a significantly reduced oviposition was recorded from insects in the presence of 1 and 5 at all concentrations evaluated (10, 100,1000 ppm) in comparison to the solvent control and this effect was not significantly different to rotenone, the positive control ( Table 4 and Fig. 2 ). The effects of the two nor -hopanes on bruchid mortality were influenced by concentration and exposure period. Mortality observed after 1, 2, 3 and 6 days exposure were described by linear regression, with significantly increasing mortality observed with increasing exposure period and increasing concentration (S1). Although the addition of untreated beans after 72 h provided a refuge from exposure to the extract and compounds, they did not prevent further mortality of insects. In fact, the mortality continued to increase after 72 h (S1). These data suggest that the nor -hopanes are toxic to bruchids but are less toxic than other highly potent plant compounds such as rotenone and deguelin identified in Tephrosia vogelii that is used as a natural pesticide in East and Southern Africa ( Belmain et al., 2012).

Hopanes, while rare, are previously known from plants, for example, from Megacodon stylophorus ( Gentianaceae ) ( Liu et al., 2014) and are elsewhere reported to be insecticidal compounds produced by entomopathogenic fungi ( Isaka et al., 2011). Nor -hopanes, however, have previously been of interest primarily due to their occurrence in crude oil albeit in highly reduced form ( Prince et al., 1994) rather than the oxidised and benzoyloxy substituted products reported here and have not been reported before as insecticidal compounds. While 1 and 5 were not potent insecticidal compounds on their own compared to rotenone, the positive control, it is likely that they contribute to the insecticidal effects of Z. africana preparations that are used by farmers. Zanha species are also rich in saponins ( Cuellar et al., 1997a,b; Lavaud et al., 2015) which occur widely in other plants used for control of a agricultural pests ( Jain and Tripathi, 1991; Mongalo et al., 2015; Nozzolillo et al., 1997; Shinoda et al., 2002) and, in some cases, also explain, biological activities against storage pests including bruchids activity ( Stevenson et al., 2009). Thus further work may reveal additional value of saponins of this species but the present work provides scientific evidence that at least partially underpins the use of Z. africana by resource-poor farmers based on the occurrence nor -hopanes that reduce the survival and oviposition of storage pests. The commercial potential of plant materials as pesticides is constrained by regulatory hurdles and plants such as Z. africana are unlikely to replace synthetic products ( Sola et al., 2014). Isman (2006), however, suggests that the value of pesticidal plants will be most important in developing countries by poorer farmers. But, this requires greater scientific information to help understand how use of pesticidal plants might be optimised. In this respect, the present work provides important knowledge for understanding more about chemical variability, persistence, residues and improving application.

Figures

Fig. 2. Relationship between bruchid mortality and number of eggs laid per female when exposed for six days to crude extracts and isolated nor-hopane compounds from Zanha africana root bark and a positive control (rotenone).