Studies were conducted to assess the biological activity of E. camaldulensis, T. indica, O. sanctum, L. camara and C. longa against cotton bollworm, Helicoverpa armigera during 2001 – 2004 at Centre for Bioresources and Biotechnology, TERI School of Advanced Studies, New Delhi. All the bioassays were conducted under controlled environmental conditions (27±10C, 65-70% RH, 14:10 L/D). The bioactivity varied with the plant species in the following order: T. indica ≈ E. camaldulensis > O. sanctum > C. longa > L. camara based on chronic feeding bioassays. T. indica and E. camaldulensis proved superior in terms of bioactivity than the other plant species having growth inhibition (GI) as the main activity, hence identified as promising plant species for further studies. The crude hexane and ethanol extracts of the promising plant species were used for conducting different insect bioassays viz. chronic feeding and antifeedancy. The results proved that the ethanol extracts were superior to the hexane extract for both the plants in reducing larval growth, feeding and oviposition of H. armigera. Insect bioassay-guided-fractionation of ethanol extracts of T. indica and E. camaldulensis were therefore conducted to identify active fraction.
The group of compounds identified from the present findings are T. indica alkaloids and E. camaldulensis tannins that were found active against H. armigera, though there may be other active groups. The effects of these plants on the performance of H. armigera emphasize the potential defensive function of T. indica and E. camaldulensis, especially alkaloids and tannins, against H. armigera in a more comprehensive and an applied way.
The salient findings are summarized:
1. The leaf/ rhizome powder of all the test plant species significantly affected the growth and development of H. armigera neonates following chronic feeding bioassay by diet incorporation method. These species significantly disrupted the moulting process thereby resulting into delayed developmental period (larval and pupal both) with the subsequent decrease in the larval and pupal weight, and in certain cases, effect on male-female ratio. Accordingly, two main groups were established. T. indica and E. camaldulensis were placed together as they showed significant growth retardation of H. armigera larvae ultimately leading to 100% mortality and restricting the growth up to 3rd instar stage. While O. sanctum, L. camara and C. longa were placed in other group where although growth retardation was observed, however larvae were not restricted to 3rd instar and also mortality was observed during the later phase of the life. In addition, T. indica at different concentrations has demonstrated growth inhibitory activity against the H. armigera larvae by prolonging larval duration at the lower concentration and by retarding growth and disrupting ecdysis at higher concentrations. Owing to the most effective IGR activity, T. indica and E. camaldulensis were selected as promising species for further studies.
2. Ethanol extract of both the plant species proved to have significant biological activity against H. armigera mediated through reduction in larval growth coupled with mild feeding inhibition and oviposition deterrency. T. indica ethanol extract was the most effective growth inhibitor with GI50 values of 0.031 & 0.027%. It also acted as a mild antifeedant to H. armigera larvae having FI50 of 2.816%. The oviposition of moths reduced by 88% at2% concentration. Whereas, E. camaldulensis ethanol extract remarkably reduced the larval growth (GI50 =0.258 & 0.203%) and feeding (FI50= 6.921%), and oviposition of moths by 75%.
3. Active fraction having up to 22 times more efficacy than the ethanol extract of E. camaldulensis and up to 155 times more efficacy than the ethanol extract of T. indica were identified by trying various combination of solvent extraction. Fraction II b of T. indica was found best in terms of growth inhibition that reduces the chances of H. armigera survival (GI50= 0.0002 & 0.0006%; LC50= 0.019%). Alkaloids of T. indica was found one of active group of compounds with GI50 of 0.016 & 0.020%, FI50 of 0.75% and OD (at 1%) of 72% working synergistically with other class of compounds as evident from relative efficacy data. Whereas, ethanol extract of E. camaldulensis leaves are considered active probably due to complex mixture of tannins (GI50= 0.015 & 0.009%; FI50= 2.06%; OD at 1%= 76%).
4. Column chromatography of E. camaldulensis ethanol extract to identify compounds other than tannins identified two fractions with more than 80% growth inhibition following chronic feeding bioassay on neonate larvae. On the basis of these observations it is evident that Tylophora indica and Eucalyptus camaldulensis has growth inhibition as the primary mode of action coupled with mild feeding inhibition and oviposition deterrent activity.
Therefore, these plants do not kill the larvae directly but arrest their growth, reduce their feeding potential by having a broader range of physiological effects that have been found ultimately responsible for Helicoverpa armigera larval inability to survive beyond early instars. Growth retardation at early stage is extremely important for management of H. armigera larvae in field because stunted larvae will do very less damage to the crop and pest larvae impaired by growth inhibition may be forced to spend more time searching for acceptable food, thereby, decreasing their chances of survival owing to natural hazards under field conditions. Further, larval inability to reach late instars will prevent further progeny development thereby minimizing the chances of crop damage due to subsequent generations of the pest. Therefore, it is concluded that these plants may have more practical relevance in insect control programmes.
The study has pointed to spectrum of action against H. armigera under laboratory conditions. The extract or derivatives of T. indica and E. camaldulensis fulfil the biological criteria of efficacy to move successfully ahead. Certainly, identification of the principal active ingredient responsible for the desired activity may provide lead to an entire class of synthetic insecticides. Also, it is worthwhile to screen these extracts firstly for other economically important pests and then judge their efficacy against these pests including H. armigera under realistic field conditions with suitable formulated product. The presence of alkaloids and tannins in crop plants may confer resistance to H. armigera thus may help in breeding H. armigera resistant varieties