MSc

Fresh and dried leaves and seeds of Thevetia neriifolia were evaluated for their antifeedant activity against Henosepilachna vigintioctopunctata, using acetone, benzene, ethanol, hexane, methanol and water as extractants. Based on the percentage of leaf protection, seed extracts were superior to the leaf extracts. Fresh leaf and dried leaf extracts were on par. Among the solvents, ethanol and methanol gave maximum leaf protection closely followed by water. Based on larval starvation, seed extracts were superior to leaf extracts and fresh leaf extracts were significantly better than dried leaf extracts. Water, methanol and ethanol were on par for extracting plant tissues.
Soaking of powered plant material well in solvents for 48 hours and filteration was found as effective as soxhlet method for extracting antifeedant components from leaves and seeds of T. neriifolia.
Leaves and seeds of the plant showed conspicuous hormonal/sterilant activity on Dysdercus cingulatus and the response was dose dependent. This was evident in post treatment larval mortality, prolonged preovipositional period, emergence of malformed adults and reduced fecundity. Adult longevity, incubation period and hatching percentage of eggs were not affected. Forty per cent and ten per cent of leaf and seed extracts as well as 20 and 5 per cent extracts of the same came on par in juvenomimetic effect. The results showed that seeds were four times more effective than the leaves in their juvenomimetic effect on D. cingulatus.
The length of ovary, number of oocytes per ovariole and size of basal, penultimate and antepenultimate oocytes were significantly lower in seed and leaf extract treated females of D. cingulatus. Closer examination of the data revealed that growth of ovary was delayed rather than suppressed by the plant extracts.
Levels of protein, glycogen and lipid contents observed on ovary, fat body and haemolymph of treated and untreated insects from first to fifth day after emergence indicated a delay in vitellogenesis and oocyte development in treated insects. Consequently previposition period was prolonged. The effect observed is similar to the effect of chemosterilants reported on some insects earlier. Results indicated that reduction in fecundity of D. cingulatus may be due to the chemosterilant action of T. neriifolia extracts.
Bioassay studies in the laboratory revealed the low contact toxicity of leaf and seed extracts of T. neriifolia to S. litura and H. vigintioctopunctata. Chromatographic isolation of different fractions of ethanol extracts of fresh leaf and seed yielded six bioactive fractions from the leaf and four from the seed. Fractions III and IV were absent in seeds. Fractions IV, V and VI obtained from leaves and V and VI from seeds had far higher activity than the remaining fractions and they were on par in their antifeedant effect and larval starvation on H. vigintioctopunctata. Available active fractions were more concentrated in seeds than in leaves. Laboratory studies showed the safety of leaf and seed extracts of T. neriifolia at field doses to Chrysocharis johnsoni, the most important parasite of H. vigintioctopunctata in Kerala.
Toxicological studies in albino mice and rats revealed that 400 mg/kg body weight was the LD50 dose of seed extract of T.neriifolia while doses upto 1000mg/kg body weight of leaf extract was not toxic to the animals.
Haematological picture of animals chronically exposed to the extracts showed marginal deviations in the haemoglobin content, RBC count, serum cholesterol, SGOT levels in seed extract treated animals from the permissible safe range. Leaf extract treatment caused deviations in SGOT and serum cholesterol.
Vacuolation, cytoplasmic acidophilia and degenerative changes in the liver and heart tissues and inflammation of kidney were observed in animals treated with seed extract. Adverse effects of leaf extract were negligible.
Replicated field experiments were conducted for two seasons on bittergourd and amaranthus to evaluate the efficacy of crude extracts of T. neriifolia in controlling their major pests. All the extracts viz., ethanol and water extracts of dried and fresh leaves and seed reduced the pest population and the extent of leaf damage. Seed extract was on par with the insecticide check, carbary 1 (0.15 per cent) in bittergourd and malathion (0.1 per cent) in amaranthus. Fresh leaf extract was on par with the seed extract and was superior to extracts of dried leaf in reducing pest population and increasing yield of both the crops. Water extracts of leaves and seed were found equally good for checking the pest population and increasing yield. On cost benefit basis water extracts were found more advantageous to the farmer. Parasitization of egg masses, grubs and pupae in treated plots were on par with control while they were totally missing in the insecticide treated plots. Extracts of T.neriifolia were thus found to be safe to the non – target organisms too.
An overall assessment of the results obtained revealed that water extracts of fresh leaf and seed in appropriate doses can be effectively and safely used for the control of important pests of bittergourd and amaranthus without any health hazards and adverse effect on the non target organisms in the agroecosystem. T. neriifolia can be considered as a plant suited for developing effective plant protection chemicals for replacing undersirable synthetic insecticides, especially in integrated pest control calendar.