Prabhakaran Vasantha-Srinivasan

Toxicity of Alangium salvifolium Wang chemical constituents against the tobacco cutworm Spodoptera litura Fab.

Widespread use of synthetic pesticides has resulted in the development of insecticide-resistant populations of pests and harmful effects on human health and the environment. There is a need to identify alternative pest management strategies to reduce our reliance on conventional chemical pesticides. In recent years the use of botanical pesticides for protecting crops from insect pests has assumed greater importance. Methanol extract of Alangium salvifolium (L.f.) Wang has potential insecticidal activity against Spodoptera litura Fab. The active fractions were identified through chromatographic techniques as F-IV (Rf value=0.45) and F-VI (Rf value=0.63) and were subjected to GC-MS (GCMATE II). Fifty, 100 and 200ppm of active fractions were applied to fourth instar larvae and the mortality increased with higher concentrations. Relative consumption rate, relative growth rate, efficiency of conversion of ingested food and efficiency of conversion of digested food values all decreased in treated larvae, but approximate digestibility rate increased after treatment. The hydrolytic enzymes, such as acid phosphatase, alkaline phosphatase and the glycolytic enzyme lactate dehydrogenase were inhibited in treated larvae compared with controls. The histopathology study revealed that the epithelial columnar cells were enlarged, completely atrophied; intercellular spaces were swollen, and also noted a cytoplasmic ooze of cell material that mixed with food column. The present study clearly showed the active fractions from A. salvifolium as potential botanicals to control the larvae of S. litura. This is the first report for nutritional indices, enzymatic activities and histological effects of A. salvifolium chemical constituents against S. litura. Thus probably, this will be used as an alternative for synthetic pesticides against the polyphagous pest like S. litura.

Target and non-target toxicity of botanical insecticide derived from Couroupita guianensis L. flower against generalist herbivore, Spodoptera litura Fab. and an earthworm, Eisenia foetida Savigny

Botanical insecticides may provide alternatives to synthetic insecticides for controlling Spodoptera litura (F.) and they are target specific, biodegradable, and harmless to mammals. Eight natural chemical compounds with larvicidal activity were identified from fraction F6 of C. guianensis flower extract. Probit analysis of 95% confidence level exposed an LC50 of 223ppm against S. litura third instar larvae. The growth and development of S. litura was affected in sub-lethal concentrations of fraction F6 (50, 100, 150 and 200ppm) compared to controls. Similarly nutritional indices values decreased significantly compared to controls. Fraction F6 also damaged the gut epithelial layer and brush border membrane (BBM). This study also resolved the effects of toxicity to non-target earthworm treated with fraction F6 and chemical pesticides (monotrophos and cypermethrin) and the results showed that fraction F6 had no harmful effect on E. fetida. Further, fraction F6 was eluted and sub fractions F6c (50ppm) showed high mortality against S. litura third instar larvae. Octacosane from fraction F6c was established and confirmed using IR spectrum and HPLC. The time of retention of fraction F6c was confirmed with the octacosane standard. Fraction F6 of C. guianensis extract caused dose-dependent mortality towards S. litura. Octacosane in fraction F6c was establish to be the prominent chemical compound associated with causing mortality but other compounds present in the fraction F6 were shown to be associated with changes in development of S. litura at low dosages. S. litura at low dosage. Therefore, these findings suggest that octacosane may be one of the major insecticidal compounds affecting S. litura survival.

Anti-dengue efficacy of bioactive andrographolide from Andrographis paniculata (Lamiales: Acanthaceae) against the primary dengue vector Aedes aegypti (Diptera: Culicidae)

The current study investigated the toxic effect of the leaf extract compound andrographolide from Andrographis paniculata (Burm.f) against the dengue vector Ae. aegypti. GC-MS analysis revealed that andrographolide was recognized as the major chemical constituent with the prominent peak area compared with other compounds. All isolated toxic compounds were purified and confirmed through RP-HPLC against chemical standards. The larvicidal assays established at 25ppm of bioactive compound against the treated instars of Ae. Aegypti showed prominent mortality compared to other treated concentrations. The percent mortality of larvae was directly proportional to concentration. The lethal concentration (LC50) was observed at 12ppm treatment concentration. The bioactive andrographolide considerably reduced the detoxifying enzyme regulations of α- and β- carboxylesterases. In contrast, the levels of GST and CYP450 significantly increase in a dose dependent manner. The andrographolide also showed strong oviposition deterrence effects at the sub-lethal dose of 12ppm. Similarly, the mean number of eggs were also significantly reduced in a dose dependent manner. At the concentration of 12ppm the effective percentage of repellency was greater than 90% with a protection time of 15-210min, compared with control. The histopathology study displayed that larvae treated with bioactive andrographolide had cytopathic effects in the midgut epithelium compared with the control. The present study established that bioactive andrographolide served as a potential useful for dengue vector management.

Potential mode of action of a novel plumbagin as a mosquito repellent against the malarial vector Anopheles stephensi, (Culicidae: Diptera).

Plumbagin was isolated and characterized from the roots of Plumbago zeylanica using chromatography: TLC, Column chromatogram, HPLC, FTIR and 1H NMR. The isolated pure compounds were assayed for potency as inhibitors of: acetylcholine esterase (AchE), glutathione S-transferases (GST), superoxide dismutase (SOD), cytochrome P450 and α, β-esterase, and for repellency with Anopheles stephensi at four different concentrations (25, 50, 75 and 100ppm). The enzyme assay against the pure compound reveals that the level of esterase and SOD was decreased significantly in contrast the level of GST and cytochrome P450 was increased significantly. Our results suggests that novel Plumbagin has significantly alters the level of enzyme comparable to the control. Evaluations resulted in Plumbagin producing maximum repellency scores against An. stephensi mosquitoes in dose dependent manner with highest repellence was observed in the 100ppm. Histological examination showed that the midgut, hindgut and muscles are the most affected tissues. These tissues affected with major changes including separation and collapse of epithelial layer and cellular vacuolization. The results support the utility of plant compound Plumbagin for vector control as an alternative to synthetic insecticides, however, more vigorous field trials are needed to determine viability under natural conditions.

Impact of Terminalia chebula Retz. against Aedes aegypti L. and non-target aquatic predatory insects

Aedes aegypti Linn is one of the most important mosquito species. The vectors are responsible for causing deadly diseases like dengue and dengue hemorrhagic fever. Several chemical pesticides used to control these dengue vectors caused severe toxic significances on human health and other non-target beneficial insects. Therefore the current investigation has been made to access the bio-efficacy of the crude seed extracts of T. chebula against the dengue vector Ae. aegypti. The GC-MS analysis of crude seed extracts of T. chebula identified nine chemical compounds with major peak area in the 1,2,3-Benzenetriol (61.96%), followed by Tridecanoic acid (09.55%). Ae. aegypti larvae showed dose dependent mortality rate was observed between the treatments. Prominent protection rate at greater concentrations of 100ppm and moderate protection at 75 and 50ppm was observed in the repellent assay. Lethal concentration (LC50 and LC90) of fourth instar larvae of Ae. aegypti was observed in 138 and 220ppm concentration respectively. Similarly, the seed extracts showed 100% adulticidal activity at the concentration of 400ppm at 30min of exposure time. Phytochemicals present in the seed extracts of T. chebula significantly affects the major portions of the midgut tissues of Ae. aegypti at the concentration of 100ppm. The toxicological evaluation of seed extracts also proved non-toxic towards the A. bouvieri and Tx. splendens aquatic predatory insects. Hence, the present result suggest that bio-rational plant derived T. chebula could be incorporated in the dengue vector control and have no adverse effects on non-target beneficial insects.

Comparative analysis of mosquito (Diptera: Culicidae: Aedes aegypti Liston) responses to the insecticide Temephos and plant derived essential oil derived from Piper betle L.

Resistance to treatments with Temephos or plant derived oil, Pb-CVO, between a field collected Wild Strain (WS) and a susceptible Laboratory Strain (LS) of Ae. aegypti were measured. The Temephos (0.1mg/L) showed the greatest percentage of mosquito mortality compared to Pb-CVO (1.5mg/L) in LS Ae. aegypti. However, WS Ae. aegypti was not significantly affected by Temephos (0.1mg/L) treatment compare to the Pb-CVO (1.5mg/L). However, both strains (LS and WS) when treated with Pb-CVO (1.5mg/L) displayed steady larval mortality rate across all instars. The LC50 of Temephos was 0.027mg in LS, but increased in WS to 0.081mg/L. The LC50 of Pb-CVO treatment was observed at concentrations of 0.72 and 0.64mg/L for LS and WS strains respectively. The enzyme level of α- and β-carboxylesterase was reduced significantly in both mosquito strains treated with Pb-CVO. Whereas, there was a prominent deviation in the enzyme ratio observed between LS and WS treated with Temephos. The GST and CYP450 levels were upregulated in the LS, but decreased in WS, after treatment with Temephos. However, treatment with Pb-CVO caused both enzyme levels to increase significantly in both the strains. Visual observations of the midgut revealed cytotoxicity from sub-lethal concentrations of Temephos (0.04mg/L) and Pb-CVO (1.0mg/L) in both strains of Ae. aegypti compared to the control. The damage caused by Temephos was slightly less in WS compared to LS mosquito strains.

Effect of Aspergillus flavus on the mortality and activity of antioxidant enzymes of Spodoptera litura Fab. (Lepidoptera: Noctuidae) larvae

Insects have developed tolerance against mycoses caused by entomopathogenic fungi through several humoral and cellular mechanisms. Antioxidant enzymes such as superoxide dismutase, lipid peroxidase, and peroxidase can play a role in defense against mycosis, but the physiological interactions between the fungus and the insect are not well characterized. In this study, the effects of infection by entomopathogenic fungus, Aspergillus flavus on the antioxidant defense system of Spodoptera litura, were investigated. The fungi, A. flavus exposure resulted in modification of the levels of antioxidant enzymes, as well as significant decline in phenoloxidase titers and the total hemocyte count 48 h post exposure. A significant increase was observed in detoxifying enzymes. All these results suggest that A. flavus infects S. litura by directly acting on the immune system, resulting in decreased immune function. Bioassay results showed that A. flavus affects third and fourth instar larvae of S. litura. This report supports the importance of A. flavus as a candidate for biological control of S. litura.