Keshav R. Kranthi

Insecticide resistance in five major insect pests of cotton in India

Insecticide resistance to representatives of commonly used insecticide groups (pyrethroids—cypermethrin; organophosphates—chlorpyriphos; cyclodienes—endosulfan) was determined in five major insect pests of cotton from the main cotton growing regions of India with emphasis on Andhra Pradesh and Maharashtra. The cotton bollworm Helicoverpa armigera (Hubner) exhibited widespread resistance to cypermethrin with 23–8022-fold resistance being recorded in field strains. Resistance to endosulfan and chlorpyriphos was low to moderate in H. armigera. The overall resistance of the pink bollworm Pectinophora gossypiella (Saunders) to pyrethroids was low. However, high resistance levels of 23–57-fold to endosulfan were recorded in some areas of Central India. Resistance to chlorpyriphos was high in the Medak, Bhatinda and Sirsa strains from North India. The majority of the Spodoptera litura (Fab.) strains collected in South India exhibited high resistance levels of 61–148-fold to cypermethrin. Resistance to endosulfan was high only in two strains, collected from Bhatinda and Karimnagar in North India. The S. litura strains from South India exhibited high levels of resistance at 45–129-fold to chlorpyriphos. Insecticide resistance in Earias vittella (Fab.) was low to moderate in the Sirsa and Sriganganagar strains from North India. Bemisia tabaci (Genn.) exhibited moderately high levels of resistance to cypermethrin, but resistance to endosulfan and chlorpyriphos was negligible in the field strains tested. The implications of resistance for cotton pest management in India are discussed.

Pyrethroid Resistance and Mechanisms of Resistance in Field Strains of Helicoverpa armigera (Lepidoptera: Noctuidae)

Abstract Pyrethroid resistance was found in 54 field strains of Helicoverpa armigera collected between 1995 and 1999 from 23 districts in seven states of India. LD50 values of the field strains ranged from 0.06 to 72.2 μg/larva with slopes of 0.5–3.1. Resistance was highest in regions where pyrethroid use was frequent (four to eight applications per season). Resistance to deltamethrin was exceptionally high with resistance ratios of 13,570 and 27,160 in two strains collected during February 1998 in central India. Resistance to cypermethrin, fenvalerate and cyhalothrin also was high with resistance ratios of >1,000 in four strains collected from central and southern India. Resistance ratios were below 100 in >50% of the strains tested. Pyrethroid resistance was high in strains collected from the districts in Andhra Pradesh where a majority of the cotton farmer suicide cases in India were reported. Resistance to pyrethroids appeared to have increased over 1995–1998 in most of the areas surveyed. Studies carried out through estimation of detoxification enzyme activity and synergists indicated that enhanced cytochrome p450 and esterase activities were probably important mechanisms for pyrethroid resistance in field strains. Pyrethroid nerve insensitivity also was found to be a major mechanism in some parts of the country where the use of pyrethroids was high. The information presented illustrates the importance of proper insect management programs to avoid the consequences associated with improper insecticide use.

Mitochondrial DNA analysis of field populations of Helicoverpa armigera (Lepidoptera: Noctuidae) and of its relationship to H. zea

BackgroundHelicoverpa armigera and H. zea are amongst the most significant polyphagous pest lepidopteran species in the Old and New Worlds respectively. Separation of H. armigera and H. zea is difficult and is usually only achieved through morphological differences in the genitalia. They are capable of interbreeding to produce fertile offspring. The single species status of H. armigera has been doubted, due to its wide distribution and plant host range across the Old World. This study explores the global genetic diversity of H. armigera and its evolutionary relationship to H zea.ResultsWe obtained partial (511 bp) mitochondrial DNA (mtDNA) Cytochrome Oxidase-I (COI) sequences for 249 individuals of H. armigera sampled from Australia, Burkina Faso, Uganda, China, India and Pakistan which were associated with various host plants. Single nucleotide polymorphisms (SNPs) within the partial COI gene differentiated H. armigera populations into 33 mtDNA haplotypes. Shared haplotypes between continents, low F-statistic values and low nucleotide diversity between countries (0.0017 – 0.0038) suggests high mobility in this pest. Phylogenetic analysis of four major Helicoverpa pest species indicates that H. punctigera is basal to H. assulta, which is in turn basal to H. armigera and H. zea. Samples from North and South America suggest that H. zea is also a single species across its distribution. Our data reveal short genetic distances between H. armigera and H. zea which seem to have been established via a founder event from H. armigera stock at around 1.5 million years ago.ConclusionOur mitochondrial DNA sequence data supports the single species status of H. armigera across Africa, Asia and Australia. The evidence for inter-continental gene flow observed in this study is consistent with published evidence of the capacity of this species to migrate over long distances. The finding of high genetic similarity between Old World H. armigera and New World H. zea emphasises the need to consider work on both pests when building pest management strategies for either.

Influence of foliar chemical compounds on the development of Spodoptera litura (Fab.) in interspecific derivatives of groundnut

Abstract:  Tobacco armyworm, Spodoptera litura (Fab), a polyphagous insect, is an important pest of groundnut (Arachis hypogaea L). It is one of the insect pests which had developed resistance to insecticides. Currently there are no cultivars of groundnut which express high level of resistance to S. litura. Wild species of groundnut, which show high levels of resistance, have been identified. Arachis kempff‐mercadoi is one such wild species, which is reported to be resistant to S. litura, and indicated that in wild species three flavonoids chlorogenic acid, quercetin and rutin are involved in the components of resistance. In the present study, although these flavonoids had an effect on larval mortality, statistical analysis revealed that quercetin had a major effect due to high correlation of quercetin with chlorogenic acid and rutin. Interspecific derivatives were obtained as a result of crossing cultivated groundnut with A. kempff‐mercadoi. In vitro studies showed high percentage of neonate larval mortality when fed on the foliage of interspecific derivatives, majority of interspecific derivatives were detrimental to larval development and had considerable effect on its subsequent progeny. Resistant derivatives were found to have high levels of flavonoids and antibiosis mechanism prevented larval growth. Susceptible derivatives and the female parent, A. hypogaea have low levels of flavonoids.

Seasonal Dynamics of Metabolic Mechanisms Mediating Pyrethroid Resistance in Helicoverpa armigera in Central India

Very high cypermethrin and fenvalerate resistance frequencies were recorded in Helicoverpa armigera (Hiibner) populations in central India during the 1993-94, 1994-95 and 1995-96 cropping seasons. Synergism assays and biochemical analyses of detoxification enzyme levels indicated that mono-oxygenases and esterases were important metabolic mechanisms mediating pyrethroid resistance. Piperonyl butoxide- (PBO) and profenofos-suppressible pyrethroid resistance were correlated with enhanced levels of cytochrome P450 and general esterases respectively. Enzyme assay data indicated that high cytochrome P450 levels generally coincided with low esterase activity and vice versa. Similarly, synergist bioassays showed that PBO-insensitive resistance was frequently associated with profenofos-sensitive resistance and vice versa. Oxidase- and esterase-mediated mechanisms evidently alternated in a reciprocal manner, with perceptible shifts in relative importance occurring during mid-October in all three seasons and in late January in 1995. Apart from metabolic mechanisms, a synergist-insensitive resistance mechanism (believed to be nerve insensitivity), accounted for an average of 51, 30 and 28% of cypermethrin resistance during the 1993-94, 1994-95 and 1995-96 seasons respectively.

Influence of semilooper damage on cotton host-plant resistance to Helicoverpa armigera (Hub)

Abstract The semilooper, Anomis flava Fab is a foliage feeding lepidopteran insect, often found occurring in the cotton ecosystem during the first 60–75 days of sowing. Its occurrence overlaps or just precedes the incidence of Helicoverpa armigera . This study reports the impact of mechanical wounding or of prior herbivory by semilooper larvae on cotton host-plant resistance to H. armigera. It is proposed that cotton plant damaged by semilooper or by mechanical means is less preferred for the growth of H. armigera. This is probably due to a reduction in the nutritive quality, induction of allelochemicals, such as chlorogenic acid and of oxidative enzymes, such as lipoxygenases and peroxidases in the damaged plants.

In-season changes in resistance to insecticides in Helicoverpa armigera (Lepidoptera: Noctuidae) in India

Abstract Discriminating doses of fenvalerate, cypermethrin, quinalphos, and endosulfan were determined with an insecticide-susceptible Helicoverpa armigera (Hübner) strain. In-season changes in insecticide resistance were monitored with discriminating dose assays at weekly intervals throughout the cropping season for 6 yr from 1993 to 1999 in central India. Resistance to pyrethroids was high throughout all seasons. Resistance to 0.75 μg of quinalphos was consistent, with seasonal averages ranging from 23 to 27% survival over the 6 yr. Resistance to 10.0 μg of endosulfan was moderately high at an average of 40–47% survival during 1993–1994 and in 1997–1998. It was lower in 1996–1997 at 27%, and in 1998–1999 at 33%. The weekly monitoring data for all seasons were pooled and the consolidated 6-yr seasonal average profile indicated that resistance to quinalphos and endosulfan was low during September at 21 and 27% survival, respectively, but increased to 28 and 37% by the end of November. Resistance levels to organophosphates and endosulfan increased during the season, depending on the use of these compounds. At almost all monitoring sites, the within-season changes in quinalphos resistance for all seasons through the study period followed a trend similar to that of endosulfan. The results suggest the possibility of cross-resistance between these compounds. Based on this study and the existing information on cotton pest management, we have developed a “window strategy” for cotton pest management with specific emphasis on the management of insecticides for effective control of H. armigera. This strategy has contributed to improved control at reduced costs in extensive trials.

Population Genetic Structure of the Cotton Bollworm Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae) in India as Inferred from EPIC-PCR DNA Markers

Helicoverpa armigera is an important pest of cotton and other agricultural crops in the Old World. Its wide host range, high mobility and fecundity, and the ability to adapt and develop resistance against all common groups of insecticides used for its management have exacerbated its pest status. An understanding of the population genetic structure in H. armigera under Indian agricultural conditions will help ascertain gene flow patterns across different agricultural zones. This study inferred the population genetic structure of Indian H. armigera using five Exon-Primed Intron-Crossing (EPIC)-PCR markers. Nested alternative EPIC markers detected moderate null allele frequencies (4.3% to 9.4%) in loci used to infer population genetic structure but the apparently genome-wide heterozygote deficit suggests in-breeding or a Wahlund effect rather than a null allele effect. Population genetic analysis of the 26 populations suggested significant genetic differentiation within India but especially in cotton-feeding populations in the 2006–07 cropping season. In contrast, overall pair-wise F ST estimates from populations feeding on food crops indicated no significant population substructure irrespective of cropping seasons. A Baysian cluster analysis was used to assign the genetic make-up of individuals to likely membership of population clusters. Some evidence was found for four major clusters with individuals in two populations from cotton in one year (from two populations in northern India) showing especially high homogeneity. Taken as a whole, this study found evidence of population substructure at host crop, temporal and spatial levels in Indian H. armigera, without, however, a clear biological rationale for these structures being evident.

Baseline toxicity of Cry1A toxins to Helicoverpa armigera (Hubner) (Lepidoptera: Noctuidae) in India

The baseline toxicity of Cry1A δ-endotoxins (derived from Bacillus thruringiensis (Kurstaki)) on field populations of the cotton bollworm, Helicoverpa armigera (Hub.), was determined through log dose probit analysis. All the three Cry1A proteins were found to be toxic to the bollworm larvae. Cry1Ac was found to be the most toxic followed by Cry1Aa and Cry1Ab. LC 50 values ranged from 0.07 to 0.99 μg/ml (14-fold) for Cry1Aa, 0.69 to 9.94 μg/ml (14-fold) for Cry1Ab and 0.01 to 0.67 μg/ml of diet (67-fold) for CrylAc. The LC 50 values deduced from the cumulative log dose probit response of the data pooled from all assays, were 0.62 μg/ml for Cry1Aa, 4.43 μg/ml for Cry1Ab and 0.100 μg/ml of diet for Cry1Ac. The respective LC 50 values represent the baseline susceptibility indices for resistance monitoring through the conventional log dose probit assays. The LC 99 values derived from the cumulative data were 515 μg/ml for Cry1Aa, 13385 μg/ml for Cry1Ab and 75 μg/ml of diet for Cry1Ac. These values represent the diagnostic doses for routine monitoring of resistance to the respective toxins through discriminating dose assays after the introduction of Bt transgenic crops.

Changing Trends in Cotton Pest Management

The cotton crop sustains more insects than any other crop grown commercially world-wide. Any single insecticidal intervention to control a particular pest invariably sets up a chain reaction causing short-term imbalances in the ecosystem, mostly in favour of the pest in the long run. Thus over the years, insecticide use was establishing undesirable ecological and economic consequences for cotton cultivators and administrators in many countries. Individual insecticide molecules when first introduced have always been impressive in their rapid efficacy in controlling target insect pests. As long the target pests are effectively controlled with the pesticide, cultivators do not care for the naturally occurring predators and parasites in their ecosystems. Unfortunately almost all the insecticides have inadvertent adverse effects on naturally occurring beneficial insects. However, phytophagous target pests usually develop resistance much faster than entomophages, thereby causing pest populations to survive the pesticide, increase in numbers in the absence of natural control, and so generate outbreaks. The cotton crop has been subjected to more pesticide exposure than any other crop, in all cotton growing countries of the world. Intense insecticide use has resulted in insect resistance to insecticides, pesticide residues, and the resurgence of minor pests causing immense problems to cultivators. With the most reliable tools turning redundant, pest management experts started exploring the utility of naturally occurring pest control components as alternatives to replace the chemical insecticides. Thus, Integrated Pest Management (IPM) programs began to take shape as ‘intelligent selection and use of pest management tactics which results in favorable ecological, sociological and environmental consequences’ as defined by Rabb (1972). Insecticide Resistance Management (IRM) strategies have strengthened pest management systems by identifying appropriate insecticides, rates and timings so as to delay resistance, ensure effective control of target pests, and conserve naturally occurring biological control for enhanced sustainability of ecosystems. With the recent introduction of Bt-cotton, novel eco-friendly pesticides and IRM strategies, coupled with the trends in technology dissemination through area-wide farmer participatory approaches and farmer field schools, IPM programs all over the world have improved their sustainability and economic success.