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Estimation of resistance in Helicoverpa armigera (Hubner) to carbaryl and its effect on biology
Munazah Yaqoob, R.K.Arora
and R.K.Gupta
Division of Entomology,
Sher-e-Kashmir University of Agricultural Sciences & Technology-J,
Faculty of Agriculture,
Udheywalla, Jammu-180 002 (India)
ABSTRACT Application of sub-lethal concentrations of carbaryl to the 3rd instar larvae of Helicoverpa armigera up to five generations registered survival at the highest concentration in each successive generation after exposure. The resistance ratio increased up to 4.44-fold as compared to first generation. Out of seven biological parameters studied for carbaryl-selected, parental and susceptible strains, three parameters viz. oviposition period, fecundity and female adult longevity were significantly affected.
KEYWORDS Resistance, Carbaryl, Helicoverpa armigera, Biology
Helicoverpa armigera (Hubner) (Lepidoptera: Noctuidae) is charismatic and one of the most dominant insect pests in agriculture, accounting for the consumption of over 55 percent of total insecticides used in India (Puri,1995).The problem of this pest is magnified due to its direct attack on fruiting structures, voracious feeding habits, high mobility and fecundity, multivoltine, overlapping generations with facultative diapause, nocturnal behaviour, migration, host selection by learning and propensity for acquiring resistance against wide range of insecticides (Satpute and Sarode,1995; Sarode,1999). Loses solely due to this pest up to Rs. 10,000 million have been reported in crops like cotton, pigeon pea, groundnut, sorghum, pearl millet, tomato and other crops of economic importance (Raheja, 1996).
H. armigera has assumed such proportions in India that for the past decade, farmers and plant protection agencies of Central and State governments have virtually become perplexed regarding its control which ultimately has lead to an array of social, economical and political problems. Of these, the major concern is the development of resistance in this pest to almost all known insecticides. H. armigera has, thus, been documented as the best example holding multiple resistant mechanism as well as cross resistance characters attributed to repeated exposures for longer periods to a succession of insecticide groups (Horowitz and Denholm, 2000).Therefore, it became necessary to assess the potential of development of resistance to a widely recommended carbamate insecticide i.e. carbaryl, besides evaluating the comparative fitness costs of this pest.
MATERIALS AND METHODS
Rearing of test insect: A mass culture of H. armigera was
initiated by collecting about 200 larvae from the farmer's fields in and
around Jammu in scintillation vials and brought to the laboratory and
maintained at 26± 2º C in a BOD. The larvae were provided with the leaves
of Rumex sp. to feed upon till they attained pre-pupal stage. After
the emergence of adults, one generation of the pest was reared on artificial
diet (Singh and Rembold, 1992) prior to segregation. After multiplying
the culture in the laboratory for two successive generations, the whole
stock was divided into two lots. One lot was marked as parental stock
where as the other was used for exposure to carbaryl. Hundred pupae of
susceptible strain of H. armigera were also procured from the Division
of Entomology, IARI, New Delhi. These pupae were also reared till the
emergence of adults and thus the culture was further multiplied to be
used in the bioassay studies with the same method as that of field collected
H. armigera.
Preparation of insecticidal concentrations: The proprietary product of carbaryl was used to prepare one percent stock solution in acetone from which further dilutions were prepared subsequently.
Bioassay and Laboratory Selection: Sub-lethal concentrations of insecticide prepared based on the recommended concentration of insecticide as per package and practices of SKUAST-J for the control of H.armigera (Simwat, 1994) were applied to the thoracic dorsum of each 3rd instar larva of susceptible as well as field collected strain of the test insect separately with the help of a Merck micropipette @ 1.0µl per larva. Six to seven concentrations of carbaryl were utilized for exposure in each generation. Besides, a set of control (with acetone only) was also maintained with each exposure to work out the correct mortalities. Ten larvae per replicate were treated with one concentration and three replications were maintained for each concentration. The treated larvae were housed in the Petri plates (9 cm diameter) individually and were kept in B.O.D. incubator (26 ± 2°C; 12: 12 L/D period). The mortality data was recorded 24 hours after the treatment. The survivals obtained at higher concentrations were shifted to clean rearing trays consisting of hundred cells each and provided with fresh artificial diet until pupation. The progeny of the first surviving lot was termed as F1 generation. The exposures and the selections were conducted subsequently up to 5 generations. The parental and susceptible strains were also maintained through out the period of study without any exposure to insecticide to observe the biological parameters.
Quantification of insecticidal resistance: The degree of development of resistance through different generations was determined by working out LC50 values in each generation by computer aided statistical programme SPSS 10.0 and computing the resistance ratio. The resistance ratio for any generation was worked out by dividing LC50 for that generation with LC50 value of the parental strain. The observations on larval mortality were recorded by considering the larvae as moribund when prodded with a fine camel hair brush.
Biological studies: The newly emerged moths from the carbaryl-selected, parental, and susceptible strains were desexed so as to get at least 10 adults of both the sexes. The desexing was done on the basis of morphological characters. These moths were kept in separate glass jars (15 cm diameter) covered with muslin cloth and provided with cotton swab soaked in 10 percent sucrose solution to serve as food to initiate the studies on the various biological and developmental attributes of carbaryl-selected, parental and susceptible strains. Statistical analysis of the data: The mortality data was analysed to work out LC50 value at 95 percent confidence interval by SPSS 10.0. The data on the various parameters of the biology of the insecticide selected, parental and susceptible strains were subjected to analysis of variance with Tukey's HSD test.
RESULTS AND DISCUSSION Third instar larvae of H. armigera were selected for five generations to various discriminating concentrations to obtain maximum mortality at the highest concentration in each generation. The minimum survival of H. armigera larvae obtained in the 1st generation at 0.35 percent concentration (Table 1) was 16.67 percent among all the concentrations used. Similarly, the lowest survival of 16.67, 12.00, 12.00 and 10.00 percent were obtained in the subsequent bioassays up to 5th generation, respectively.
After subjecting the data to probit analysis, the results obtained with the Pearson Goodness of Fit, chi square values revealed that heterogeneity in the population of the test insect was significant (p > 0.05) in the 1st generation which on the other hand, gave homogenous response of the population of H. armigera in the successive generations (p < 0.05) as was evident from the observed and expected responses. The regression lines in the 1st generation (slope 5.99±0.16) were significantly different from the other log concentration - mortality lines obtained in the 2nd to 5th generation (Fig. 1). The percent LC50 value (Table 2) recorded in the 1st generation was 0.17064 which subsequently increased to 0.75796 in the 5th generation and the insect developed 4.44 -fold resistance in the 5th generation as compared to 1st generation. It clearly indicated that the H. armigera has propensity to develop more resistance to carbaryl used on various vegetable crops in Jammu.
Though several workers (Lal, 1998; Patel and Koshiya, 1999; Kapoor et al., 2000) have come out with the report of development of resistance to carbaryl in H. armigera, yet the information is scanty on generation-wise study on the potential of development of resistance in this pest. However, it could be inferred that the potential of resistance development to carbaryl is extremely high in Jammu strain of H. armigera and any increase in the use of this insecticide for the control of this pest in field may lead to future control failures. Though the information in development of resistance to carbaryl in H. armigera is available from the field collected strains, yet no systematic study has been conducted so far to detect the resistance level in the laboratory bred generations. However, Ahmad and McCaffery (1988) observed 28-fold resistance in Thailand strain of H. armigera to carbaryl. Armes et al. (1992) advocated that up to 80 percent carbaryl consumption on cotton was responsible for the multiple cross resistance in this pest to carbamates. 3.36 to 4.83-fold (Satyavani et al. 1991) and 4.83-fold (Mehta et al., 1992) resistance to carbaryl corroborate the findings of the present study. An increase in the resistance level of H. armigera to carbaryl (9.10-fold) had been detected by Patel and Koshiya (1999) in Gujarat which increased to more than 10-fold as observed by Patel et al. (2000). The differential resistance reactions at geographically different places could be attributed to repeated applications of the same insecticides and/or mixing of the different groups of insecticides and/or the quality of the insecticides used (Fakrudin et al., 2003).
COMPARATIVE BIOLOGY Out of seven biological parameters, only three parameters, viz., oviposition period, fecundity and adult-female longevity (Fig.2) had been adversely affected in the carbaryl-selected H. armigera. On the other hand, no significant differences in other parameters were observed in all the three strains of test insect.
Oviposition period: The mean oviposition period of carbaryl-selected and parental strains was 8.15 ± 0.35 and 7.92 ± 0.36 days, respectively, whereas the corresponding figure for susceptible strain was 6.74 ± 0.35 (Table 3).Both the carbaryl-selected and parental strains were significantly different from susceptible strain of Helicoverpa armigera.
So far, the literature lacks any report regarding the effect of carbaryl selection on this parameter. However, the differences in the oviposition period of H.armigera reported by Dong et al.(1996) and Xia et al. (2001) have been attributed probably to the reproductive disadvantages in the resistant female moths caused due to pesticide selection.
Fecundity: The average fecundity of carbaryl selected and parental strains were 313.7 (305-320) and 379.4, respectively (Table 3), whereas, the corresponding values for the susceptible strain was 703.8 (670-719). It is evident from the data that the selection pressure of carbaryl resulted in the considerable reduction of egg laying capacity of H. armigera as compared to susceptible strain. The number of eggs laid by the females of carbaryl selected strain significantly different from that of the parental strain (Fig.3) which revealed that the selection with carbaryl for five generations had ostentatious effect on the fecundity of this insect. Campanhola (1988) recorded a significant reduction in the fecundity of resistant females of H.armigera who produced 1200 eggs each as compared to 2500 eggs produced by one susceptible female. It was reported that the reduced fecundity was the consequence of metabolic resistance to insecticides. Further, it was advocated by various workers (Forrester et al., 1993; Glenn et al., 1994) that the females of this pest have relatively higher tendency for the development of resistance to insecticides. Recently, these observations got further strengthening by striking differences observed on the effective fecundity of resistant females of this pest (Xia et al., 2001) which was found decreased significantly. The results obtained in the present findings are in accordance with the results of these workers.
Adult longevity: The average female longevity of the carbaryl-selected, parental and susceptible strains was 13.70 ± 0.76, 12.77± 0.78 and 11.61± 0.55 days, respectively (Table 3). The significant difference in the female longevity exhibited by carbaryl-selected, parental and susceptible strains of H.armigera might have occurred due to variations in the rearing conditions in the laboratory. Since no information is available in literature on this aspect, it is suggested that repetitive work is required to be undertaken to authenticate the data.
ACKNOWLEDGEMENT: Authors are thankful to Dr.R.M.Bhagat, Professor & Head, Division of Entomology, SKUAST-Jammu for providing facilities to conduct the present work.
REFERENCES
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