Potential of entomopathogenic nematodes for the management of weevil pests of banana (musa SP)

Abstract

A study was taken up, to isolate native entomopathogenic nematodes from banana rhizosphere, to standardize artificial media for the wax moth Galleria mellonella L., the trap insect of EPN, to identify cheap medium for the mass production of native EPN, to establish the potential of native isolates of EPN in comparison with the standard cultures by screening and to assess the potential of native isolates for the management of two major weevil pests of banana viz. , pseudostem weevil Odoiporus longicollis Oliv. and rhizome weevil Cosmopolites sordidus Ger. in the laboratory of Department of Agricultural Entomology, College of Agriculture, Vellayani during 2005-06. Native isolates of EPN were collected from banana rhizosphere in six blocks of the Instructional Farm, Vellayani covering an area of 75 ha. Out of the hundred samples processed, ten native isolates were obtained. Based on the mortality of grubs of pseudostem weevil in the laboratory, the native isolates N1 and N2 were selected for further studies. The G. mellonella larvae were fed with standard medium along with cheap media viz., M1, M2, M3 and M4. The result revealed that among the cheap media, maximum survival of larvae was recorded in M1 medium. The cost of preparation of standard medium used for the rearing was high as compared to other three media. The percentage reduction of cost of ingredients of M1, M2 and M3 media from the standard medium was 21.29, 34.87 and 21.67 respectively. Among the three media, minimum reduction in cost of ingredient was computed for M2 medium. But considering the number of larvae survived and mass multiplication rate M1 medium was promising. For mass multiplication of native EPN, the rate of multiplication of infective juveniles was assessed in G. mellonella reared in different cheap media along with standard medium. The result showed that all the EPN (H. indica, S. glaseri, N1 and N2) reared in standard medium recorded maximum emergence of infective juveniles. The studies on the rate of multiplication of native isolates along with standards revealed that initial inoculum of 10 infective juveniles per G. mellonella larva recorded maximum emergence of infective juveniles of all EPN except N2. But the native isolate N2 required an initial inoculum level of 25 IJ per G. mellonella larva for maximum emergence of infective juveniles of EPN. The results of the study revealed that maximum emergence of EPN including native isolates were recorded at 20 days after inoculation rather than 15, 10 and 7 DAI. In order to establish the potential of native isolates on the mortality of grubs and adults of pseudostem and rhizome weevil, trials were conducted in vitro condition and using infested pseudostem and banana rhizome in the laboratory condition. Maximum mortality of grubs and adults of the pseudostem and rhizome weevils were recorded at 72 hrs after treatment. Under in vitro conditions the inoculum required for getting maximum mortality of grubs of pseudostem weevil was 100 IJ per grub, while it was 200 IJ per grub in rhizome weevil. In the case of adult weevils, the inoculum level required to get maximum mortality was 200 IJ per adult in both the cases. But inside the pseudostem and banana rhizome both the grubs of pseudostem and rhizome weevils recorded maximum mortality with 100 IJ per grub. From this investigation it was evident that native EPN strains are available in different agro ecosystems and are more virulent than the standard ones. The rearing media for trap insect, G. mellonella was modified with cheap ingredients and there by reducing the cost of production to 21.29 per cent with out affecting the nutritional status and survival of different instars. Comparative efficacy of native isolates (N1 and N2) and standards were worked out for the management of grubs and adults of weevil pests of banana. An inoculum level of 100 IJ per insect recorded a mortality of above 80 per cent in vitro condition in rhizome and pseudostem weevils but it was reduced to 52 to 56 per cent inside the rhizome and pseudostem. Thus from these studies, mortality of grubs and adults of the weevils inside the banana pseudostem and rhizome required a minimum inoculum level of 200 IJ per insect.