Black soldier fly larvae as hosts for entomopathogenic nematodes and its assessment against Odoiporus longicollis (Olivier)

Abstract

The study entitled “Black soldier fly larvae as hosts for entomopathogenic nematodes and its assessment against Odoiporus longicollis (Olivier)” was conducted at the Department of Entomology, College of Agriculture, Vellayani during 2021-2023. The main objective of the study was assessment of the pathogenicity of the entomopathogenic nematode, Heterorhabditis bacteriophora (Poinar) multiplied in BSFL against various pests in-vitro and its evaluation against banana pseudostem weevil, O. longicollis. Pathogenicity of H. bacteriophora from BSFL to grubs of banana pseudostem weevil, rhinoceros beetle, hadda beetle and larvae of tobacco caterpillar was assessed with inoculum levels of 10, 50, 100, 200 and 250 IJs per insect. The concentrations viz., 200 and 250 IJs per insect recorded 100 per cent mortality at 60 and 72 hours after treatment (HAT) respectively in pseudostem weevil grubs and at 48 HAT in hadda beetle grub and the highest mortality of 88.89 per cent at 96 HAT in rhinoceros beetle grubs. 100 per cent mortality was observed with concentration of 100 and 250 IJs per insect at 24 HAT in tobacco caterpillar. The mortality percentage increased with increase in infective juvenile concentration and increase in exposure period in all the insects tested. This study systematically assessed the pathogenicity of H. bacteriophora from black soldier fly larvae against various insect pests, offering insights into the potential of these entomopathogenic nematodes for biocontrol purposes in agriculture. Under field evaluation, the cadavers were applied as per schedule of treatments in the outer four leaf axils of plants during 5, 6 and 7 months after planting after inoculation with grubs of pseudostem weevil. During the first application, the bore hole were lowest at 10 and 21 days after treatments in 3 BSFL cadavers (12.08 and 13.17 respectively) and 1 Galleria cadaver per leaf axil (11.92 and 13.08 respectively). Similar trend was observed during the second and third application of treatments wherein 3 BSFL cadavers per leaf axil and 1 Galleria cadaver per leaf axil were recorded least bore holes (21.67 and 22.17 respectively) before application of second treatment and during third application, it was 29.08 and 31.75 respectively. After 10 days and 21 days too these treatments recorded a similar trend after second application and after third application, the lowest bore hole count was with 3 BSFL cadavers per leaf axil, however it was statistically on par with 2 BSFL cadavers and 1 Galleria cadaver per leaf axil. The results of the study suggested that a decrease in borehole counts in banana plants treated with H. bacteriophora infected cadavers compared to control plants, indicating successful pest management. Destructive sampling was carried out seven days after last application and mortality of insect was recorded. The plants treated with 2 BSFL cadavers per leaf axil recorded significantly higher mortality of grubs (72.31%) which was found statistically on par with 3 BSFL cadavers per leaf axil (70.47%) and 1 Galleria cadaver per leaf axil which caused 69.95 per cent mortality of insects. There was no significant difference between the treatments in the biometric observations of plant such as basal girth and girth at 1m height. Highest pseudostem height was observed in the plants treated with, 2 BSFL cadavers per leaf axil (182 cm) followed by 3 BSFL cadavers leaf axil (181 cm) and control plants recorded least pseudostem height (169.50 cm). Leaf length was highest in the plants treated with 1 Galleria cadaver per leaf axil (167 cm) and was on par with 2 and 3 BSFL cadavers per leaf axil and the least in control plants (149.25 cm). The plants treated with 1 BSFL cadaver per leaf axil recorded highest leaf breadth (60.92 cm) which was least in control plants (54.50 cm). Leaf area was also recorded highest in the plants treated with 1 BSFL cadaver per leaf axil (7457.71 cm2) compared to control plants (6149.26 cm2). Biometric observations showed positive effects on plant growth parameters, affirming that the pest control strategy positively influenced overall plant health. The result of the study revealed the potential of utilizing BSFL as hosts for entomopathogenic nematode, H. bacteriophora and the efficacy of these nematodes from BSFL cadaver to cause pathogenicity in the grubs of pseudostem weevil, rhinoceros beetle, hadda beetle and larvae of tobacco caterpillar under in vitro condition. This can be recommended for the management of pseudostem weevil grub as leaf axil application of H. bacteriophora infected BSFL cadaver @ 2 cadavers per leaf axil at 5, 6 and 7 months after planting. The outcomes of this research could contribute to the development of eco friendly strategies for pest management in agriculture.