In vivo mass production of nosema bombycis spores and their efficacies against diamondback moth and beet armyworm larvae in laboratory connnditions

Abstract

Diamondback moth (DBM), Plutella xylostella and beet armyworm (BA W), Spodoptera exigua are major insect pests of crucifer crops found worldwide. Since these pests are highly resistant to many chemical insecticides, using biological control agent(s) or biopesticides might be a good choice to overcome this problem. Nosema bombycis is a known microsporidial pathogen with the potential to control insect pests, however it needs to be produced massively in order to function as an alternative to chemical insecticides. This study was aimed at mass producing N. bombycis ???spores in vivo and to test their efficacies against DBM and BA W in laboratory conditions. Mass production of this pathogen was carried out by feeding both DBM and BA W larval ins tars with artificial diet inoculated with viable spores of N. bombycis. Spore concentrations in the larvae were determined every 24 hours. The spores produced in each cycle were purified alld accumulated until a concentration of 1.56 x 108 spores/ml were obtained and later kept in liquid nitrogen until further use. For laboratory efficacy tests, artificial diets with differem spore concentrations namely iao'. Ixl05 and lxl 06 spores/ul were fed to different larval ins tar groups with four replicates per concentration. Mortality rates were observed recorded and analysed using the Two-way ANOVA test. For DBM, the highest mean mortality rate (46.16%) was caused by the highest spore concentration (lxJ06 sporeslpl, and the instar with the highest mean mortality rate was instar 3 (46.55%). Mortality rates caused by the different spore concentrations were significantly different from one another (F=1l.46, P=O.OO) and so did the mortality rates in different ins tars (F=32.10, P=O.OO) However, there was no interaction between both factors, where a combination of spore concentrations and ins tars did not show any significant differences (F=0.70, P=0.598) For BA W, the highest mean of mortality rate (32.91%) was also caused by the highes spore concentration and the ins tar with the highest mean of mortality was also ins tar 3 (40.98%). Mortality rates caused by the different spore concentrations were significantly different from one another (F= 10.80, P=O.OO) and also did the mortality rates in differen instars (F=27.29, P=O.OO). However, there was no interaction between both factors where a combination of spore concentrations and instars did not show any signtf/can differences (F=0.38, P=0.884). These results indicated that N. bombycis spores prepared in vivo caused more deaths in DBM larvae than in BA W larvae and that larval instal' 3 fa both species and that I x 106 spores/ 'ul treatments demonstrated the highest efficacy rates.