Bitrophic Effects of Artificial Diets of American Bollworm (Helicoverpa armigera H.) on Different Biological Aspects of Bracon Hebetor Say

Abstract

Helicoverpa armigera (Hubner) is a notorious pest of various fields and horticultural crops worldwide. Insecticides application ultimately responsible for resistance development in H. armigera, unsafe for the environment and natural enemies of insect pests. To withstand the pesticides resistance, biological control successfully helps in reducing pest population. Bracon hebetor is a cosmopolitan species that have the ability to parasitize a wide range of lepidopteran and few coleopteran hosts. As H. armigera fed on wide host range so, quality and quantity of food nutrients provided at the time of Bracon hebetor larval development is a crucial step. The need of the current research project is to focus on which best suited semi-synthetic artificial diet of H. armigera; B. hebetor developmental parameters are good and further, how it can effectively be used under the scenario of ecological conditions after getting reared on efficient host for the control of H. armigera. So we can utilize B. hebetor as an efficient biological control agent for H. armigera management. Study objectives were to screen out the best suited artificial diet of H. armigera for the rearing of B. hebetor. Investigation of B. hebetor life history traits on most efficient hosts from the screened host species for the effective rearing of B. hebetor. Determination of the effect of temperature, relative humidity regimes and host larval densities on the life history traits of B. hebetor parasitizing H. armigera. B. hebetor parasitization, reproduction, development, fitness related traits and life table parameters were observed on different pyralids, noctuids, gelechiids and curculionids under laboratory conditions (25±2°C and 70±5%) in order to find out the most suitable host for the mass rearing and inundative release purposes to control H. armigera. Results indicated that among four semi-synthetic diets (gram, rice, wheat and corn) of H. armigera, gram flour mediated diet was responsible for good biological and developmental parameters of B. hebetor under laboratory conditions. It can be successfully used for the rearing of H. armigera as a host for good parasitism and development of B. hebetor. Though, H. armigera itself was not an excellent host for the continuous rearing of parasitoid therefore, seven different hosts (six from Lepidoptera and one Coleoptera) were utilized to screen out the suitable one. Results showed that the pyralids hosts (E. kuehniella, C. cephalonica and G. mellonella) gave the best results for daily fecundity, parasitoid survival to adulthood, development time and female biased sex ratio followed by H. armigera and the worst reproduction occurred on H. postica. Among the most efficient screened pyralid hosts, B. hebetor biological parameters and fitness related traits were excellent on E. kuehniella as compared to C. cephalonica. So production of good fitness traits (wing area, tibia size and dry mass), excellent biological parameters (good immature numbers and adults emergence), short egg-adult development time and changes in THCs provide a good means for the immature development of B. hebetor within the parasitized E. kuehniella for several days. Temperature relative humidity regimes and H. armigera larval densities significantly affect the biological and developmental parameters of B. hebetor. Excellent life history parameters of B. hebetor parasitizing gram diet fed H. armigera were observed at 25°C and 60% R.H. combination. Among various larval densities of H. armigera good biological parameters of B. hebetor like production of immature numbers, short egg-adult development time, survivorship and female progeny production was significantly good on five larval density of H. armigera offered for parasitization. Therefore, it is recommended that gram mediated diet is best suited for the rearing of H. armigera in order to find out the best suited temperature, humidity combination and host larval density in the field release planning and maintenance of B. hebetor population for long term field crops management to control H. armigera. On the other hand B. hebetor quality progeny production on E. kuehniella make this insect of great potential to serve as valuable material for laboratory rearing and inundative field release to control H. armigera larval populations.