Abstract: 【Aim】 In the present study, we assessed the functional response of the 4th instar larvae of a coccinellid species, Menochilus sexmaculatus (Fabricius) on aphid species, Aphis craccivora Koch and Lipaphis erysimi (Kaltenbach) under prey fluctuating conditions. L. erysimi is a defended prey that contains allyl isothiocyanates. We hypothesized that under experimental prey conditions, larvae reared on optimal/abundant prey would show better functional response than scarce prey reared larvae. But the slope of the functional response curves would be lower on L. erysimi than on A. craccivora. 【Methods】 The 4th instar larvae of M. sexmaculatus were abruptly shifted from their rearing conditions (viz. scarce/optimal/abundant prey) to different experimental conditions (i.e., extremely scarce/scarce/sub-optimal/optimal/abundant prey) and their functional response curves were extrapolated. 【Results】 The results revealed that despite prey resource fluctuations, the 4th instar larvae of M. sexmaculatus exhibited the type-II response on A. craccivora, but a modified type-II response on L. erysimi. Further, irrespective of prey species, the 4th instar larvae that were reared on optimal/abundant prey exhibited normal predation rates under five experimental conditions. But, the 4th instar larvae reared on scarce prey compensated for a shortage of food by consuming higher prey biomass than usual when suddenly switched over to the optimal/abundant prey experimental conditions. However, they exhibited the highest prey consumption on abundant prey and the lowest on extremely scarce prey experimental conditions. Moreover, higher attack rate but lower prey handling time on A. craccivora were recorded. 【Conclusion】Our results suggest that fluctuations in the availability of L. erysimi may modify the functional response curves of M. sexmaculatus larvae. However, A. craccivora is more suitable as a prey for M. sexmaculatus than L. erysimi.    

Key words: Ladybird, aphid, biocontrol, functional response, resource fluctuation