Role of Physical and Chemical Stimuli of Legume Host Seeds in Comparative Ovipositional Behaviour of Callosobruchus Maculatus (Fab.) and C. Chinensis (Linn.) (Coleoptera: Bruchidae)

Abstract

In India, pulses like Cajanus cajan (pigeon pea), Vigna radiata (mung bean), Cicer arietinum (chickpea) and Vigna mungo (Urd bean) are split into two halves known as “Daal” and have been stored in this manner since time immemorial. Split pulses with or without seed coats thus stored afford excellent protection against bruchid attack. This fact, coupled with the reported observations that C chinensis exhibits a preference for curvature and observations that aqueous fractions of the seed coats of Phaseolus vulgaris and Vigna unguiculata contain oppositional stimulant activity for C maculatus, formed the basis for the present investigation. We assessed the role of physical (curvature) and chemical stimuli (aqueous and/or methanol fractions of cowpea seed coat) in a quantitative and on a comparative basis for ovipositional preference of C. chinensis and C. maculatus. For standardization of the bioassay, several natural and artificial oviposition substrates were tested and glass beads with 6 mm diameter were found most suitable. The concentration of the stimulant material was expressed in terms of bean equivalent. The oviposition responses of gravid C. maculatus revealed that the curvature of oviposition substrate does not have any role in preferential oviposition and that the gravid females are solely guided by a chemical stimulus perceived from the oviposition substrate. Whereas for C. chinensis a physical stimulus (curvature) is a prerequisite for normal oviposition. However, once the requirement for the physical stimulus is met, then the chemical stimulus alone exerts its influence on oviposition. The chemical stimulus alone does not trigger normal ovipositional response.