A detailed investigation of the feeding behaviour of starved and fed Sitophilus granarius L. of both sexes as well as the oviposition behaviour of granary weevils in response to stored wheat grains, wheat extracts and dummies was undertaken. Male and female granary weevils were found to respond to pentane, acetone or water extracts of stored winter wheat by a definite sequence of behaviour patterns ending up by arrestance and sustained feeding. The occurrence of olfactory sensilla is confined to the tip region of the terminal segment of the antennae of granary weevils. In S. granarius, antennectomy led to a reduced frequency of approaching 103 μg of wheat extract (pentane) which was 70–75% less than the respective parameter in untreated granary weevils of both sexes.
Arrestance and probing of granary weevils were induced by water alone, indicating that the latter is a feeding stimulant. The aqueous extract of wheat was found most active and the pentane extract least active, while the acetone extract of wheat revealed intermediate activity in causing arrestance and probing of male and female S. granarius. The extent of previous feeding has a marked influence on the duration of arrestance of granary weevils on wheat extract. The maximal time of arrestance (probing included) on wheat extract was induced by a starvation period of 120 hours. Female granary weevils were usually more responsive than male granary weevils to wheat extracts of different polarity, offered in levels ranging from 10 to 103 μg.
When exposed to wheat grains (stored for ˜ 1 year), gravid female S. granarius revealed a definite sequence of behaviour leading to oviposition in egg laying cavities followed by sealing of their openings. The process extending from food selection to oviposition was found to take about two hours. Oviposition behaviour of S. granarius was investigated by exposing gravid females to dummies made of white chalk (resembling the shape of wheat grains), which were impregnated with graded levels of wheat extract. Female S. granarius produced significantly more egg laying cavities in grains of stored wheat than in dummies impregnated by either 10−1, 10***0 or 102 grain equivalents of wheat extract, whereas the number of egg laying cavities in wheat grains and dummies was similar when the latter comprised 10 grain equivalents of wheat extract. The formation of egg laying cavities increased from the second through the eighth day on both dummies and wheat grains. Interestingly, antennectomized and intact females laid eggs in similar numbers.
Female granary weevils failed to deposit eggs in egg laying cavities of dummies containing wheat extract, whereas they laid the usual number of eggs in wheat grains. It was concluded that the prerequisites for oviposition of S. granarius include (1) a medium of compressed particles (2) presence of the feeding stimulants available in wheat extract and (3) provision of a diet which is adequate for growth of the offspring. The feeding and oviposition stimulants which accumulate in wheat d...