OSMIA CORNUTA LATR (HYM, MEGACHILIDAE) AS A POTENTIAL POLLINATOR IN ALMOND ORCHARDS - RELEASING METHODS AND NEST-HOLE LENGTH

Three Osmia cornuta Latr. populations were released in each of three almond orchards near Reus (NE Spain). In each orchard, bees were offered the same choice of paper straws of four different lengths (12, 15, 18, and 21 cm) inserted in milk-carton nesting materials. Releasing methods used differed between orchards: bees of orchard A were released within their natal straws; bees of orchard B were released by inserting individual cocoons in new straws; bees of orchard C were mass-released in emergence boxes. Nesting bees at each orchard were counted, and the results showed that significantly more females dispersed in orchard C (79.10%), than in orchard B (51.04%) or orchard A (28.34%). Females preferred to nest in longer straws (15 to 21 cm) than in 12 cm straws in the three orchards. In general, the number of cells built per nest increased, and sex ratio (males/females) of progeny decreased with straw length. Previously occupied straws were preferred to previously unoccupied straws in both orchards A and B. Comparison of these results with results obtained in other studies indicates that O. cornuta females select new holes when attractive nesting materials are offered, and previously occupied holes when less attractive nesting materials are available.