Influence of methoprene and temperature on diapause termination in adult females of the over-wintering solitary bee, Osmia rufa L.

Females of Osmia rufa, as most species in this genus, enter an obligatory diapause, overwintering as an imago inside a cocoon until the ensuing spring when after emergence – mating, egg development and oviposition occur. Diapause in this species is initiated in November, undergoes 2 months of a pre-wintering period that is terminated at the end of January, after 1 month of maintenance. In this study, factors that affect the termination of adult diapause in the female of this species were investigated. The experimental material consisted of bees that were brought from nests kept in natural conditions 1 month prior to natural termination of diapause. Three different experimental treatments were planned to evaluate the potential effect of methoprene and temperature on diapause termination. During the 5 day experimental period the first group of females was kept at 4 °C, the second group at 15 °C and the last group of females was kept at 20 °C. All groups of females were treated with methoprene topically at a dose of 200 μg. After methoprene application a significant increase (p < 0.001) in the size of terminal oocytes was recorded in the three experimental groups. However, no changes in the size of terminal oocytes between acetone treated and untreated control groups were observed. The number of oocytes progressively increased following topical application of methoprene compared to non-treated or acetone treated females. In successive applications of 200 μg methoprene gradual changes in ovary and fat body protein concentration were observed. As compared to controls, protein content in ovaries isolated from methoprene-treated females increased, whereas it decreased in fat body. The least differences in oocyte size and protein concentration in ovary and fat body between control groups and with methoprene application occurred at 4 °C. Differences increased and were higher in females kept at 20 °C and increased rapidly after methoprene application. Exposure to increasing temperature regimes accelerated the juvenile hormone (JH) induced termination of diapause. Taken together, our results indicate that temperature may play an important role in termination of diapause in O. rufa, but its role is secondary to that played by JH.

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► The influence of methoprene and temperature on diapause termination was investigated in Osmia rufa.
► Methoprene application terminated adult diapause.
► Methoprene stimulated ovarian development and changes in protein content.
► Exposure to increasing temperature regimes accelerated the JH-induced termination of diapause.