Etiology and pathogenesis of chalkbrood in the alfalfa leafcutting bee, Megachile rotundata Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/41687m103

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  • Chalkbrood is a mycosis of larvae of the alfalfa leafcutting bee, Megachile rotundata (Fabricius). It is a serious threat to alfalfa seed growers in western North America who use populations of this bee for pollination. Basic studies were conducted to determine the etiology of this disease and the course of pathogenesis in afflicted larvae. Spores of Ascosphaera aggregata Skou were found to cause chalkbrood in one- to nine-day-old larvae of M. rotundata. A distinctive syndrome accompanied infection of older larvae. The host hemolymph became cloudy with fungal mycelium at the time of death. A change from normal cream color to pink or tan occurred in larvae soon thereafter. Sporulation beneath the host cuticle took place within 2 weeks after death. Younger larvae did not undergo a color change but remained cream colored and the fungus did not sporulate. Ascosphaera aggregata spores germinated in the midgut and penetrated the hemocoel of fourth instar larvae within 2 days after inoculation. Invasion of epidermis, tracheae and muscles followed within 3 more days. The entire hemocoel was nearly filled with mycelium at the time of death, 3 - 7 days after inoculation. Conditions of reduced redox potential existed in the midguts of fourth instar larvae. A. aggregata spores germinated in vitro under conditions of reduced potential but mycelium proliferated under the oxidized conditions of an open petri dish. Events in vivo reflected preferences shown in vitro: germination occurred in the reduced midgut and mycelium proliferation occurred in the more oxidized tissues of the hemocoel. Other Ascosphaera species were able to infect M. rotundata larvae. Spores of both A. apis (Maassen ex Claussen) Olive and Spiltoir and A. proliperda Skou caused a syndrome distinct from that caused by A. aggregata. No color changes were evident following infection by A. apis or A. proliperda. White mycelia erupted through the cuticle within 2 days after death. Sporulation outside the host cuticle followed within 1-2 weeks. The time to death was significantly shorter for larvae infected with A. proliperda than for those infected with A. apis. Spores taken from cadavers grew less readily on the pollen bee diet and appeared to be more virulent than spores produced in vitro. Ascosphaera atra Skou and Hackett and A. major (Prokschl and Zobl) Skou did not infect larvae but grew saprophytically on the pollen diet.
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