Transmission of microsporidia, especially Orthosoma operophterae (Canning, 1960) between generations of winter moth Operophtera brumata (L) (Lepidoptera: Geometridae)

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Abstract
The transmission mechanisms of microsporidia between generations of Operophtera brumata, a geometrid moth with an univoltine life-cycle, have been investigated. The parasites, Orthosoma operophterae, Pleistophora operophterae and Nosema wistmansi were present in moths collected from Wistman's Wood on Dartmoor. An infection rate of 100% was found in eggs laid by moderately or heavily infected female moths. Thus, 100% infection was found in eggs laid by 78 out of 90 infected females. Lower prevalences were found in eggs laid by the remaining 12 and these moths were subsequently shown to have very light infections. Male moths do not pass on this infection to progeny. The development of O. operophterae was followed during embryonation. The microsporidia do not invade the cells of the developing larva but are restricted to the yolk, where spores accumulate in abundance after earlier merogonies. When the larva is mature and ready to hatch, spores are ingested with the remains of the yolk, as the larva eats its way through the egg-shell. Spores are then found in the meconium in the lumen of the gut. These spores are infective and, once the gut of the larva has become functional, are responsible for infecting the tissues of the larvae which harbour them, as well as healthy larvae reared with them. Spores of O. operophterae held in the natural environment for 7 months from winter until mid-summer still retained infectivity and those contaminating the environment from the bodies of one generation must contribute to transmission between generations.