A Third Way for Entomophthoralean Fungi to Survive the Winter: Slow Disease Transmission between Individuals of the Hibernating Host.

Jørgen Eilenberg,Lene Thomsen,Annette Jensen

doi:10.3390/insects4030392

Abstract

In temperate regions, insect pathogenic fungi face the challenge of surviving through the winter. Winter is a time when hosts are immobile, low in number or are present in a stage which is not susceptible to infection. Fungi from Entomophthoromycota have so far been known to survive the winter in two ways: either as (1) thick-walled resting spores released into environment from dead hosts, or as (2) structures inside the dead host (e.g., hyphal bodies). Here we report, from the Danish environment, a third way to survive the winter, namely a slow progression and transmission of Entomophthora schizophorae in adult dipteran Pollenia hosts that hibernate in clusters in unheated attics, sheltered areas outdoors (under bark etc.). Fungus-killed sporulating flies were observed outside very early and very late in the season. By sampling adults at the time of their emergence from hibernation in late winter/early spring we documented that the fungus was naturally prevalent and killed flies after a period of incubation. Experimentally we documented that even at the low temperature of 5 °C, the fungus was able to maintain itself in Pollenia cohorts for up to 90 days. From these observations the full winter cycle of this fungus is elucidated. The three types of winter survival are discussed in relation to fungus epidemic development.

Highlights

The life cycles of fungi from Entomophthoromycota are closely associated with the life cycles of their hosts
Our studies provided a full overview of the winter survival strategy of E. schizophorae in its hosts, Pollenia flies, and revealed a, yet undescribed, strategy with slow disease development and transmission between individuals of hibernating adult hosts at cold temperatures
The identity of the fungus killing the Pollenia flies was confirmed by morphology of the primary conidia which fell well within the range for E. schizophorae [2,8]

Summary

Introduction

The life cycles of fungi from Entomophthoromycota are closely associated with the life cycles of their hosts. Resting spores are important in temperate climates for survival during periods when hosts are not present in the susceptible stage and/or temperatures are much too low for a conidial infection cycle to take place. In M. domestica the fungus can be transmitted by conidial infections year-round [11], with rapid cycles at room temperature (lethal time of less than one week) This host-pathogen system is characterized by environmental conditions (stables), which can be buffered against extremes throughout the year, and there is basically no need for specific winter survival structures. An anthomyiid fly that lives outdoors and whose larvae feed on cabbage roots In this host, resting spores instead of conidia are produced late in the summer in order to ensure the winter survival of the fungus [1]. This connection to the life cycle of the host adds a third, previously undescribed way in which a fungus from Entomophthoromycota is able to survive the winter

Experimental Section

Results and Discussion

Conclusions