Abstract text (incl. figure legends and references)

Introduction

For biological control of economically important herbivorous insects, entomopathogenic fungi are efficiently used. They are established as an eco-friendly control tool in organic farming. Products of Metarhizium brunneum are successfully used against several pest insects, such as Western Corn Rootworm (Diabrotica virgifera), Japanese Beetle (Popillia japonica), Black Wine Weevil (Otiorhynchus sulcatus), and thrips (Frankliniella occidentalis) (https://ec.europa.eu/food/plant/pesticides/eu-pesticides-database/active-substances/?event=search.as). This fungus can be produced on selective solid and liquid artificial media, resulting in different spore types. On solid media, areal conidia are formed, while in liquid media often so-called blastospores are produced.

Objectives

Conidia and blastospores differ in their physiology, e. g. germination of blastospores is much faster compared to conidia. Ultrastructure of both spore types was investigated in transmission electron microscopy to elucidate differences in their metabolism and will be presented in several micrographs.

Materials & methods

For best results of observing ultrastructure of conidia and blastospores, the two spore types had to be fixed and embedded in different ways. Conidia were fixed in 3.5% glutaraldehyde, 2% osmium tetroxide and embedded in Methacrylate, while blastospores were fixed in 4% glutaraldehyde, 2% osmium tetroxide and embedded in Epon. Materials and methods protocol with detailed information will be provided.

Results

Studies on the ultrastructure of the two spore types illustrated that blastospores are in a physiologically active state and therefore capable of immediate germination, while conidia are in a kind of dormant condition. In contrast to conidia, blastospores have numerous mitochondria, many ribosomes and endoplasmic reticulum as well as densely enriched storage substances (Fig. 1).

Fig. 1. Micrograph of an ultrathin section of a Metarhizium brunneum blastopore. Active metabolism is obvious by numerous ribosomes (r), mitochondria (m), droplets of storage substances (o = oil, l = lipid) and by endoplasmic reticulum (er), n = nucleus.

Perspective

Production of blastospores of M. brunneum in liquid media is fast, and enables a high number of spore yield. One advantage of blastospores is their ability to germinate within a few hours. Therefore, penetration through insect cuticle and killing of its host can proceed in shorter time compared to conidia. Entomopathogenic fungi are susceptible to UV-light, high temperatures and dryness. The application of M. brunneum blastospores could therefore be of great advantage for biocontrol of pest insects.