Invasive and developmental processes of Entomophthora species infecting aphids

Abstract

Entomophthora aphidis Hoffmann and E. thaxteriana (Petch) Hall & Bell on Acyrthosiphon pisum Harris, and E. fresenii (Nowakowski) Gustafsson and E. planchoniana Cornu on Aphis fabae Scopoli penetrate the cuticle at any point, probably by enzymic and mechanical processes. Only E. fresenii forms an appressorium. Hyphae or hyphal bodies of each fungus first invade the haemocoel where individual blood cells appear to engulf some but do not agglomerate and encapsulate them; each species invades the solid tissues sequentially, with slight differences between fungal species, but all invade sclerotized tissues and embryos last, often after the host dies. Each pseudocystidium of E. aphidis develops from a swollen hyphal body and breaches the aphid cuticle allowing the surrounding conidiophores to emerge. These, and the conidiophores of E. planchoniana and E. thaxteriana, begin to emerge in groups, but eventually those of all species cover the cuticle in an even layer. Weakly differentiated rhizoids sometimes develop around the mouthparts of aphids killed by E. thaxteriana; rhizoids of E. aphidis develop from greatly swollen hyphal bodies, and the compound ones of E. planchoniana develop from bundles of elongated hyphal bodies in the thorax and abdomen of the host; rhizoids of both of these species sometimes emerge before the aphid dies.