Variation of larval susceptibility to Lagenidium giganteum in three mosquito species.

A significantly greater number of Lagenidium giganteum zoospores were found encysting on the dorsal thoracic surface of Anopheles gambiae larvae than on the larvae of Aedes aegypti and Culex pipiens. However, germ tube penetration in the cuticle of A. gambiae provoked an intense and diffuse melanization which encapsulated the fungus and protected 56% from death. Although a small number of zoospores attach to and penetrate the cuticular surface of A. aegypti and C. pipiens approximately 99% of both species succumb to fungal infection. Melanization in A. aegypti is slower, weaker, more localized, and generally ineffective against L. giganteum infection compared to A. gambiae. Upward migration of L. giganteum zoospores to the water surface favored encounters with mosquito larvae and was speculated to be due to negative geotaxis rather than positive aerotaxis and phototaxis. Otherwise, initial contact between larva and zoospore was random.

[Histology of insect infection by entomopathogenic fungi: interest in a new procedure for the study of precocious phases]. / Histologie de l'infection des insectes par les champignons entomopathogènes: intérêt d'une nouvelle procédure pour l'étude des phases précoces.

Infective and invasive processes of entomopathogenic fungi expected to be used in biological control shall be analysed in detail. Histological and ultrastructural studies are therefore essential. This paper is devoted to the field of light microscopy; its purpose is to show that coating, in a 4% agar solution, the insects infected by a fungus, prior to fixation and embedding, allows the preservation of the fungal structures present on the cuticle, especially the germinating conidia. This is useful when observing the initial stages of infection. The relevance of this coating technique is illustrated with the example of Pea aphid, Acyrthosiphon pisum, infected by Conidiobolus osmodes. This contribution is the first histological study of infective processes of this entomophthoralean fungus in an insect.

Defense reactions by larvae of Aedes aegypti during infection by the aquatic fungus Lagenidium giganteum (Oomycete).

The adherence of zoospores of Lagenidium giganteum to the cuticle of mosquito larvae is the initial step in the infection process. Subsequently, a germ tube penetrates the integument, inducing a rapid melanization of the injured cuticle and epidermis. After entering the hemocoel the developing hyphae are occasionally encapsulated locally. This process is slow (6 to 12 h postincubation) and most frequently cell-free, although it can be mediated by circulating hemocytes. Sporadic hemocyte mediation of the humoral encapsulation process in larval stages of Culicidae adds a previously unreported dimension to this unusual type of defense reaction. The defense reactions of larvae of Aedes aegypti were ineffective against observed infection by Lagenidium giganteum.

[Taxonomic reconsideration of Entomophthora obscura Hall & Dunn]. / Reconsidération toxonomique de Entomophthora obscura Hall et Dunn.

A set of slides and strains designated as Entomophthora thaxteriana Petch, E. obscura Hall et Dunn and E. ignobilis Hall et Dunn were compared on morphological, physiological, pathological and biochemical bases. It is concluded that E. thaxteriana (name invalidated by Humber, 1978) must be replaced by E. obscura and not by E. ignobilis, this species being considered as nomen dubium. Some intraspecific variations within the species E. obscura were found particularly with regard to the ability to produce resting spores and cystidia.

[An example of research on biological control: Entomophthora fungi pathogenic for aphids]. / Un exemple de recherche en lutte biologique: les champignons entomophthora pathogènes de pucerons.

The results obtained in 15 years of research on the Entomophthorales pathogen of aphids showed the importance of the action of these fungi in the regulation of natural aphid populations and their possible use in agriculture as a biological control agent. Recent ecological studies on natural populations of aphids established the seasonal variation of the different fungal species and the diverse degrees of specificity between the species or groups of species of aphid and the various species of Entomophthora. The study of populations dynamics of an aphid species on a cultivated plant permitted the determination of the way a certain number of biotic and abiotic factors, such as temperature, humidity, thresholds of the insect population and of the infecting fungus lead to an epizootic development. If the air propagation of the disease by conidia is understood for a long time, the role of the soil as a reservoir for the infecting fungus has been demonstrated recently. Under favourable climatic conditions, the use of industrially produced resistant resting spores would allow the regulation of aphid populations in nature.