Know Your Friends

The Entomopathogen Paecilomyces fumosoroseus

Among the many fungal insect pathogens encountered in nature, the genera Beauveria and Metarhizium are probably the most widely known by entomologists. Both of these belong to the same subdivision (Deuteromycotina or Fungi Imperfecti) as Aschersonia, Hirsutella, Nomuraea and Paecilomyces. A common attribute of these fungi is the lack of sexual forms, thereby the name Fungi Imperfecti. The infection cycle of entomopathogenic fungi, including enzyme production on the cuticle, remains one of the most fascinating areas of research involving these organisms. In this cycle, a spore is deposited on the insect cuticle followed by the development of a germ tube (and in some cases by a structure called appresorium) which penetrates the insect. Fungal penetration can also occur through the spiracles, the mouth, and anal opening. Development occurs in the hemolymph with mycelia eventually exiting from the insect, and producing conidia. Insect mortality due to fungal infection has been ascribed to the destruction of insect tissues, the release of toxins, and nutritional deficiencies.

Paecilomyces fumosoroseus, or PFR, is considered a very promising biological control agent due to its extensive host range which includes insects in over 25 different families, including the diamondback moth (Plutella xyllostella), Russian wheat aphid (Diuraphis noxia), and silverleaf whitefly (Bemisia argentifolii). Some members in the genus, such as P. lilacinus, are active against plant parasitic nematodes. At one point, 31 species were recognized in the genus.

One major impediment towards the widespread use of PFR is the lack of a commercial formulation in the USA, although one is being marketed in Europe under the trade name PreFeRal. A commercially-viable formulation should have low production costs in order to compete advantageously with conventional insecticides. The formulation should also maintain a high viability, virulence, and long shelf-life. Ideally, materials used in commercial formulations should also provide some protection against high temperatures, low humidity, and ultraviolet light, the major environmental obstacles towards the establishment of entomopathogenic fungi after application.

PFR, just as Beauveria bassiana, produces conidia in solid culture and blastospores in liquid culture. Experiments conducted at the USDA-ARS Bioactive Agents Research Unit in Peoria have shown that blastospores start germinating at a faster rate on the cuticle of silverleaf whiteflies than do conidia, which indicates that the use of blastospores for the development of formulations would be advantageous over the use of conidia. In addition, blastospores grown in a specific medium developed at the Fermentation Biochemistry Unit have been shown to resist desiccation, which improves the chances of producing a formulated product. Several different agricultural products have been tested as formulation ingredients and some have shown promise in maintaining blastospore viability through time when the formulations have been freeze-dried.

Different Paecilomyces strains are available from the ARS Collection of Entomopathogenic Fungi in Ithaca, New York, and the American Type Culture Collection in Rockville, Maryland.

- Fernando E. Vega, Bioactive Agents Research Unit, USDA-ARS, Peoria, Illinois

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