Germination of conidia and blastospores of Paecilomyces fumosoroseus on the cuticle of the silverleaf whitefly, Bemisia argentifolii.

Abstract

The mitosporic fungus Paecilomyces fumosoroseus(Wize) Brown & Smith infects more than 40 insectspecies [1] and produces blastospores in liquid me-dium [2] and conidia on solid media [3]. Because twotypes of propagules can be used for formulating P.fumosoroseus as well as many other fungal entomo-pathogens, it is important to determine which formmight be best suited for use in commercial formula-tions. For example, a study comparing the bioactivityof P. fumosoroseus blastospores and Beauveria bas-siana (Balsamo) Vuillemin conidia has shown thatblastospores were four times more effective in infect-ing and killing the silverleaf whitefly ( Bemisia argen-tifolii Bellows & Perring) [2]. In contrast, Vandenberget al. [4] showed that the relative efficacy of blasto-spores or conidia of P. fumosoroseus against Russianwheat aphids (Diuraphis noxia(Mordvilko)) was sim-ilar. Hall [5] reported that blastospores ofVerticilliumlecanii (Zimmermann) Viegas were twice as patho-genic to an aphid as conidia when based on a numer-ical basis, but only0.6timesas pathogenicwhen basedon a volume basis, due to their larger size.Germination rates of fungal propagules could playan importantrolein determiningvirulence. Infact, fastspore germination has been shown to increase insectpathogenicity for germinated conidia ofP. fumosoros-eus[6] andconidiaofMetarhizium anisopliae (Metch-nikoff) Sorokin used against three different insects[7–9]. Most studies on spore germination rates haveexaminedconidialgerminationinvitro, with onestudyassessing time to conidial germination on the cuticleof an insect in an indirect manner, not using scanningelectron microscopy [SEM, 9].We used SEM to assess germinationrate of conidiaand blastospores of P. fumosoroseus (strain 612; My-cotech Corp., Butte, MT, USA) on the cuticle of B.argentifolii, a devastating insect pest throughout theworld. P. fumosoroseus blastosporeswere obtained us-ing the method of Jackson et al. (1997), yielding 1.2 10