Submitted to: Journal of Invertebrate Pathology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 12/23/2002
Publication Date: 8/21/2003
Citation: James, R.R., Buckner, J.S., Freeman, T.P. 2003. Cuticular lipids and silverleaf whitefly stage affect conidial germination of Beauveria bassiana and Paecilomyces fumosoroseus. Journal of Invertebrate Pathology.84:67-74. Interpretive Summary: Beauveria bassiana and Paecilomyces fumosoroseus are insect pathogens that have been used as biological control agents for the silver leaf whitefly. The silverleaf whitefly is a serious pest of a variety of crops in the southern US, as well as other warm climate regions of the world. The two pathogens have similar biologies, yet only P. fumosoroseus causes naturally occurring outbreaks. We further investigated these pathogens to determine any differences that might enhance or inhibit their ability to control outbreak populations of the pest. We found that mid-development stages of the whiteflies (the 3rd instar) are the most susceptible stage to B. bassiana (lowest LD50), and young nymphs (2nd instar) were the most susceptible to P. fumosoroseus. Spores from the fungi germinated more readily on the exoskeleton of these two life stages than on eggs and mature larvae. Fourth instars had low susceptibility to these pathogens, and spore germination on the cuticle of 4th instars was very low for B. bassiana (7%) and intermediate for P. fumosoroseus (33%). Whitefly larvae are covered in a thick layer of wax that is excreted by the insect. We found that these waxes act to inhibit spore germination for these two pathogens. B. bassiana is more affected by the waxes than is P. fumosoroseus. In fact, when the P. fumosoroseus spores are incubated in water (with no nutrients) and the waxes, and more germination occurs than when the waxes are omitted. The waxes may be acting as a food source for that fungus when no other food source is available. Thus, the thick coating of long-chain wax esters produced by whitefly nymphs may serve, in part, as a first line of defense against fungal pathogens-especially against B. bassiana.
Technical Abstract: Beauveria bassiana and Paecilomyces fumosoroseus are generalist entomopathogenic fungi that are pathogenic to the silver leaf whitefly (Bemisia argentifolii). We found third instar whiteflies to be the most susceptible larval stage to B. bassiana (lowest LD50), and 2nd instars were the most susceptible to P. fumosoroseus. Conversely, conidia of B. bassiana germinated most readily on the cuticle of 2nd instars (54% germinated), and P. fumosoroseus germination was highest on 3rd instar cuticle (45%). Fourth instars (the ultimate instar) had low susceptibility to these pathogens, and spore germination on the cuticle of 4th instars was very low for B. bassiana (7%) and intermediate for P. fumosoroseus (33%). Cuticular lipids were found to have toxic or inhibitory effects on conidia of B. bassiana and P. fumosoroseus. In the absence of added nutrients, P. fumosoroseus conidial germination increased in the presence of the lipids; therefore, the cuticular lipids may have acted as a nutrient source for this fungus. Synthetic long-chain wax esters inhibited germination of P. fumosoroseus to a degree that was similar to the effect of the cuticular lipid extracts. The synthetic lipids did not have a significant effect on B. bassiana. Thus, the thick coating of long-chain wax esters produced by whitefly nymphs may serve, in part, as a first line of defense against fungal pathogens.