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ARS Home » Northeast Area » Ithaca, New York » Robert W. Holley Center for Agriculture & Health » Emerging Pests and Pathogens Research » Research » Publications at this Location » Publication #310275

Title: Conidial vigor vs. viability as predictors of virulence of entomopathogenic fungi

item FARIA, MARCOS - Embrapa
item Wraight, Stephen

Submitted to: Journal of Invertebrate Pathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/27/2014
Publication Date: 1/5/2015
Citation: Faria, M., Lopes, R.N., Souza, D., Wraight, S.P. 2015. Conidial vigor vs. viability as predictors of virulence of entomopathogenic fungi. Journal of Invertebrate Pathology. 125:68-72.

Interpretive Summary: The principal ingredients in most biopesticide products based on insect pathogenic fungi (mycoinsecticides) are dry spores called conidia, and viability of these conidia is one of the most commonly reported indicators of product quality (insecticidal activity). Yet, there is no standard protocol for determining viability. In particular, the various protocols in use today do not standardize the incubation time, and thus do not account for speed of germination, which typically decreases with conidial age or with exposure to stressful storage environments. In this study, we tested the hypothesis that fast-germinating conidia (those capable of germinating within 16 hours) are more virulent than conidia exhibiting slower germination. Preparations of the common fungal pathogen Beauveria bassiana with variable proportions of slow-germinating conidia were tested against fall armyworm larvae. The common measure of virulence, LD50 (Dose lethal to 50% of treated insects), remained constant, regardless of the proportion of slow-germinating conidia comprising the preparation, indicating that fast-germinating (vigorous) conidia were responsible for nearly all mortality observed in the tests. Also, as expected, larvae treated with vigorous conidia succumbed to infection much sooner than larvae treated with slow-germinating conidia. These results confirm our previous hypothesis that scoring of slow-germinating conidia as viable can lead to overestimation of the quality (potency) of biopesticide preparations and support our recommendations for use of short incubation periods for assessing viability/product quality. It is anticipated that these results will lead to greater standardization of viability protocols among biopesticide researchers and producers.

Technical Abstract: We tested the hypothesis that debilitated conidia exhibiting slow-germination (requiring greater than 16 hours to germinate) are less virulent than vigorous conidia exhibiting fast germination (requiring less than or equal to 16 hours to germinate). Preparations of Beauveria bassiana s.l. strain CG 1027 with variable ratios of vigorous to debilitated conidia were assayed against third-instar larvae of Spodoptera frugiperda. As the proportion of debilitated conidia in test preparations increased, LC50 (lethal concentration 50) expressed in terms of total viable conidia increased, while LC50 expressed solely in terms of vigorous conidia remained constant, indicating that vigorous conidia were responsible for nearly all mortality observed in the assays. Larvae treated with conidia from low-quality batches (with high proportions of debilitated conidia) survived consistently longer than those treated with comparable doses of conidia from high-quality batches. These results confirm our previous hypotheses that inclusion of debilitated conidia in viability assessments can lead to overestimation of the quality (potency) of mycoinsecticide preparations and support our recommendation for use of short incubation periods for assessing viability whenever viability is relied upon as an indicator of product quality. Incorporating short incubation times (less than or equal to 24 hours) and fast rehydration may provide the most reliable assessments of overall mycopesticide quality.