Submitted to: Journal of Invertebrate Pathology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 12/12/2003
Publication Date: 2/1/2004
Citation: Vandenberg, J.D., Cantone, F.A. 2004. Effect of serial transfer of three strains of paecilomyces fumosoroseus on growth in vitro, virulence and host specificity. Journal of Invertebrate Pathology. 85:40-45. Interpretive Summary: Control of insect pests of crops costs farmers billions of dollars every year. However, many pests have become resistant to conventional, as well as some biological, insecticides. Alternative control measures being investigated include the use of insect-pathogenic fungi, but basic studies on the variable nature of fungal strains are needed for effective commercial development of fungi for insect management. We evaluated changes among three strains of an insect-pathogenic fungus, Paecilomyces fumosoroseus, following long-term cultivation of these strains in one nutrient medium and two insect hosts. We showed that none of the strains lost virulence toward either insect after cultivation on a laboratory growth medium. This means that commercial cultivation of these strains is not likely to result in a loss of virulence for a target host. We also showed that two strains lost virulence toward one insect after being cultivated in the other. Consequently, when developing fungi for insect control, attention must be paid to changes in virulence depending on the target insect. These findings improve our understanding of the factors that make insects susceptible to fungi and will help us design effective strategies for their use as safe biological control agents.
Technical Abstract: We evaluated virulence, host specificity, biomass production, conidiation, conidal germination and a genetic fingerprint of 3 strains of Paecilomyces fumosoroseus after passage in vitro or in vivo in Diuraphis noxia or Plutella xylostella. Strain 4461 did not change in virulence after 30 passages in vitro. However, it lost virulence toward D. noxia after 15 passages in P. xylostella and did not regain virulence after 5 passages in D. noxia. Passage in D. noxia resulted in a loss in conidiation for strain 4461, and passage in vitro resulted in a reduction in the speed of germination. Strain 4481 was the least variable and did not change in any of our tests. Strain 4491 did not change in virulence after passage in vitro. It lost virulence toward D. noxia after passage in P. xylostella but regained virulence after re-passage in D. noxia. Mycelial dry weight and conidiation were both reduced after passage in vitro, but were increased after passage in D. noxia. Germination speed was reduced after in vitro passage of strain 4491. These results demonstrate the interspecific variability and phenotypic plasticity of strains of P. fumosoroseus. Stability of virulence after in vitro passage is clearly a desirable trait for a mass-produced biocontrol agent. However, a change in host specificity or productivity in vitro, as we observed for some strains, must be monitored and minimized.