Effect of relative humidity on viability of primary conidia of Zoophthora radicans

Formulation and application of the entomopathogenic fungus: Zoophthora radicans (Brefeld) Batko (Zygomycetes: Entomophthorales)

AIMS

To isolate and formulate a native strain of Zoophthora radicans naturally infecting larvae of diamondback moth, Plutella xylostella, existing in South Australia and to provide evidence that formulation of the fungus is effective against P. xylostella larvae, and therefore, it could be used as a tool in pest management of this insect.

METHODS AND RESULTS

Dose-response bioassays using formulated and unformulated forms of the fungus strain were carried out against third instar larvae of P. xylostella. Results obtained have indicated a significant increase in the larval mortality when higher concentrations of a formulated form of the fungus strain were applied compared to the treatments with the unformulated form (85·0 vs 57·5% of larval mortality, respectively, at the top concentration of 10(7) conidia/ml). The median lethal concentration (LC50) for a formulated form was 100 times less than that of the unformulated form when they were applied against the third instar larvae of P. xylostella. In addition, the formulation used in the present bioassays has preserved the viability of introduced fungus conidia for longer time in comparison with the unformulated conidia.

CONCLUSIONS

The effective application of a formulated fungus strain against P. xylostella larvae constitutes the first step towards its use in pest management of this insect.

SIGNIFICANCE AND IMPACT OF THE STUDY

The formulated fungus in inverted emulsion could be used as an alternative tool to insecticides in pest management of P. xylostella larvae because of the development of resistance to insecticides in the treated larvae.

Experimental epizootiology of Zoophthora anhuiensis (Entomophthorales) against Myzus persicae (Homoptera: Aphididae) with a description of a modified Gompertz model for aphid epizootics

Epizootiological features of Zoophthora anhuiensis, a fungal pathogen specific to aphids in southern China, were studied in six aptera colonies of Myzus persicae at 16 regimes of temperature (T = 10, 15, 20 and 25 degrees C) and relative humidity (H = 90%, 95%, 98% and 100% RH) with initially infected proportion (Ip) of 0.5 in experiment (Expt) 1 or at a fixed regime of 15 degrees C and 100% RH with a variable Ip of 0.17-1.00 in Expt 2. Mycosis-caused mortalities (Mp) varied with aphid densities (D) over time after colony initiation (t) were well fitted to a Gompertz growth model modified to include the variables T, H, Ip and D in the form of Mp = 91.72exp[-5.282exp[-(0.0095T + 0.0128H/T-0.5407D2/H)t]] for Expt 1 (r2 = 0.94) or Mp = 95.49exp[-4.314exp[-(0.1479Ip + 0.1636/D)t]] for Expt 2 (r2 = 0.97). These variables were effective in determining the rate of epizootic development in the M. persicae colonies. Based on the observed and fitted results, Z. anhuiensis was most adaptive to cool climate or season and epizootiologically was subject to a minimal effect of RH at an optimum of approximately 15 degrees C. However, the humidity became determinant for epizootic progress at higher temperatures. Interactive effects of the variables and application of the fitted models for predicting epizootic development in field situations are discussed.