Assessing winter-survival of Pandora neoaphidis in soil with bioassays and molecular approaches

Pandora neoaphidis (Entomophthoromycotina, Entomophthorales) is a specific pathogen of aphids with a great potential for use in biological control. The development of effective biological control strategies requires detailed knowledge of its biology and ecology. However, little is known on the overwintering strategies of this fungus. It is believed that natural areas may play an important role for survival and soil may serve as an inoculum source for new populations in spring. This study aimed to investigate winter-survival of P. neoaphidis in topsoil layers in a field experiment by assessing fungal persistence and potential to infect and control pea aphid ( Acyrthosiphon pisum) populations in spring. For this purpose, a selected P. neoaphidis strain was introduced in the form of living infected pea aphids into caged plots containing defined pea aphid populations. Within 3 weeks the aphid populations decreased massively and a significant amount of P. neoaphidis inoculum accumulated on the soil providing optimal conditions for investigating the winter-survival of this fungus. Prevalence and pathogenicity of P. neoaphidis in plot soils were assessed from fall 2006 until spring 2007 using a combination of bioassay, quantitative PCR, and single nucleotide polymorphism genotyping approaches. Results indicated that the introduced strain did not survive the winter on the soil and that the strains present in the plots in the following spring were derived from outside the plots. However, the introduced strain had a beneficial effect on plant survival. The tools developed and applied in this study proved powerful and reliable for tracking specific target strains of P. neoaphidis in the environment.