Improved sporulation of alginate pellets entrapping Pandora nouryi and millet powder and their potential to induce an aphid epizootic in field cages after release

Abstract

Pandora nouryi is an obligate aphid fungus (Entomophthorales) forming resting spores in vivo but little is known about its epizootic potential. We manufactured small uniform alginate pellets (∼1 mg/dry pellet) entrapping ∼1% P. nouryi mycelia and ∼5% millet powder to facilitate sporulation and evaluated their epizootic potential in Myzus persicae population developing on cabbage plants inside meshed field cages (2 × 2 × 2 m). The pellets sporulated 8.5-fold more (28.4 × 10 4 conidia/pellet) than the millet-free pellets at 20 °C, and 3.6–13.6 fold more and much longer than mycosis-killed adult aphids at 4–28 °C under saturated humidity. These pellets, irrespective of drying or not, retained sporulation capability after 120-day storage at 6 °C. The releases of 5–20 pellets/plant in different field cages resulted in successful colonization of P. nouryi in aphid cohorts during 4-month field trial from late autumn through spring. Aphid mortalities attributed to P. nouryi mycosis were consistently detectable but very low in all cages, irrespective of the timing of a release and the density of released pellets, until ⩽13% mortalities occurred in late spring. Of those aphids infected with P. nouryi, high proportions (32–95%) were found producing resting spores on most of weekly sampling occasions. The low-level prevalence of P. nouryi in the released cages was thus attributed to the frequent resting spore formation in vivo, which may interrupt the infection cycle. The alginate pellet formulation with millet powder is recommended for improving preparations of fungal pathogens without resting spores, such as Pandora neoaphidis, for biological control of aphids.