Diurnal patterns and conidial dynamics of Batkoa major, a generalist entomophthoralean pathogen

The generalist entomophthoralean insect pathogen Batkoa major was recorded causing epizootics in populations of a new invasive fulgorid in North America, the spotted lanternfly (Lycorma delicatula). We conducted studies on the basic biology and ecology of B. major using Galleria mellonella larvae exposed to conidial showers. Death of G. mellonella followed a diurnal cycle with most larvae dying within 4 h before or after the end of photophase. Time for initiation of rhizoid emergence also followed a diurnal rhythm and, on average occurred 3.6 h after host death. While B. major sometimes began producing rhizoids to attach cadavers to substrates while G. mellonella were alive (but moribund), often hosts were dead before rhizoids began emerging. On average, conidial discharge began 18.6 h after host death and was greater 4–8 h before the end of photophase, compared with 4–8 h after scotophase began. At 20 °C under high humidity, initiation of conidial discharge was 95% complete within 24 h after host death. To evaluate B. major activity by temperature, we tested percent conidial germination over 24 h from 5 to 35 °C. When showered onto water agar, all primary conidia produced secondary conidia. At 20 and 25 °C, at 3 h ≥89% of primaries had produced and discharged secondaries and from 10 to 30 °C, secondaries were produced by over 75% of primary conidia within 12 h. When cover slips were placed over primary conidia to force production of germ tubes, germination was much slower, with >85% germination from 20 to 30 °C only by 24 h. Batkoa major therefore times host death and initiation of conidial discharge for night-time hours and conidial germination occurs within 24 h over a broad temperature range (10–30 °C).