Susceptibility of the Velvetbean Caterpillar and Soybean Looper (Lepidoptera: Noctuidae) to Nomuraea rileyi: Effects of Pathotype, Dosage, Temperature, and Host Age

Abstract

Comparisons among four Nomuraea rileyi isolates tested against Anticarsia gemmaatalis and Pseudoplusia includens demonstrated distinct differences in both the susceptibility of the two noctuid hosts and the virulence of strains. Host susceptibility to N. rileyi infection was directly influenced by larval age. Maturation of host larvae resulted in increased Lc50 values and in the mean days to death of the N. rileyi strains. The in vivo developmental rate of N. rileyi was affected by the ambient temperature. Temperatures below the calculated optima (26°C) for N. rileyi decreased the developmental rate of the infection process but did not alter the percent N. rileyi-induced mortality of treated A. gemmatalis larvae. A. gemmatalis, with a developmental optimum of 31°C, became increasingly resistant to N. rileyi infection when reared at temperatures above 26°C. The assay and food substrate influenced both the development of the host and the relative virulence of N. rileyi to A. gemmatalis larvae. Comparisons of concentration and time responses demonstrated that larvae assayed on the soybean leaflets were more resistant to N. rileyi infection than larvae assayed on agar substrate and fed artificial diet. TheA. gemmatalis feeding on mature soybean foliage (pod set stage) appeared to be more resistant to N. rileyi infections than did the larvae feeding on prebloom and bloom soybean stages.