Submitted to: Journal of Invertebrate Pathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 18, 1998
Publication Date: N/A
Interpretive Summary: The diamondback moth is a worldwide pest of cabbage, broccoli, canola, and other crucifers. Each year, farmers spend more than $1 billion to control this pest, primarily with chemical insecticides. However, many populations of diamondback moth have become resistant to conventional, as well as some biological, insecticides. Alternative control measures being investigated include the use of insect-pathogenic fungi, but basic studies of fungal infection processes are needed for selection of appropriate strains. We compared the early stages of fungal infection of diamondback moth using eight strains of the fungus Paecilomyces fumosoroseus. We found dramatic differences among strains in relative virulence, spore size, germination speed, and the ability of spores to attach to the surface of test insects. A fungal strain that was not able to infect diamondback moth had the smallest spores, germinated most slowly, and did not attach well to cuticle. In contrast, highly virulent strains had larger spores that attached well and germinated quickly. These findings will allow more efficient selection of useful fungal strains for safe, effective biological control of diamondback moth and other pests.
Technical Abstract: Infectivity to larvae of the diamondback moth, Plutella xylostella, was compared among eight Paecilomyces fumosoroseus isolates. Isolate infectivity was assessed for correlation with spore length and germination speed. Four isolates applied to P. xylostella cuticle were also compared for number of spores remaining on the cuticle after washing and for percentage germination after 36 h. Infection of larvae inoculated with the different isolates at an average dosage of 4,000 conidia per square cm ranged from 1% to 49%. The correlation of infectivity with spore length and germination speed in broth was highly significant. Fewer spores of the least infective isolate, ARSEF 1576, attached to larval cuticle compared to spores of the more infective isolates ARSEF 3682, 4461, and 4482. After 36 h on larval cuticle, the percentage of spores germinated for isolates 1576 and 3682 was 3% and 95%, respectively. Spores of isolate 1576 were smaller, germinated more slowly, and attached to cuticle in smaller numbers than spores of the more infective isolates. However, these conidial traits are not sufficient to explain the virtual lack of pathogenicity of isolate 1576, because larvae survive dosages that yield an abundance of germlings attached to the cuticle. Preliminary observations suggest that isolate 1576 may not be able to penetrate the cuticle of some Lepidopteran larvae.