Pnyxia scabiei (Hopkins) Feeding on Sclerotia of Rhizoctonia solani Kühn and Its Population Changes in Sugarbeet Root Rot Field

Abstract

Observations of sugarbeet flelds in Sapporo in 1985 revealed that larvae of Pnyxia scabiei (Hopkins) (Diptera, Sciaridae) were often feeding on sclerotia and hyphae of Rhizoctonia solani Kühn AG-2-2, the causal organism of the sugarbeet root rot. Results of laboratory feeding tests showed that P. scabiei aggregated on and around sclerotia, mycelia of R. solani grown on autoclaved barley grains and pieces of diseased sugarbeet roots, but not on sterile barley grains or healthy roots. Larvae actively fed on sclerotia; 15 sclerotia were almost completely eaten up by 30 larvae within 24 hr in 6-cm diameter petri dishes. Optimum temperature for feeding on sclerotia was 25 C. Among 6 anastomosis groups of R. solani, sclerotia destruction were severe in AG-2-2, AG-3 and AG-5, but less in AG-1 (culture type IA and IB), AG-2-1 and AG-4. Populations of larvae in sugarbeet fields increased rapidly as root rot severity increased. Occasionally up to 5, 000 larvae per dead root were counted. They were found mostly on lesions or in the soil within a distance of 2-cm from the root surface. When sclerotia of R. solani AG-2-2 were buried in soil around dead or healthy roots of sugarbeet for 3 weeks in autumn, using small plastic containers with 10-or 250-μm mesh, sclerotial destruction occurred only in 250-μm containers around the dead roots, and larvae of P. scabiei were often inside these containers. These results suggest that P. scabiei decreases sclerotial density of R. solani in soil and thereby reduces Rhizoctonia root rot incidence of sugarbeets.