Evaluation of novel fungal and nematode isolates for control of Conotrachelus nenuphar (Coleoptera: Curculionidae) larvae

The primary objective was to identify potential nematode and fungus pathogens for control of plum curculio (Conotrachelus nenuphar) larvae. Initial bioassays were conducted in the laboratory. Seventeen isolates (13 wild-types and four color mutants) of the entomopathogenic fungus Metarhizium anisopliae var. anisopliae were screened. Seven of the isolates were highly virulent for plum curculio larvae (LT50 = 4.0-5.4 days at a concentration of 10^7 conidia ml^-1). Of these isolates, four were color mutants induced by UV-B irradiation exposure (DWR 180, 145, 142, and 62), and three of these showed significantly higher virulence than their parental wild-types (ARSEF 2575 and 23). The most virulent wild-type was the isolate ARSEF 1187. Surprisingly, higher virulence was not correlated with higher conidial adhesion to the cuticle of plum curculio larvae. A Utah-collected isolate of Heterorhabditis bacteriophora was as effective as a commercial strain of Steinernema feltiae in killing plum curculio larvae in laboratory bioassays. Larval mortality ranged from 21 to 89% at inoculum concentrations of 0.125 to 4.0 × 10^6 infective juveniles (IJs) per m^-2. A diapausing (northern) population of plum curculio was less susceptible to S. feltiae (LC50 = 8.6 × 10^5 IJs m^-2) than a non-diapausing (southern) population (LC50 = 3.6 × 10^5 IJs m^-2). A time delay in adding plum curculio larvae to sterilized soil treated with S. feltiae significantly reduced insect mortality after 2 days and reduced mortality below 50% after a delay of 7 days. In field tests, S. feltiae killed 22-39% of northern plum curculio larvae at concentrations of 0.5-2.0 × 10^6 IJs m^-2.