Field efficacy of Heterorhabditis bacteriophora (Nematoda: Heterorhabditidae), Metarhizium brunneum (Hypocreales: Clavicipitaceae), and chemical insecticide combinations for Diabrotica virgifera virgifera larval management

A two-year field study using a blend of entomopathogens in conjunction with chemical insecticides was carried out to determine to which extent they affect western corn rootworm (WCR), Diabrotica virgifera virgifera, survival, maize root damages, and grain yield and to assess the potential for side-effects on natural arthropod enemies. The products tested were conducted on a maize crop and included maize seeds dressed with the neonicotinoid clothianidin (Poncho™), Belem™ (ai: cypermethrin), dianem™ (entomopathogenic nematode: Heterorhabditis bacteriophora), and a granular formulation of the insect pathogenic fungus Metarhizium brunneum. Trials were conducted at four (2014) and two (2015) sites heavily infested with WCR in the Southeast of Styria, Austria. The lowest number of WCR adults were recorded in 2015 at sites where the nematode and fungal biocontrol agents were used with clothianidin dressed maize seeds. The treatments did not significantly influence plant lodging and grain yield compared with the untreated (negative) control. The WCR population density rose during the investigation period and the average number of beetles in all treatments and in both study years far exceeded the stipulated economic threshold value for continuous maize of around one beetle per plant. Non-target evaluation using pitfall traps did not show any significant impact of the treatments on the abundance or diversity of indigenous predatory beetles and spiders. The lack of damage suggests that the WCR threshold is considerably higher than the stipulated economic threshold under favourable growing conditions, but a systematic crop rotation is recommended as an integral part of the WCR management both as a prophylaxis and as a background for the best treatment.