Biological control of Spodoptera cosmioides pupae by the entomopathogenic nematode Heterorhabditis bacteriophora in soybean crops

Abstract

Spodoptera cosmioides has become animportant pest to soybean in the Americassince the incorporation of transgenic cultivars. The entomopathogenicnematode (EPN) Heterorhabditisbacteriophora establishes a symbiotic association with bacteria of thegenus Photorhabdus, acting as insect control agent. The aim of this work was to determinethe pathogenicity of H. bacteriophoraand its symbiotic bacterium against S.cosmioides pupae. In the laboratory, insect pupae were placed in containerswith sterile sand and then infective juveniles (IJs) of H. bacteriophorawere applied to the surface. After 48 h, pupa tissues were enzymaticallydigested and penetration of IJs into the insect haemocoel was observed in alltreatments. In pot assays, pupal mortality of 13.3 and 26.7% was recorded 7days after the application of 100 and 1000 IJs, respectively. Photorhabdus laumondii(25, 250, 2500 or 25000 CFUs) were injected into the haemocoel of pupae using amicro-syringe. After 96 h, bioluminescence revealed bacterial growth in more than70.0% of pupae. Field efficacy trials were conducted during three crop seasons.In entomological cages, emergence of insect adults was reduced by 41.8% withrespect to the control treatment using a dose equivalent to 2.5×109 H.bacteriophora IJs ha-1. The effects of EPN application did notdiffer significantly from those of the chemical treatment. We confirm thepathogenicity of H. bacteriophora and its symbiotic bacterium against S.cosmioides pupae. The application of EPN reduced the emergence of S.cosmioides adults; hence, this strategy can be used to decrease their populationsin the field.