Comparison of Entomopathogenic Nematode Efficacy via Infected Cadavers or Aqueous Applications for the Management of Cylas formicarius and Various Sweetpotato Insect Pests

Abstract

Entomopathogenic nematodes (EPNs) are deadly to certain insects and consequently have great potential as biological control agents. The potential of two local EPN isolates and their efficacy in the management of the sweet potato weevil, Cylas formicarius, was investigated. The development of a novel application method by releasing EPN-infected cadavers to improve the efficacy and ease of handling of EPN was investigated. Insect cadavers infected with EPN followed by slow desiccation of the cadavers to 85% relative humidity has been reported to protect the nematodes from ultraviolet radiation and desiccation, but the effects on the EPN are not known. Field experiments were established and aqueous applications of Heterorhabditis indica OM160 and Steinernema feltiae were made when the sweetpotato roots began to swell. Sweet potato weevil populations were low and little weevil damage occurred to the sweetpotato yield in the experiments, masking the efficacy of EPN treatments. In experiments comparing aqueous vs infected-cadaver applications of EPNs, sweetpotato yield was greater in plots treated with 2 cadavers of Galleria mellonella infected with H. indica OM160 compared to plots receiving 0, 3, or 6 cadavers (P<0.05). Desiccation of infected cadavers worked well with H. indica OM160 but S. feltiae MG14 migrated out of the drying cadavers. Heterorhabditis indica OM160 tolerated desiccation well with no effect on the nematode’s reproduction in the cadaver nor in the IJs’ ability to emerge and infect new insect larvae. IJs of H. indica OM160 in desiccated cadavers of G. mellonella were able to infect and kill 100%, 45% and 95% of Galleria mellonella, Oryctes rhinoceros and Tenebrio molitor larvae, respectively. Desiccated cadavers took on a “beef jerky” appearance and were not as fragile as non-desiccated cadavers. Desiccation of EPN-infected insect cadavers improves the ease of EPN handling for growers and decreases breakage of cadavers which can lower the efficacy of the EPNs.