Efficacy of Heterorhabditis species against Sweet Potato Weevil (Cylas formicarius Fabr.) in tropical islands: a case study of Visayas, Philippines

Abstract

Sweet potato weevil (Cylas formicarius Fabr.) is a serious threat to sweet potato production in the world. This insect pest can cause damage as high as 97% yield reduction when not managed. The use of biological control agents like entomopathogenic nematodes (EPNs) can be an alternative to the use of synthetic chemicals to manage this insect pest.  The aim of this study was to determine the pathogenicity of the four EPN isolates from the Visayas islands in the Philippines in controlling the sweet potato weevil in–vitro and in greenhouse pot trials. All four isolates were initially identified under the Heterorhabditis genera based on the coloration of the cadavers. Of these four, three isolates, namely UBD, AVD, and CP were identified as H. indica and HO1 as H. baujardi. All of these four (4) EPN isolates were equally promising for the control of both larvae and prepupae. Among the four isolates, UBD at 300EPN/ml recorded the highest mortality on larvae (99.4%) and prepupae (88%), highest percent infection on larvae (95%) and prepupae (83.4%), and fastest to achieved LC₅₀ which was 2 days after application on larvae while 4 days on prepupae. At higher concentration, LC₅₀ were achieved faster than in lower concentration across isolates. A lower percent mortality was recorded on prepupae compared to larvae. In the greenhouse experiment, tubers inoculated with UBD isolate at 300EPN/ml obtained the least damage with no apparent weevil feeding damage on the tuber and no adult exit holes. The four isolates were found effective in controlling sweet potato weevil under the tropical island situation in Visayas, Philippines.