Reduced emergence of cylas formicarius elegantulus(Coleoptera:curculionidae) from sweet potato roots by heterorhabditis indica

Authors: Myers, Roxana; Sylva, Charmaine; Mello, Cathy; SNOOK, KIRSTEN - University Of Hawaii

Submitted to: Journal of Economic Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/7/2020
Publication Date: 3/29/2020
Citation: Myers, R.Y., Sylva, C.D., Mello, C.L., Snook, K.A. 2020. Reduced emergence of cylas formicarius elegantulus(Coleoptera:curculionidae) from sweet potato roots by heterorhabditis indica. Journal of Economic Entomology. 113(3):1129-1133. https://doi.org/10.1093/jee/toaa054.
DOI: https://doi.org/10.1093/jee/toaa054

Interpretive Summary: The purple fleshed Okinawan sweet potato is an important food staple and high valued export crop for Hawaii Island. One of the most destructive insect pests hindering maximum crop production is the sweet potato weevil, Cylas formicarius. It negatively affects the quality of the product when larvae feed and tunnel inside the storage root causing malformation and resulting in an off taste. It is common practice for farmers to leave these culled roots in the field after harvest resulting in a large reservoir of weevils for subsequent crops. Due to high labor costs, farmers rarely practice sanitation and prefer to move to virgin fields for the next cropping cycle. Entomopathogenic nematodes have been used to reduce weevil populations in other sweet potato growing regions. Local Hawaiian strains of these nematodes were collected and tested against the weevil in laboratory and cage bioassays. High mortality occurred in larvae and pupae when exposed to these nematodes directly. In tub trials, the nematodes were able to enter the sweet potato root to search and kill weevils of all life stages. In a simulated field trial, applying the nematodes to infested roots prevented the infestation of adjacent clean roots. Utilizing the local strains of entomopathogenic nematodes could allow the farmers to replant in existing fields without increasing damage by the weevils.

Technical Abstract: Okinawan sweet potato, Ipomoea batatas, is an important food staple and export crop for the Island of Hawaii. Cylas formicarius elegantulus, sweet potato weevil, is a major quarantine pest that causes severe destruction to the crop. Root malformation and a bitter taste occur when larvae feed and tunnel within the storage root. Off-grade roots are often left in the field after harvest and serve as a reservoir for the weevils. Current management involves the unsustainable practice of moving to virgin land for the next cropping cycle. Strains of Heterorhabditis indica isolated from the Hawaiian Islands were tested for their efficacy at causing mortality of C. formicarius and reducing the emergence of adults from infested roots. In well plate assays, H. indica caused mortality of 88% larvae, 96% pupae, and 4% adults after 48 h. When applied to infested roots, the nematodes caused an average mortality of 78% larvae, 66% pupae, and 32% adults. Greater mortality was observed at the highest inoculum levels (10,000 infective juveniles per storage tuber) but a reduction of 90% inoculum density was still effective at weevil management. In simulated field trials, infestation of storage roots was reduced by 42–99.6% when planted among infested roots that had been inoculated with H. indica. Rates of 2.5 billion IJs/hectare were just as effective as 5 billion IJs/hectare. Application of local H. indica strains in sweet potato production has the potential to manage C. formicarius populations and allow for consecutive cropping seasons.