Potential of endemic entomopathogenic nematodes against emerging insect pests in Hawai‘i

Abstract

Entomopathogenic nematodes (EPNs) parasitize insects utilizing mutualistic bacteria to infect and kill the host, allowing the nematode to feed and reproduce within the insect cadaver. Consequently EPNs are highly sought after for their biological control potential. A survey for EPNs was conducted on O’ahu and Hawai’i Island using a modified baiting method. One hundred seven soil samples were collected and baited with Tenebrio molitor (mealworm) larvae. Forty-seven of the 107 sites contained at least one nematode infected mealworm containing nematodes. Mealworm mortality was attributed to EPNs, fungal contamination, parasitoids or an unknown variable in 16%, 10%, 1% and 73% of samples respectively. Eighty-two EPN isolates were passed through two subsequent inoculations in order to confirm their entomopathogenic nature. A total of 41 EPN isolates were recovered through three rounds of reinoculation and recovery. PCR analysis and sequencing were conducted on third generation nematodes, targeting the ITS region. Sequencing analysis suggested three groups of Oscheius. Oscheius was recovered from 96% of locations sampled on Hawai’i Island and O’ahu. Four Oscheius isolates (BI 1a, BI 12a, OJ 4a, OJ 5b) recovered from the soil survey were tested alongside endemic Steinernema feltiae and Heterorhabditis indica to evaluate their efficacy against Tenebrio molitor larvae at three inoculum levels (3, 9 and 15 nematodes/mealworm). The most effective nematodes as indicated by LT50 were: S. feltiae at 3 nematodes/mealworm (46.4 hours), Oscheius BI 12a at 9 EPN/mealworm (39.7 hours), and S. feltiae at 15 nematodes/mealworm (35.2 hours). Three Oscheius isolates, S. feltiae and H. indica, were further evaluated against all stages (larvae, pre-pupae, pupae and adults) of Hypothenemus hampei, the coffee berry borer (CBB). CBB mortality attributed to nematodes was highest with H. indica and S. feltiae at 17.6% and 16.7% respectively. Coffee berry borer mortality not attributed to EPNs or Beauveria bassiana may have resulted from the EPNs symbiotic bacteria physiological or physical causes. This series of experiments suggests that Oscheius is a common EPN found in Hawai’i that displays behavior and levels of biological control similar to established EPNs. The presence of endemic Oscheius isolates in Hawaii is a welcomed finding that will add to the sparse list of biological control organisms in Hawaii. The utilization of the prospective endemic EPN Oscheius is an excellent option as Heterorhabditis and Steinernema spp. (aside from Steinernema carpocapsae) are currently on Hawaii's List of Restricted Animals and cannot be imported into the state of Hawaii.