Persistence of widespread moderate Spinosad resistance among wild melon fly (Zeugodacus cucurbitae) and oriental fruit fly (Bactrocera dorsalis) populations on the major Hawaiian Islands

Authors: STOCKTON, DARA - Animal And Plant Health Inspection Service (APHIS); KRAFT, LAURA - Washington State University; DOMBROWSKI, PATRICIA - University Of Hawaii; DOUCETTE, LAURA - University Of Hawaii; BOSCH, MICHAEL - Orise Fellow; GUTIERREZ-COARITE, ROSEMARY - University Of Hawaii; MANANDHAR, ROSHAN - University Of Hawaii; UYEDA, JENSEN - University Of Hawaii; SILVA, JOSHUA - University Of Hawaii; HAWKINS, JENNIFER - University Of Hawaii; SHIKANO, IKKEI - University Of Hawaii

Submitted to: Pest Management Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/17/2024
Publication Date: 7/12/2024
Citation: Stockton, D.G., Kraft, L., Dombrowski, P., Doucette, L., Bosch, M., Gutierrez-Coarite, R., Manandhar, R., Uyeda, J., Silva, J., Hawkins, J., Shikano, I. 2024. Persistence of widespread moderate Spinosad resistance among wild melon fly (Zeugodacus cucurbitae) and oriental fruit fly (Bactrocera dorsalis) populations on the major Hawaiian Islands. Pest Management Science. https://doi.org/10.1002/ps.8279.
DOI: https://doi.org/10.1002/ps.8279

Interpretive Summary: Invasive fruit flies are one of the biggest risks associated with Hawaiian Agriculture, affecting crop yields within the state, as well as limited exports to the continental US due to biosecurity concerns. This study aimed to track the extent of Spinosad resistance throughout the islands' fruit flies populations, where much of the current management relies on Spinosad efficacy. We tested 20 sites on the five major islands and two species of tephritid fruit flies. We found that while levels of resistance were lower than at their peak in 2018, resistance remains elevated on Oahu and parts of the Hawaii island. Resistance was detected in both melon flies and oriental fruit flies, confirming earlier reports that resistance is a multi-species problem. Our results suggest that continued efforts to scale back Spinosad use may be warranted, as well as the incorporation of a rotational insecticide program to prevent further resistance from developing.

Technical Abstract: BACKGROUND: Insecticide resistance among invasive tephritid fruit flies poses a great risk to national food security and has the potential to disrupt quarantine and eradication programs, which rely on the efficacy of Spinosad to prevent widespread establishment in North America. During 2022-2023 we surveyed the extent of Spinosad resistance of two key species, Oriental fruit fly Bactrocera dorsalis, and melon fly Zeugodacus cucurbitae, from 20 sites across five Hawaiian Islands including Kaua’i, Oahu, Maui, Molokai, and the Big Island of Hawai’i. RESULTS: We used topical thoracic applications of 8 concentrations of Spinosad ranging from 0.028-3.6 mg/ml to evaluate the lethal concentration (LC50 and LC99) required to kill wild caught males. Resistance ratios (RR) were calculated by comparing the LC50 of wild flies to lab susceptible lines maintained in colony. Our results identified at least two new sites of concern for melon fly resistance on the Big Island, and at least 4 sites of concern for Oriental fruit fly, all of which were located on the Big Island. At these locations RRs exceeded 5. On Oahu, melon fly RRs exceeded 10. CONCLUSIONS: While the persistence of Spinosad resistance is concerning, it is a reduction compared to the values reported previously and prior to changes to Spinosad use recommendations by local extension agents beginning in 2017. For Oriental fruit fly, these RR values are the highest levels that have been detected in wild Hawaii populations. These data suggest that expanded Spinosad reduction and replacement programs are warranted given the ongoing issues with Spinosad resistance in Hawaii and expansion in the number of species affected.