Evaluating the Persistence of Pyrethrin and Pyrethroid Insecticides in Coconut Palm (Cocos nucifera) Canopies for Coconut Rhinoceros Beetle (Oryctes rhinoceros) Management

Abstract

Canopy application of contact insecticides is emerging as a short-term tool for protecting against coconut rhinoceros beetles (CRB; Oryctes rhinoceros) in Hawai‘i. Pyrethrin and pyrethroid formulations are primarily labeled for use on ornamental plants, which represent a large portion of Hawaiʻi’s public palms. To provide guidance on the frequency for which insecticides should be applied, we evaluated five products with labeling that allowed applications to coconut palm crowns as ornamentals and/or fruit production in Hawaiʻi. Burlap was soaked in spot-treatment or minimum-volume per area rates according to product labels, with water as the control. After drying, burlap was used immediately in bioassays or affixed to coconut palm canopies at a research station in Mānoa Valley, Oʻahu, exposed to local weather. Sub-samples were collected every four weeks for bioassays, where 4 to 5 g of treated burlap was placed in a perforated 500-mL bottle with four colony-reared adult CRB added (12 total per treatment). After a 24-h exposure, insect health was rated on a 5-point scale (1 = dead, 5 = healthy) via double-blind evaluation. CRB ratings were analyzed using KruskalWallis and Dunn’s tests with Bonferroni correction. Burlap treated with synthetic pyrethroids deltamethrin, prallethrin and lambda-cyhalothrin, cypermethrin, and zeta-cypermethrin retained toxicity against CRB for at least 12 weeks. The organic pyrethrins did not retain toxicity. Since many palms in Hawaiʻi are routinely serviced by arborists and landscapers every 3 to 5 months, a canopy application of the longer-lasting formulations may serve as an immediate management option for protecting palms from CRB.