Palm Treatments Show Promise in Reducing CRB Populations (December 2021 Newsletter)

As coconut rhinoceros beetle (CRB) detections increased overall this summer, promising results from palm treatments are a welcome and hopeful sign in the fight against CRB. Palm treatments offer a complementary approach to earlier methods of manually searching breeding sites for CRB, minimizing available breeding material through processing and heat treatment, and trapping adult beetles. After observing reductions in CRB populations in treated areas over the past year, here’s a look back on how the treatment was developed and applied, plus a look forward to the possibility of future applications.


CRB Response field technician Leland Williams applies treatment to a coconut palm in a CRB-infested area.


Dr. Zhiqiang Cheng, principal investigator of the Turfgrass and Landscape Pest Management Lab at the University of Hawaiʻi at Mānoa, led the effort to research effective chemicals to control CRB, and recommendations for field application techniques. After five years of lab and field experiments, Dr. Cheng and his team demonstrated that imidacloprid and acephate were effective in killing CRB that fed on treated palms without measurable adverse effects on the treated tree or surrounding soil. Rather than applying the chemical to the crown of the tree or performing a soil drench application, this study explored systemic injection (into the palm trunk) which has several advantages over other application methods. Systemic injections incorporate the chemical into the tree tissue, concentrating it in the juices that CRB feed on. The chemical is delivered directly to the plant so it has no soil leaching and fewer off-target effects compared to a soil drench. Since the pesticides used degrade in sunlight, crown (foliar spray) applications will require frequent reapplication whereas the chemical continually moves into the crown from the trunk providing effective treatment for many months following injection.



Field technicians from the CRB Response are able to prepare smaller palms for systemic treatment by trimming fruits and flowers, but taller palms require contracted landscape professionals.


Once the early trials demonstrated their effectiveness, areas were selected for landscape-scale treatment. Sites were chosen based on high concentration of CRB in the area and the accessibility and maintenance of palms. Before treatment, fruits and flowers are removed from palms to minimize any potential adverse effects to pollinators or accidental ingestion of fruits during the treatment interval. Palms that were already on a six-month trimming schedule required lower startup costs pre-treatment. After palms were treated, sites showed reductions in CRB trap detections, especially relative to comparable infested areas. The greatest impact of treatments was observed seven to nine months after application. We expect that this delay is caused by the time it takes for the chemical to reach the crown of the tree and for earlier CRB life stages to emerge from nearby breeding sites and feed on a treated palm.



CRB Response field technician Leland Williams measures the diameter at breast height (DBH) of a palm in preparation for treatment.


Results from these field applications demonstrated that treatments are not effective in protecting individual palms, but instead show better results when applied to a large majority of palms. If there are pockets of nearby untreated host trees, adult beetles may continue to feed and reproduce. This broad scale treatment approach means that its success in reducing CRB detections depends on the trust and support from a majority of tenants, owners, and stakeholders in the area. The CRB Response recognizes and deeply appreciates the time and effort individuals and communities put in to learn about treatment and coordinate site access with us.


Palm treatments were applied in two distinct areas, Iroquois Point (Sept - Dec 2020) and Pearl City Peninsula (May - Sept 2021). In both locations, the CRB populations were on an upward trend for several years before injection and both show sustained population decreases within 1-2 months from the midpoint of injections while surrounding untreated areas had increased trap detections over the same period.


Looking ahead to new sites for palm treatments, select areas in Central Oʻahu have recently seen an increase in CRB detections and are an upcoming priority. Coordinating and applying palm treatments takes a significant portion of the CRB Response team’s capacity, but based on promising results, this treatment approach is a key piece of our strategy in reducing or eliminating CRB populations. Still, palm treatments cannot replace existing methods to stop CRB from spreading or multiplying, such as trapping, treating breeding sites, preventing movement of infested mulch, palms, or green waste, and stakeholders adopting best management practices.


 

Pearl City Peninsula, Waipio Peninsula, West Loch, Mililani, and Kunia are coconut rhinoceros beetle hot spots on Oʻahu as of November 2021. If you are in one of these areas, please keep an eye out for CRB, capture any you find, and report them here. Although all CRB populations are a concern, these hot spots are not equally infested.



Traps in Waipio Peninsula and Pearl City Peninsula catch the majority of CRB on Oʻahu (marked in bright yellow). In these areas, significant breeding sites have been identified and are undergoing treatment. In less infested hot spots and areas with occasional detections such as the West Side, our teams are in the process of identifying active breeding sites and developing treatment plans with tenants and land managers. While these areas continue to detect beetles, recent treatments in Pearl City Peninsula are beginning to reduce catches in the area. We are continuing to monitor the population to determine if the decrease will be sustained.