Trapping protocols for South American palm weevil in California

Ivan Milosavljević, Christina D. Hoddle, and Mark S. Hoddle, Department of Entomology, University of California, Riverside.

South American palm weevil (SAPW), Rhynchophorus palmarum (L.) (Coleoptera: Curculionidae), is an invasive pest in California (CA) (Fig. 1). Native to parts of Mexico, Central and South America, and the Caribbean (Löhr 2013), this large black beetle, about 1.25 inches in length, was first detected in San Ysidro, in southern San Diego County CA, in 2011. Weevil populations likely established in or around San Ysidro in 2014 or earlier. Founding populations probably invaded San Diego County from Tijuana, Baja California, Mexico (~5 miles south of San Ysidro), where this beetle was first collected from infested Canary Islands date palms, Phoenix canariensis, in 2010. In addition to detections in San Ysidro, SAPW was trapped in Alamo, Texas in 2012 (USDA-APHIS 2012), and Yuma, Arizona in 2015 (USDA APHIS 2015). There are no reports of established populations of SAPW in Arizona and Texas or palm mortality caused by this weevil. Estimates suggest that SAPW has killed nearly ten thousand Canary Islands date palms in San Diego Co. (APC 2020).

Fig. 1. An adult male Rhynchophorus palmarum, the South American palm weevil.
Photo by Mike Lewis, Center of Invasive Species Research, University of California, Riverside

Palm Mortality

Infestations of SAPW can kill palm trees in about 9 weeks once feeding damage becomes apparent. Feeding larvae cause irreversible damage to the apical meristem also known as palm heart (Giblin-Davis 2001). This relatively soft and fleshy growing material is found in the crown of the palm tree, and it is responsible for generating new fronds. Feeding in this region can lead to crown drop and palm death. Generally, a few months after initial infestation, palm trees decline and die from larval feeding damage. Damage symptoms include crown tilt and collapse, and significant loss of fronds that results in a halo of fronds attached to the top of the trunk (Fig. 2). The bases of dropped fronds are usually heavily tunneled indicating construction of pupal chambers within which pupating weevil larvae make cocoons from palm fibers. Occasionally, fibrous cocoons containing pupae or unemerged adults will drop to the ground beneath affected palms (Fig. 3).

Fig. 2. Canary Islands date palms killed by SAPW in San Diego County. Photos by Mark Hoddle, Dept. of Entomology, UC, Riverside

The Economic Threat

SAPW presents significant threat to the ornamental and edible date palm industries in CA (Milosavljević et al. 2019). The urban landscape in CA is defined by palms, especially the ubiquitous Canary Islands date palm, and to a lesser extent, the edible date palm, Phoenix dactylifera, of which the latter is also known host for SAPW. The ornamental palm industry in CA is estimated at $70 million (US) per year. Canary Islands date palms have been valued at $500 (US) per 12 inch of trunk length and individual ornamental edible date palm costs around $5,000 (US). The risk SAPW presents to the edible date industry in Coachella Valley is substantial. The value of this crop is estimated at $100 million (US), growers produce approximately 47,000 tons of fruit grown on about 10,000 acres, and the industry employs around 6,000 people. In 2019 at Balboa Park in San Diego, the first confirmed report of SAPW killing Brahea edulis, the Guadalupe palm, native to Guadalupe Island in Mexico, was made.

Red Ring Nematode

The impact of SAPW on palms in CA may eventually be amplified by a palm-killing nematode, Bursaphelenchus cocophilus, commonly known as red ring nematode (RRN), the causative agent of red ring disease in palms. SAPW vectors RRN. Nematodes acquired by larvae render the weevils infectious, and movement of RRN from palm to palm is mainly by flying adults (Giblin-Davis 2001). Palms infected with RRN can die in 4 months after infection symptoms become noticeable. RRN has not been detected in SAPW captured in California and this nematode is not known from other locations in the USA either. However, RRN is known from parts of Mexico. As is the case with many invasive vector-pathogen systems in CA, the vector is detected first, then several years later, the pathogen is recorded (Milosavljević et al. 2017). A similar situation for SAPW-RRN may eventually develop in CA.

How Far can SAPW Fly?

The northern most urban areas in San Diego Co. infested with SAPW are a linear distance of approximately 80 miles from edible date production areas of the Coachella Valley. Adult SAPW are strong fliers and may be able to disperse naturally into date production areas from these infestation foci. In the lab, flight mill studies indicate that field-captured male and female weevils can fly on average 25 miles or more a day if they elect to do so (Hoddle et al. 2020). It is unknown if weevils undertake such long-distance flights in nature. However, flight mill studies indicate long distance flights are possible should weevils choose to do so, and this could occur in areas where there are no suitable hosts to attack. Field and lab studies indicate that SAPW flight activity is restricted primarily to daylight hours. Another way weevils can travel long distances is through accidental movement by humans when infested palms are moved into new areas (Milosavljević et al. 2019). Movement of live ornamental palms out of infested areas should be avoided to reduce the chances of unintended weevil introductions into new areas.

Which Traps to Use for Monitoring SAPW?

Incipient management programs for invasive SAPW continue to evolve in response to new infestations in CA. These programs rely upon trapping adult weevils in combination with insecticide applications which kill larvae and adults infesting palms, and removal and destruction of infested palms (Milosavljević et al. 2019). SAPW trapping utilizes either bucket traps hung from tree branches or staked to the ground or cone-shaped traps placed on the ground. All traps need to be loaded with commercially available SAPW aggregation pheromone and baited with fermenting bait (Fig. 4). Ethyl acetate can be used as a synergist to increase the combined attractiveness of the pheromone and bait. Details on building bucket traps, loading traps with aggregation pheromone, ethyl acetate synergist, and fermenting bait are available online (see Hoddle 2020). We recommend using cone traps over bucket traps for detecting and monitoring SAPW. In the field, cone traps were more effective than bucket traps for capturing and retaining SAPW (Milosavljević et al. 2020a). SAPW captures were five times greater in cone traps than in bucket traps that were suspended 1.5 m above the ground or placed on the ground. Cone traps captured almost 90% of SAPW that entered the trap (Milosavljević et al. 2020b). By contrast, of those weevils attracted to bucket traps, 89% entered, and of those entering traps, 82% escaped and only 18% that entered traps were retained and killed. Traps baited with the SAPW aggregation pheromone, ethyl acetate synergist, and dates with water or in combination with baker’s yeast were superior to other tested baits (e.g., fermenting molasses solution) and traps loaded with pheromone only.

Fig. 3. Development stages of SAPW. (A) eggs, (B) feeding larva, (C) a larva extracted from an infested palm, and (D) pupa fully and (E) partially removed from cocoon. Photos by Mike Lewis, Center of Invasive Species Research, University of California, Riverside

Suggestions for Effective Trap Deployment

Trap placement is important for detecting SAPW activity in an area of concern. Studies on red palm weevil in Europe have contributed to the following recommendations for trap placement for SAPW in California. Traps should not be hung from palm trees or placed near (i.e., within 500 yards) palms of interest. Traps are not 100% efficient in capturing weevils. If traps are placed too close to palms adults that are attracted to traps but are not retained in traps may start infestations in palms that are being monitored. If detecting low levels of palm weevil activity in the general vicinity is the goal of the monitoring program, traps should be deployed outside (perhaps > 0.5 mile away) of the immediate area of concern. If weevils are captured at this distance from the palms of concern it likely indicates that weevil activity is close by and steps should be promptly considered and implemented for protecting those palms. Trap efficacy is maximized if traps are placed in areas with partial or full shade. Full sun exposure, especially during the hottest parts of the day, rapidly diminishes trap potency.

References

Fig. 4. (A) Picuan trap, (B) hanging bucket trap, dead weevils floating in antifreeze in, (C) a bucket trap, and (D) fermenting bait in a plastic container with a perforated lid sitting in the capture pan of bucket trap. Holes in the container are for release of fermentation volatiles. Photos by Ivan Milosavljević and Mark Hoddle, Dept. of Entomology, University of California, Riverside

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