The palmetto weevil blues

“Oh no!” – was I all I could say as I noted the telltale brown fronds and slightly tilted, leaking canopy.  Our prized Bismarck palm was marked for death as masses of unseen three-inch long palmetto weevil larvae tunneled inside eating the heart out of this majestic palm.   Growing up to fifty feet tall, the stunning palm known as the Bismarck palm has become a popular landscape subject throughout Southwest Florida.  Planted as an accent specimen in many landscapes, beautiful silver-blue Bismarck palms tower over other plantings to the pride of homeowners.  Is this a good choice for your landscape? 

Originally from the island nation of Madagascar, Bismarckia nobilis have been popular landscape subjects in Southwest Florida for some time.  Potentially growing to an enormous size of up to sixty feet tall and twenty-feet wide, the Bismarck palm is widely sold in local garden centers and nurseries. The growth seen in these palms is fairly rapid and the one planted at our East Port Environmental Campus Demonstration Garden in Port Charlotte grew from a five-gallon, three foot plant to well over twenty-feet in about six years.  Site selection is very important and you should definitely “plan before you plant”.   Keep in mind that this palm often outgrows small residential lots as its gigantic proportions make everything around it look puny.  Bismarck palms are best planted in full sun on well-drained soil.  Once established, they are highly drought- tolerant and moderately salt-tolerant – good characteristics for our area.  A recommended fertilizer program for all palms includes the use of a granular 8-2-12-4 (or 8-0-12-4) in November, February and May, and a 0-0-16-6 in August, applied as per label directions.

Bismarck palms are best suited to Hardiness Zones 10A and 11.  Charlotte County has some zone 10A right along the coast, but further inland the zone changes to 9B – a potentially cooler area.  However, the Bismarck palm has been planted well out of its hardiness zone range all the way north to Orlando.  While there may be some micro-climates in these cooler areas, there is the real risk of freeze damage.  Some Bismarck palms in Charlotte County visibly suffered from freezes several years ago.  Months after these events, the Extension Office began to receive calls from heart-broken clients with failing Bismarck palms.  While some were associated with lightning strikes, many seemed to show evidence of stress due to cold damage to the bud.  This damage then led to palmetto weevil invasion and eventual collapse of Bismarck specimens in the landscape.  While we have not had a severely cold winter in a few years, Bismarck palms are still randomly dying from time to time.   Unfortunately, recent observations have indicated that even “apparently healthy” Bismarck palms may be attacked by palmetto weevils.  This was the case with our specimen.  Palmetto weevils are large beetles which are normally attracted to palms under stress.  Stressed palms give off a chemical scent picked up by passing palmetto weevils.  Once they find a suitable host, the weevils release chemicals that attract more weevils to the feast, and an infestation is born. 

It is still considered a good management practice to keep your palms healthy and stress-free (no over-pruning for example) – this should keep palmetto weevils at bay in most cases.   One band-aide approach may include a chemical option.  A root drench with Imidacloprid (a systemic insecticide) as per label directions may afford some degree of limited protection, but there are no guarantees. 

The funeral for our Bismarck palm has come and gone.  We enjoyed it as a crown jewel in our Demonstration Garden for years, but knew that we could lose it someday.  So, if you decide to plant a Bismarck in your landscape, check your hardiness zone or micro-climate options, consider the ultimate height and width of this behemoth, fertilize it properly and keep in mind that there are some factors out of your control that could limit this palm’s long-term success.  Those who already have Bismarck palms established in the landscape, keep them stress free, keep your options open on palmetto weevil deterrents, and appreciate these big blue giants!  For more information on all types of palms, please call our Master Gardener volunteers on the Plant Lifeline on Mondays, Wednesdays and Fridays from 1 to 4 pm at 764-4340 for gardening help and insight into their role as an Extension volunteer.  Don't forget to visit our other County Plant Clinics in the area.  Please check this link for a complete list of site locations, dates and times - http://charlotte.ifas.ufl.edu/horticulture/Plant%20Clinics%20Schedule.pdf.     

Resources:

Broschat, T. K. (2016) Cold Damage on Palms.  The University of Florida Extension Service, IFAS.

Broschat, T. K (2015) Bismarckia nobilis: Bismarck Palm.  The University of Florida Extension Service, IFAS.

Weissling, T. J. & Giblin-Davis, R. M. (2016) Palmetto Weevil, Rhynchophorus cruentatus Fabricius (Insecta: Coleoptera: Curculionidae).  The University of Florida Extension Service, IFAS.