Published Friday, August 28, 2020
The Dreaded Palm Beetle
By Victoria Torley
The final line in last week’s column on coconut palms sparked a reader’s question: how does the coconut palm beetle destroy the plant and how do we control them?
First, the beetle itself. A mature beetle can be four centimeters long, about 1.5 inches but the larvae we found (we never found an adult) were bigger, about twice that size. The fertilized female flies to the crown of the palm, an open wound on the plant or the base of the fronds where she can deposit two hundred eggs. Fortunately, many are eaten by predators. Adult beetles do some palm damage, but it is the hatched larvae that will kill the palm. All the larvae we have seen have been at the base of the fronds. When they have finished off your palm, the larvae fall to the ground to pupate in a cocoon they build of palm fibers. The cocoon we found was about five inches long. When they hatch as beetles, the cycle begins again. The entire cycle of destruction takes from seven to ten weeks.
The larvae tunnel through the palm for a month, destroying the delicate tissues of the plant. In our case, the first sign that we had a problem was the browning of one of the palm fronds. We had no idea what was causing the problem. When more fronds died, we pulled them from the palm and found an enormous larva. By that time, it was too late. We had planted five palms and none of them survived.
So, what is to be done about the beetle?
We had planted our palms in a grove, perhaps ten or fifteen feet apart. That was our first mistake. The adult beetles are not great fliers and we were giving them palms within easy reach. In fact, if your palm makes it to twenty years old, it is usually so tall that the beetle can’t fly high enough to reach the crown. Sadly, it can find other ways into the plant. When we put in palms now we separate them by 100 meters.
In commercial groves, they have interesting ways of locating the larvae – they use stethoscopes or dogs! A dog can actually hear the larvae chomping away inside the palm and the stethoscope confirms their presence. Insecticides are immediately applied.
Insecticides are your best line of defense against these voracious beetle larvae. First, to kill the adults so they can’t lay eggs in the palm and then to kill the larvae. Pheromone baited beetle traps are commercially available although I haven’t seen them here in Costa Rica. If you decide to use them – and find some – make sure you set them up far away from the palms. No sense enticing them toward your plants.
Insecticides can be used at the crown of the palm, at the base of each frond, and on any damaged portion of the trunk that might provide beetle access. Any number of insecticides, from organics to chemicals, are available. You can also use a sealant to stop-up any damage you can see on the trunk of the plant.
Finally, clean up around the base of your palms frequently. The larvae need fibers from the palm in order to make a cocoon so leave them nothing to work with and burn the debris you collect.
I miss my adult palms; they were just beginning to produce fruit when they were killed. Now we have another long wait – it takes seven years before a palm produces coconuts.
Plant of the Week
This is my new baby palm. Fingers crossed!
Editor's note: More information on this article or about gardening, Ms. Victoria Torley, gardener columnist, can be reached at firstname.lastname@example.org