Insect Infestation That Is Killing Off San Diego Palm Trees

Palm trees are a staple in San Diego’s native and natural landscape. They come in all shapes and sizes and grow unabated throughout the entire San Diego region due to the region’s mild climate and arid conditions. It’s impossible to think of North County San Diego without conjuring up an image of palm trees and the beach.

While only one palm tree is native to California, the California fan palm, it’s natural for many individuals to assume that many species are native to the area. But the reality is that many of the commonly encountered species originate in Mexico, South America, Africa, the Middle East, Australia, and Asia. Despite the origins of these species, they have found ideal growing conditions in and around San Diego.

But while these trees are appreciated as an integral feature in San Diego’s natural and manmade landscapes, they are under threat. The invasive South American palm weevil presents a formidable challenge when they infest palm trees. These weevil infestations kill off many trees.

History & Origins of the Palm Weevil

The South American Palm Weevil, or Rhynchophorus palmarum, is native to parts of Mexico, Central and South America, and the Caribbean. It is not native to Southern California. Unfortunately, the insect migrated to the region in infested Canary Island date palms transported from Tijuana, Mexico, in 2010. It was first captured in San Diego County in 2011. Since that time, it has become an established invasive species in Southern California, near Alamo, Texas, and Yuma, Arizona.

The South American palm weevil presents an enormous threat to ornamental and edible date palms in California. The date palm industry in California is worth approximately $70 million per year, although the palm weevils threaten the viability of this industry.

In addition to date palms, the weevils reproduce on several other palm species, including coconut trees, African oil palms, acai palms, coco de palmito, and sago palms. They will also complete their development on planted sugar cane.

Further, adult palm weevils may feed on ripe fruit of non-palm tree hosts, such as avocados, pineapple, custard apples, breadfruit, papayas, citrus fruits, mangoes, bananas, guavas, and cocoa. Many of these agricultural products are present in the San Diego region, making them vulnerable to damage from adult palm weevils. While the insects cannot reproduce on these trees or their fruit, they can derive sustenance from them.

Damage to San Diego’s Palm Trees

With a South American palm weevil infestation, the primary damage to the tree occurs during reproduction. Once the adult weevil reproduces on a palm tree, the larvae feed on the palm heart. The palm heart is composed of soft, fleshy material growing at the top of the palm tree. It is responsible for generating new palm fronds. Once the plant has been infested, the feeding generally results in collapse of the heart, with no new frond growth and death of the tree. In some infestations, the larvae will feed near the base of the trunk, leading to trunk collapse and the death of the palm tree.

Infestations are often not caught until it is too late to save the tree. When the larvae feed on the palm heart, the damage will appear as a crown tilt or a flattened top. Advanced stages of an infestation have a fully flattened top with the remaining fronds down the trunk dry. If the larvae feed extensively on the midsection, the top of the palm may become detached and fall to the ground. As infested fronds fall from the tree, you can often spot the tunnel holes at the base of the frond where the larvae have moved.

Detection & Disposal

Unfortunately, there is no way to treat a South American palm weevil infestation. But once an infestation has been detected, the palm tree must be removed immediately. These removals can be dangerous since the extent of internal damage to the tree is often unknown, which presents the risk that it can collapse without warning.

Once removed, the palm tree should be chipped, the material should be contained and disposed of at a landfill that can cover it immediately. This process reduces the risk of spreading adult weevils into new areas near the disposal site. Adult palm weevils can fly up to 15 miles to find a new host. Since the South American palm weevil presents such a tremendous risk for the longevity of North County San Diego’s beautiful palm trees, UC Riverside is tracking suspected infestations, which may be reported here.

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