Exotic Super Pests Decimating Native St. John Agave Trees

The once common sight of bright yellow blossoms bursting from towering century plants, which used to dot the hillsides of St. John, are quickly becoming a thing of the past.

The population of Love City’s native Agave missionum, more commonly called century plants, has been decimated in recent years by an invasive super pest, explained Dr. Gary Ray, a botanist at the University of the Virgin Islands and St. John resident who specializes in native plant species.

The Mexican Agave Weevil was first discovered on St. Croix back in the early 1980s and made its way to St. Thomas by the 1990s, said Ray. The weevil showed up on St. John in 2000 most likely by way of Tortola, he explained.

Carried By the Wind
“Lots of people and plants go back and forth between Tortola and St. Thomas which is how we believe the weevil got to the British Virgin Islands,” Ray said. “The first place on St. John we discovered the weevil was on Haulover Bay which is only about a mile from the shores of Tortola. We think the weevil was probably carried by the wind over here.”

Once the first weevil was found at Haulover Bay, the island’s century plants quickly succumbed to the pest.

“After we found the pest on a plant in Haulover Bay, we saw this devastation of the population there,” said  Ray. “Within six months to a year, the weevil, which was discovered in the east, had progressed all the way to the west side of St. John.”

The weevil decimated Love City’s century plants so quickly because it altered its own cycle to adapt to the islands, according to Ray.

From Parasite to Parasitoid
In its natural habitat, the Mexican Agave Weevil was far less harmful than it has proved to be to the Agave Missionum. Back in its natural environment, the weevil targeted fruit trees, where it would bore a hole in the fruit and decimate a plant or two, but not the whole crop, Ray explained.

“Because it was a fruit eater, it would get some of the fruit crop but not all of it, in order to ensure its own fecundity,” said the botanist. “Since coming here, it changed its life cycle and is now a parasitoid, which is an insect that, during its immature stage as a larvae, kills its host. Most parasites don’t kill their host, but parasitoids do — they’re known as super pests.”

Importing Plants And Insects Too
The weevil’s change from a parasite to a parasitoid was probably due to the international plant trade where pests evolve to adapt to survive various sprays, Ray added.

“The pests which survive are the ones which adapt to some level of spraying and what you have then are those individuals who have evolved some kind of way to survive the toxicity,” he said. “That genetic information gets passed from generation to generation and before you know it, you have a super pest.”

Super Pest
“Under those conditions an insect population can pretty quickly become a super pest which is what we have on our hands now,” said Ray.

Since evolving into a parasitoid, the Mexican Agave Weevils have made short work of the century plants, which have only one growing  point. The only cells involved with the growth of the century plant are located on the top middle of the plant, explained Ray.

“Female weevils bore a hole right in that area and lays their eggs,” he said. “When those eggs hatch they eat out that area. Since that area is the very center of all the growth on the plant, once it’s eaten out by the larvae, the entire plant dies.”


The plant’s sole flowering event produced an enormous amount of nectar and pollen.

One Blooming Event
The weevil’s attack on the century plant population is made even more lethal when coupled with the natural cycle of the Agave Missionum, which bloom and spread its seeds only once in its lifetime.

Living up to 35 years and blooming only once, century plants produce an enormous amount of nectar and pollen, Ray explained.

“Because it flowers and fruits only once at the very end of its life cycle, which is very unusual for plants, that means the energy it gathers for 25 to 35 years is put into the flower event,” he said. “That is the remarkable nature of this plant. If there is such a thing as a keystone plant, this is one.”

Birds and insects gorge on the nectar, which comes in especially handy since century plants usually bloom during the dry season, added Ray.

Dry Season Relief
“Our two species of humming birds and a whole host of native insects gorge on the nectar and pollen which is produced in extremely large quantities,” said the botanist. “Because the species flowers during the dry season, it’s an extremely important a source of sugar for humming birds and insects during a time of scarcity.”

The natural cycles of the host plant and the parasitoid can easily spell disaster for the century plant.

“Rather than producing a crop of several hundred seeds with only 10 percent not taken, you have zero seeds, meaning the fecundity is effectively zero,” Ray said. “You have that repeated many, many times across a densely distributed population and you can wipe it out.”

Rapid Attack
In a little more than nine years, about two-thirds of the island’s century plants have been wiped out from the weevil, explained Ray.

“We had somewhere over 10,000 on the island, and probably even a lot more than that, before the attack,” he said. “I would be surprised if we haven’t lost at least two-thirds of the plants. That is less than a decade it took for the century plant to go from a very common plant of semi-arid conditions to something that is becoming uncommon.”

And the outlook doesn’t look promising.  

Entire Species at Risk
“When you combine the manner of the attack of the agave weevil and the  life cycle of the agave, it means death to the whole species,” Ray said.

While century plants are not yet critically endangered, Ray foresees this happening in a very short amount of time. To prevent genetic extinction, he’s been collecting seeds and shipping them to botanical gardens throughout the Caribbean.

“If you were to graph the population decline, you could project that in as little as a decade, agaves will be in a range of critical endangerment,” said Ray. “It’s better now to distribute where we can because you want to maximize the genetic information if you want to save the species.”

Saving Genetic Information
Ray’s seeds will go into botanical gardens’ living collections and be recognized as plants endemic to the Virgin Islands and Puerto Rico. The botanist is also trying to transplant agave trees to off-shore cays, but is unsure of how effective that plan will be.

“The plants will still be fairly vulnerable since the weevil was transported from St. John to Tortola by the wind so it could really go in any direction,” said Ray.

While it remains to be seen if anything can be done to save the Agave Missionum species, stricter regulations on the importation of plants, might stave off a future super pest infestation, according to Ray.

“Whenever you bring in plants, there is a chance of bringing in insects,” said Ray. “We have a multi-tiered problem to deal with and I don’t think we’re event at square one yet. Hawai’i has adopted some pretty strong regulations and they are doing much better than we are at  stopping the threat.”

More Regulations Needed
“I think we have to start heading in that direction,” Ray said.

As Ray scrambles to do his part to ensure the survival of the island’s native Agave Missionum, he’s also enjoying the plant’s beauty.

“I stood at Ram Head recently and looked across the south side of the island for evidence of the agave and we have precious few in flower this year,” said Ray. “This is one of the most beautiful plants we have anywhere in the Caribbean.”