University of California Entomologist Mark Hoddle

University of California, Riverside Entomologist Mark Hoddle says close to a dozen new invasive species are discovered each year in California. Some of these pests have wide-spread damaging impacts to the environment and agricultural production.

Hard work continues against California invasive species

Biological control methods appear to control some dangerous pests Researchers still looking for natural enemies to other invasive species Nearly half of nation's threatened species are impacted by invasive organisms    

While much attention has been garnered by one particular invasive pest sweeping its way through California there are others just as damaging and costly, according to two University of California entomologists.

Mark Hoddle and Richard Stouthamer are professors at UC Riverside in southern California. They have been studying various invasive pests introduced into California in hopes of finding methods to control and perhaps eradicate them from their new homes.

Some of these pests have strictly urban and wilderness ramifications, such as the Red palm weevil and the Goldspotted oak borer; others, like the Asian citrus psyllid, Glassy-winged sharpshooter, Diaprepes root weevil and Polyphagous shot hole borer, to name a few of many, have potentially devastating ramifications to commercial agriculture.

The battle against these organisms relates to the various diseases they spread. Pierce’s Disease, West Nile Virus, Newcastle Disease, citrus greening, and foot & mouth disease are just some of the maladies invasive pests bring with them.

Hoddle and Stouthamer spoke at the recent CAPCA annual conference in Anaheim, telling professional crop advisors and others in the room that the introduction of invasive pests to California seems to be increasing.

Prior to 1989 California acquired about six new invasive species per year, Hoddle said. Since then that number has increased to about 10 per year. Hawaii and Florida are acquiring about 15 new invasive species per year, he continued.

While cost estimates to battle these pests in California are about 20 years old, Hoddle estimates that the economic impact of fighting invasive pests in the Golden State well exceeds $3 billion annually. The economic impact to the United States is estimated at $138 billion per year, which Hoddle calls a conservative estimate.

Invasive organisms are also to blame for the decline in certain protected species, which are now considered “threatened” under the Endangered Species Act. Hoddle says 42 percent of threatened US species are due to invasive organisms.

ACP commands attention

Hoddle is well-known in California for his work with biological control methods for the Asian citrus psyllid (ACP), which quickly swept across southern California starting in about 2008. Because of the pest’s ability to spread a lethal bacterial disease among citrus trees, officials are on high alert for the pest.

“We’re very worried about the Asian citrus psyllid now that it’s in California,” Hoddle said.

According to Hoddle, a promising biological control method for the ACP has been the Tamarixia radiata, a tiny parasitic wasp that parasitizes ACP nymphs by laying its eggs in them and has, in Hoddle’s quarantine lab at UC Riverside, been seen attacking adult psyllids as well.

Hoddle is studying a second parasitic wasp he hopes to enlist in the fight against the ACP. The U.S. Department of Agriculture allowed Hoddle to study the Diaphorencyrtus aligarhensis and is considering releasing the parasitoid in California.

The public comment period on proposed releases of the Diaphorencyrtus aligarhensis in California closed in late October and approval to begin releases in California is pending.

What makes the Diaphorencyrtus aligarhensis and Tamarixia radiata promising as biological control methods is they are host-specific parasites that kill ACP nymphs. Current releases of the Tamarixia in southern California show promising results, according to Hoddle.

Hoddle is also working on biological control projects related to the Glassy-winged sharpshooter (GWSS), which can transmit Pierce’s disease in grapes.

Hoddle recently traveled to Tahiti to help local officials there with a particularly large and troublesome population of GWSS, a large leafhopper that was as much a public nuisance as it was damaging to vegetation.

Based on Hoddle’s work, agricultural officials in Tahiti released the parasitoid Gonatocerus ashmeadi to combat GWSS populations. Surveys suggest the GWSS is being held in check on the island under very low densities, thanks to the parasitoid.

Work to address GWSS populations in California has been under way for about 10 years in some unsprayed lemons at UC Riverside. GWSS populations there have dropped by almost 95 percent because of the parasitoid, Hoddle reports.

Lethal pest in avocados

Stouthamer is studying a particularly troublesome pest in California called the Polyphagous shot hole borer (PSHB).

In the survey of a Los Angeles botanical garden, Southamer found that the pest appears to attack about 200 different tree species. Trees that appear most susceptible to PSHB include the Box elder, maple, coast line oak, sycamore.

While many of these tree species line California streets, also concerning is that the PSHB likes avocado and persimmon trees. Even worse is there currently appears to be no effective chemical or biological control methods for the PSHB, according to Stouthamer.

While the PSHB has also been found in olives, Stouthamer suspects that in those instances the presence of the PSHB was due largely to an extremely large population of pests and not because they were particularly attracted to the olives.

The PSHB kills its host trees by boring through the bark and into the cambium layer just below the bark. It is here that it lays eggs and spreads a fungal disease in the tree. The fungus attacks the tree and blocks the movement of water through the tree, Stouthamer says.

There are several instances in southern California where PSHB infestations are large and growing. A large infestation in avocados in Escondido, Calif. was discovered in September. Noteworthy in this discovery was the absence of PSHB in the same grove in June, while a September survey showed trees were “riddled with them.”

Because PSHB hosts are far and wide, Stouthamer says the ability for the insect to move from urban to agricultural zones is significant.

California can be a magnate for invasive pests, according to Hoddle. That’s because of the large number of international travelers who come to California and the sea ports which can attract aquatic species as well as land-inhabiting organisms.

Troubling to Hoddle is a growing and vocal minority of ecologists who apparently believe that invasive species are not the problems they’re made out to be.

“They think we should just relax and let them install themselves in the environment and do whatever they like,” Hoddle says. “I think that’s a wrong viewpoint to be taking with a lot of these organisms.”

For instance, one pest Hoddle has been studying in a wilderness setting is the Goldspotted oak borer. This pest appears to be decimating oak trees in the Cleveland National Forest of southern California and could be spreading elsewhere as oak trees are common throughout much of the state.

The movement of oak as a firewood source is also an effective method of moving the pest from one end of the state to the other, he says.

Also making California ripe for various invasive pests is its varied climate zones that range from desert to alpine to near rain forest conditions.

That California urban planners have imported countless species of plants and trees to landscape cities and urban neighborhoods are also attractive to pests who manage to hitchhike from foreign lands into California, Hoddle says.

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