Invasive root weevil spreads to Southern California

Diaprepes root weevil feeds on more than 270 plant species, incuding citrus and nursery crops.

Entomologist Beth Grafton-Cardwell knew it was just a matter of time before the Diaprepes root weevil, a serious foe of citrus and nursery crops, showed up in California. Grafton-Cardwell’s instincts were excellent because the destructive weevils that feed on more than 270 plant species were recently found in southern California.

Until then, the pest was found only in Florida and Texas. It is native to the Caribbean region and was first found in Florida in 1964. Today, more than 30,000 acres of citrus in 23 counties in Florida are infested. In September 2005, the root weevil was discovered in an urban area of Newport Beach, and a month later the pest surfaced 18 miles away in Long Beach.

The weevil has been intercepted and destroyed numerous times in California in shipments of plants, truck trailers, and cargo holds of aircraft. The California Department of Food and Agriculture (CDFA) is surveying these infestations in Los Angeles and Orange counties and developing an eradication plan. An emergency eradication response is necessary now to ensure the root weevil does not continue to multiply and spread to other areas of the state.

With three-year funding from the Exotic/Invasive Pests and Diseases Research Program (EPDRP), Grafton-Cardwell, a researcher from University of California, Riverside, traveled to Florida in 2001 to work with researchers to find ways to prevent the spread of this insect to citrus groves and other crops in California.

" It’s imperative that we prevent this pest from migrating from urban areas into agricultural areas because the result could be devastating to California crops,” says Grafton-Cardwell. 

The root weevil larvae plunge underground and feed on the roots of the plant. They will often encircle the taproot, impeding the ability of the plant to take up water and nutrients, killing the plant. This type of injury also provides an avenue for root rot infections. A single larva can kill young plants while several larvae can cause serious decline of older, established plants. Because larvae are below ground, it is difficult to detect them before decline of above ground portions of the host are observed. In the Florida studies, researchers used a special trap called a “Tedders trap” to collect adult weevils.

This trap acts like a tree trunk, and the emerging beetle climbs up it and is caught in a container at the top. Researchers also applied a strain of a fungus to the surface of the Tedders trap to kill adult weevils. This way, the trap monitors the pest and also distributes the fungus into the weevil population to control it biologically. In the Florida project, researchers also traveled to the Caribbean to collect tiny, stingless wasps (parasites) that attack the weevil’s eggs.

Researchers released two parasites in groves and ornamentals that were infested with the pest in 10 counties in southern and central Florida in 1999 and 2001, respectively. Currently, the parasite, Aprostocetus vaquitarum, seems to have established well, however, it is sensitive to some insecticides.  Establishment and recovery of the wasps appears to be more successful in pristine habitats like ornamental plant nurseries than in citrus groves. It is likely that the pesticides that are applied to citrus for other pests are hindering the parasites. New species of parasites that attack the weevil eggs were collected from islands in the Caribbean during 2002. These new species appear to be promising when tested under quarantine conditions, and permission from federal agencies to release these species in the field is pending. 

Studies in Florida of natural enemies such as the fungi and parasites will help with California efforts to manage this pest. Although a strong flyer, Diaprepes root weevils generally only fly up to 300 yards to find food. The real threat of long distance spread of this pest could come from humans moving infested plants or soil. 

Grafton-Cardwell is working with other agencies to educate citrus growers, nursery workers and others. Through funding from the EPDRP of the University of California Statewide Integrated Pest Management Program and Center for Invasive Species Research, a team of researchers from the CDFA, the University of California, and the University of Florida developed a brochure describing the biology of this pest and the management program used in Florida.  The booklet, Diaprepes Root Weevil (ANR Publication 8131), is available for free online at http://anrcatalog.ucdavis.edu/pdf/8131, and a Web site, http://citrusent.uckac.edu/Diaprepeshomepage.htm, features the pest in relationship to citrus.  

If you see this pest, call your local Cooperative Extension office or agriculture commissioner.

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