Editor’s Note: California Department of Food and Agriculture has established two quarantines zones in southern California to stop the spread of Diaprepes root weevil. Left unchecked, the pest could damage urban and rural landscaping, the nursery industry, and significant portions of California’s fruit and vegetable output, including citrus, a favorite of the pest. Beth Grafton-Cardwell, Extension specialist and entomology researcher, UC Riverside, stationed at the Kearney Ag Center, Parlier, Calif., prepared this report on the exotic pest found for the first time in California.
Diaprepes root weevil is a pest of ornamentals, citrus and other crops (cassavas, papayas, sweet corn, peanuts, potatoes) in the Caribbean region and Florida. It arrived in Florida in 1964 and has spread throughout the southern two-thirds of that state. The adult stage of the beetle is fairly large, three-eights to three-quarters of an inch and is black in color with bands of yellow, orange, or gray on its back. The adult feeds on leaves of many different host plants. It lays its eggs in clusters of 50-100 glued between two leaves. The eggs hatch, and the neonate larvae drop to the ground and burrow into the soil.
The larval stage is a grub that feeds on roots of plants and trees. They initially feed on the smaller fibrous roots of the plant, moving to larger roots as they mature. The larvae complete 10-11 instars over a period of 8-15 months. The pupa remains in a chamber for 15-20 days and then the adults emerge. In Florida, heavy populations of grubs can girdle citrus trees. Diaprepes root weevil damage to roots can also make them more susceptible to root rot organisms such as Phytophthora. Citrus growers in Florida are spending as much as $400 per acre to control Diaprepes and Phytophthora.
In September 2005, a dead adult Diaprepes root weevil was found in a gypsy moth trap in an urban area of Newport Beach, Calif. Surveys were conducted by CDFA. Approximately 40 live adult beetles were found within a one-fourth mile radius of the initial find. A few weeks later, a homeowner in Long Beach reported finding a beetle. This finding is approximately 18 miles from the Newport Beach location.
Diaprepes root weevil can feed on and/or develop on more than 270 species of plants from 59 plant families. In all likelihood, the beetles arrived in California on ornamental plants from Florida and were planted into the landscaping of these housing complexes. Molecular testing of individuals in the two locations will be conducted to determine the origin of the beetles.
The Newport Beach and Long Beach infestations are the first known established populations of Diaprepes root weevil in California.
Monitoring for Diaprepes root weevil is done primarily by visual surveys, looking for notching of the leaves (similar to fuller rose beetle damage), frass, egg masses glued between leaves, and the presence of adult beetles. The adults are fairly poor fliers, and drop to the ground and play dead when disturbed. Thus, a good method of sampling for that stage is to place a beating sheet on the ground under a plant and shake foliage over it.
Diaprepes root weevil is well established in Florida, and in that state they use a combination of tactics used for control of this pest. In citrus, the pest is controlled primarily by insecticides. The adult stage is best controlled by foliar pyrethroid or carbamate insecticides. The neonate larvae can be killed using a soil application of bifenthrin that acts as a barrier when they drop from the eggs on the leaves to the soil and attempt to burrow in. Later larval instars are the most difficult to kill because they are protected by the soil. Treatments of thiomethoxam and treatments of imidacloprid in combination with nematodes (biological control agent) have helped to reduce larval stages.
The egg stage is most effectively controlled by the insect growth regulator diflubenzuron. If the eggs are laid on treated leaves or the female feeds on treated leaves, the eggs do not hatch. Insecticides can reduce but not eradicate Diaprepes in Florida because the adults have a wide host range that is difficult to treat on an area-wide basis and the weevil has spread throughout a large area of that state.
The University of Florida has ongoing research programs studying the use of biological control agents. There are commercial formulations of parasitic nematodes, Steinernema riobrave (BioVector) and Heterorhabditis bacteriophora (Grubstake) that are used as a treatment to help reduce the larval stage of Diaprepes in Florida. However, soil type influences their efficacy as the nematodes do not move well through pore spaces in heavier, clay soils with small particle size.
Fungus for control
A fungus (Beauveria bassiana) that affects the adult stage has been utilized to control Diaprepes, however, the fungus has limited persistence in the soil and application rates are very expensive. A number of Hymenopterous parasitoids have been collected from the Caribbean region and one in particular has established in Florida, Aprostocetusvaquitarum. Parasites attack the egg stage of Diaprepes and are less effective where winter temperatures are low and where pesticides are used. Additional releases of new parasite species in Florida are planned.
A scientific advisory panel, including researchers from the University of Florida, was convened by the California Department of Food and Agriculture to discuss an eradication program concluded that eradication is possible because of the relatively confined nature of the two infestations. Eradication is desirable because of the high cost of this pest to the agricultural and ornamental industries. The panel recommended that a combination of insecticide treatments be employed n the infested areas targeting the various life stages of the pest.
Because of the long lifecycle of the pest and the protection of the soil for the larval and pupal stages, it was concluded that an eradication program is likely to take five to seven years.
For more information on the biology and damage potential of this pest for citrus, and color photos, print out the free, downloadable pdf file brochure 8131: Diaprepes Root Weevil http://anrcatalog.ucdavis.edu/pdf/8131.pdf
Quarantine areas around the infested sites in Newport Beach and Long Beach have been established, restricting the movement of plant parts, soil, and other materials that may harbor any life stage of the Diaprepes root weevil. This is being done to limit the spread of the weevil out of these areas.
Despite the fact that the adults are capable of flight, they tend to use man’s activities to move around. The quarantine area extends one-half mile out in all directions from the known infested sites. County and state personnel are carefully monitoring the quarantine areas and compliance has been good. Work is continuing on obtaining funding for the eradication effort, surveying additional areas to insure that Diaprepes is not more widespread, and educating those in the infestation areas as to why Diaprepes is such a threat to California agriculture.