California's bustling international trade and tourism combined with its moderate climate and diversity of crops create opportunities for exotic pests to invade the state and take up residence. In this issue of California Agriculture, scientists from the U.S. Department of Agriculture, California Department of Food and Agriculture, and University of California report on the threats posed by imported plants and insects, and how these pests can be controlled.
A new sharpshooter threatens plants. An insect that has recently invaded California is creating serious problems for both agricultural and ornamental plants. The glassy-winged sharpshooter's greatest threat is its ability to spread the bacterium Xylella fastidiosa, which induces Pierce's disease in grapevines, almond leaf scorch disease, and a new disease known as oleander leaf scorch. If oleander leaf scorch were to kill all the oleanders along the state's freeways, Caltrans would suffer a $52 million setback.
Because the glassy-winged sharpshooter feeds on a broader variety of plants than other insects that carry X. fastidiosa, UC scientists say other diseases caused by different strains of the bacterium may hurt even greater numbers of crops and ornamental plants.
Sticking it to starthistle. Yellow starthistle now infests 22% of the state, covering about 22 million acres. This fast-spreading weed replaces desirable vegetation. In natural settings, its prickly thorns can make hiking paths unpassable and its density can help carry wild fires. Within the agricultural community, yellow starthistle most severely affects ranchers. While young shoots can be grazed by cattle, the sharp spines of older starthistle plants deter cows from feeding. Yellow starthistle also can be toxic to horses that eat it.
To control the invasive weed, scientists are attacking it with a variety of methods -- biological, chemical and mechanical.
USDA scientists have released overseas insects, including the peacock fly, to destroy yellow starthistle. The insect's larvae feed inside the seed head of starthistle, destroying its seeds. Serendipitously, another insect that looks like the peacock fly has been accidentally introduced and is significantly more widespread and more effective against yellow starthistle than the peacock fly -- or any other biocontrol insect to date. The "false" peacock fly is the only one that attacks late-blooming yellow starthistle flowers.
Of the herbicides registered for use on California rangelands and wildlands, the majority are effective only when applied to the foliage so plants that emerge after the herbicide application escape injury. In a UC study, a newly registered herbicide, clopyralid, provided excellent control when sprayed on the leaves or applied to the soil to kill germinating seedlings.
Successful control of yellow starthistle by mowing depends on both proper timing and the plant's form of growth and branching. Scientists found that a single mowing at early flowering effectively controlled erect, high-branching populations of starthistle. They say that mowing as part of an integrated approach with herbicide treatment, prescribed burning or biological control should provide effective control of starthistle.
Medflies are going but not gone. The Mediterranean fruit fly -- which attacks more than 250 types of fruits, nuts and vegetables -- could devastate the state's agricultural industry if it became established here. The state releases sterile medflies at a weekly rate of 125,000 to 200,000 flies per week per square mile over a 2,155-square-mile area of urban Southern California to prevent medflies from reproducing. CDFA scientists report that this areawide approach reduced the annual number of infestations found from 1994 to 1998 in the treated area by 93.3%, compared to infestations detected between 1987 and 1993.
Also in California Agriculture...Stinging news about Africanized honey bees and red imported fire ants on the move in Southern California.
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California Agriculture