The detection of medflies in Riverside County followed a recent finding of the Mexican fruit fly in San Diego and was expected to cause further problems for California's $26 billion agriculture industry.

Australia announced on Thursday it was suspending all imports of fruit from the United States because of the Mexican fruit fly. The move followed similar steps taken by New Zealand and South Korea, which banned importing agricultural goods from San Diego County because of infestation.

The discovery of the four medflies over the past eight days brought the total number of medflies found this year in California to 23.

As part of an eradication programme begun on Thursday in Riverside County, federal, state and local officials stepped up efforts aimed at trapping the insects andprepared to apply the insecticide malathion on host plants in a 200-metre radius of each of the four sites where the flies were found.

A spokeswoman for the California Department of Food and Agriculture said officials were also establishing a quarantine zone, but declined to comment on how much land would be affected. Officials also planned to release millions of sterile medflies as a further measure to eradicate the pests.

The medfly findings were the first in Riverside County since 1993, when the discovery of medflies prompted officials to quarantine a large part of the Los Angeles Basin.

Medflies are one of the most destructive agricultural pests, attacking more than 250 varieties of fruits, nuts and vegetables. Economists believe that if the medfly were to become established in the United States, it could cost billions of dollars in lost crops, exports, and treatment expenses.