BEIJING - Alien species are attacking crops and causing billions of dollars worth of damage to the economy even though detection rates have increased fourfold since 2002.

There were more than 320,000 detections of 3,400 harmful species at entry ports in the first nine months of last year.

"As China's global interaction grows, through trade and travel, the threat of invasive alien species is growing," Huang Guansheng, director of the department of supervision on animal and plant quarantine under the General Administration of Quality Supervision, Inspection and Quarantine, told China Daily in an interview.

"Cargos that contain harmful species will be returned or destroyed as we step up efforts to block them from China," he said.

Harmful species mainly come from major trading nations, including Brazil, the United States, Australia, South Korea and Argentina, according to Li Yuanping, a spokesman for the administration.

Insects account for about half of the harmful species detected. Other harmful species include weeds, roundworms, fungi, bacteria and viruses.

In the first six months of 2011, 46 plant epidemics broke out nationally, affecting 58 counties, statistics from the Ministry of Agriculture revealed.

Invasive foreign species were mostly to blame, experts said.

Foreign species can quickly adapt to a country as they are free from their natural predators.

"For example, a kind of caterpillar from Australia is the food of a certain bird there, but it has no enemy in China," said Xu Hongzhi, an analyst at Beijing Orient Agribusiness Consultants.

As imports of agricultural products grow, so too does the threat of invasive species, Huang said.

Agricultural imports rose from $11.85 billion in 2001 to $72.55 billion in 2011, an annual increase of 22.3 percent, statistics from the Ministry of Commerce showed.

But Huang said improved technology increased the chances of detecting harmful species.

"Quarantine dogs and X-ray machines also play a valuable role," Huang said.

Invasive species can play havoc with agriculture, Huang said.

For example, tilletia controversa, a fungus known to damage heads of wheat, can wipe out an entire crop. The Mediterranean fruit fly, from Africa, poses a threat to 300 types of fruit and vegetables.

Losses caused by 13 major invasive species, including pinewood nematode disease and water hyacinth, were up, according to Wan Fanghao, a specialist of alien invasive species at the Ministry of Agriculture.

Screening procedures at points of entry are not totally secure. "Many ports just carry out spot checks, so some enter," Huang said. "Honestly speaking, we can only slow down the invasion but we cannot stop it entirely."

But a defense system is being established. China revised its list of foreign plant quarantine pests in 2007 and the number of harmful species increased from 95 to 439. The Ministry of Agriculture also said it is committed to accomplishing a 100 percent detection rate and fighting any outbreaks with effective quarantine measures.

Invasive species, while free of natural predators, may find it difficult to survive in China, experts said.

"They will encounter a completely different environment, so the spread will be limited by natural conditions in the new country," Xu said.

Zheng Fengtian, an expert in rural studies at Renmin University of China, also said the invasion, so far, is relatively minor given the scale of global exchanges of seeds and grains.