From noon on May 22, masses of cabbage butterflies emerged in Shanghai's suburbs, and though their numbers had reduced by the afternoon of the next day, they were still widespread.

"The crowd of butterflies came suddenly at 1:00 PM like a white cloud, blocking the sunlight and blanketing the farm like a fog," said Wu Yonglin, director of the administrative department of Liaoyuan Farm in Fengxian District.

Other districts of the city to be affected were Nanhui, Jinshan, Minhang, Baoshan and Pudong, according to the Shanghai Morning Post.

"Their emergence came just like snow flakes as big as goose feathers and affected driving visibility," said a man named Chen who was driving when the butterflies appeared. 

The "snow" lasted about four hours before scattering, but the situation remained unchanged on lawns and gardens.

Ye, who has been growing watermelons for twenty years, is worried about his crop since, though the cabbage butterflies come every year, he has never seen such a large number.

The phenomenon is the result of the emergence of large numbers of Pieris rapae from the larval stage, according to Sun Xingquan, a butterfly expert and laboratory technician at Shanghai Jiao Tong University's School of Agriculture and Biology.

He and senior horticulturists He Cuijuan and Zhang Songhan at the Agricultural Technology Promotion and Service Center said the butterflies are not harmful to plants themselves, but their larvae are very harmful to brassicas, such as cabbage and radish. The caterpillars eat the leaves, sometimes leaving only the vein.

Li Huiming, a senior horticulturist at the Shanghai Vegetable Science and Technology Promotion Center, said that the cabbage butterflies' large-scale emergence was a sign that pests were at a somewhat serious level.

Apart from high temperatures suitable for the survival of the cabbage butterflies, the disappearance of natural enemies due to overuse of pesticides is the most important reason for their high numbers, said Li

Large numbers of the butterflies also emerged in other cities this month.

On the afternoon of May 17, many appeared on a square at 1 PM in Nanjing, capital of Jiangsu Province and dissipated an hour later.

On May 16, over ten thousand appeared in the county seat of Gaochun County in Jiangsu at about 8 AM, remaining for about half an hour.

On May 12, hordes appeared in the city center of Xiangfan, Hubei Province at 8 AM.

(Shanghai Morning Post, translated by Yuan Fang for China.org.cn May 26, 2005)