"You can find alternative explanations for all of this, no question about it," Paine said. "This is something we think is likely to have happened, but we don't have a smoking gun."

Paine's suspicions were raised by introductions of 16 eucalyptus pests over the past 25 years. The pests were introduced a few at a time in batches from different regions Down Under.

"They would be from Queensland, or they would be from New South Wales, or South Australia," he says.

The demise of eucalyptus in California, where they aren't native species, isn't Paine's greatest worry. 

"But if you're talking about a major food crop, or a disease organism, the prospects are very, very disturbing," he said.