Five years ago, University of California Davis entomologist Frank Zalom saw whitefly in strawberries in the Oxnard-Ventura area of Southern California for the first time. He shrugged it off as an unusual occurrence and waited for it to go away.
Instead, the pest has spread up the Northern California coast and today is adding considerably to treatment costs in the Watsonville-Salinas areas where thousands of acres of strawberries are grown each year.
"Usually you think of the whitefly as being in the subtropical family of insects," Zalom said, "and the Monterey Bay is definitely not a tropical region."
Similarly, five years ago, Corvallis-based USDA plant pathologist Bob Martin saw strawberry crinkle virus in the Totem variety of strawberrie s in Northern Washington for the first time. Because the variety was known to be resistant to common aphid-vectored viruses, it took Martin and other pathologists several years before realizing what was occurring and how to react.
Finally, in 2004, pathologists issued an advisory for growers to treat aphids as a means to control the virus - a treatment that today has become a regular part of production costs. Scientists now believe for the first time aphids live long enough to transmit strawberry crinkle virus in the colder Northern Washington climate.
In general, plant scientists are hesitant to attribute new pest pressure to global warming. But mounting evidence in recent years is prompting them to consider the possibility.
"I think it's got to be a change in temperature to allow them to survive in these types of conditions," Zalom said about the emergence of whitefly in California. "Whether that change is caused by increased urbanization or global warming, or a combination of the two, I don't know."
One thing is clear: Prior to 2001, the greenhouse whitefly was not a problem in commercial strawberries in California. The pest now is the target of an at-planting treatment of a systemic soil insecticide and in some cases a second above-ground treatment of an insect growth regulator in the weeks leading up to harvest.
Treatment costs can be high, Zalom said, but growers risk losing as much as 20 percent of their yield from a combination of damage caused by whitefly feeding on plant leaves and a disease called sooty mold, which is caused by whitefly secretion.
Martin, who works for the USDA's Agricultural Research Service, also is hesitant to attribute new pest problems to global warming. But he acknowledges that a rise in temperature caused by global warming is a likely explanation for the strawberry aphid's newfound ability to live long enough to transmit strawberry crinkle virus in Northern Washington.
Martin points out that growers in Oregon and California have long treated for the disease in Totem plantings, but in the 13 years he worked in Vancouver, British Columbia, as a plant pathologist for Agriculture Canada, he never encountered the disease. And, he said, his predecessor at Ag Canada also never saw the disease in the 25 years she worked there.
According to a commonly held belief, it takes 19 to 20 days for the virus to build up to high enough levels for an aphid to transmit the disease to a noninfected plant. Until 2000, evidence suggested the aphid simply didn't live that long.
Scientists can point to several other examples of new pests rearing their heads in international agriculture, including new problems with whiteflies in blackberries grown in the Southern United States and the expanded range of red fire ants in the West. Among explanations forwarded for the emergence are introduction of exotic pests through increased trade and mutations of pests developing resistance to pesticides. But scientists also believe global warming has a place in the explanation and many believe farmers should begin considering climate change when making planting decisions.
"The real dilemma is we cannot predict the speed or the potential impacts of global warming on agriculture," said Stella Coakley, an associate dean of the College of Agricultural Sciences at Oregon State University. "Therefore, it is very difficult to prepare for it."
Coakley notes that growers with perennial crops could be hardest hit by new pest pressure - primarily because of the cost of establishing a perennial crop.
"You can't have a crop failure one year and simply plant a new crop the next," Coakley said.
Coakley believes that if global warming were to occur suddenly, the warming trend will be accompanied with unpredictable hurricanes, floods and rains, which in addition to destroying some crops will create ideal conditions for pathogens. In such cases, diseases such as potato leaf blight could take off and cause widespread damage. Also, pathogens and insects will be better able to survive winter months as temperatures rise, creating a reservoir of pest problems for growers to deal with each spring.
One key to responding to what Coakley believes are unpredictable shifts in pest pressure is for governments to pump funds into research.
"Without that, identifying problems and solutions is going to be that much more difficult," Coakley said.