Warmer temperatures, caused by global warming, could make pest control more challenging in the Pacific Northwest, theorizes a Washington State University entomologist.
Hotter weather could mean faster development and more generations each year of the key apple pest codling moth, for example.
At one time, codling moth developed through just two generations per season in Washington State, and, occasionally, a partial third. Nowadays, the pest develops through a partial third generation most years.
Dr. Vince Jones at WSU’s Tree Fruit Research and Extension Center in Wenatchee, said temperature and daylight (photoperiod) influence when an insect goes into diapause for the winter. With higher temperatures and earlier development of the insect, the hours of daylight might be conducive to more of the codling moths continuing their development into a larger third generation in the fall. If it’s warm in the fall, there might be enough degree days for more codling moths to reach the last larval instar, which would result in an entire third generation and more phytosanitary concerns relating to exported apples.
In higher temperatures, the crop would also tend to mature earlier, so the fruit would be picked earlier, he acknowledged, but codling moth might be able to complete a third generation on later varieties. Hot summer temperatures, above about 88°F, might slow down codling moth development, however.
Dr. Stan Hoyt, retired WSU entomologist, said that when he was working with codling moth in the 1960s and 1970s, there occasionally would be a partial third generation in hot years. He began work at the research center in Wenatchee in 1958, which was a hot season, he recalled, "And we certainly had a partial third then."
However, that was rare, and most third generation moths usually didn’t develop beyond the egg stage.
Hoyt noted that an increase of a couple of degrees in the average season-long temperature could have a major impact because it means the temperature is a couple of degrees hotter every day.
Jones said spider mites thrive in temperatures up to 100°F and could be more difficult to control if average temperatures rise, particularly as the predatory mite species are less tolerant of high temperatures. At high temperatures, the spider mites reproduce faster than the predators do.
Historic weather data indicate that average minimum temperatures have been rising more than average maximum temperatures. Jones said more pests might survive if they are exposed to less extreme winter temperatures. Codling moths overwinter as full-grown larvae under leaf litter or under loose bark scales on the tree. It’s been estimated that about 20 to 25 percent don’t survive the winter, depending on the location. A 20 percent increase in the number of codling moths emerging in the spring would be significant, Jones said, as the numbers compound with each generation.
On the other hand, it’s possible that warmer temperatures might allow better survival of natural enemies to offset the higher number of pests.
Warmer temperatures might result in different cloud cover and different amounts of heating from radiation early in the spring, which might result in pests emerging at different times. Emergence of some pests and natural enemies might no longer be synchronized.
Jones said a number of growers have complained that the degree-day model for codling moth is not accurately predicting the field populations. They’re noticing that the first generation of codling moth has two peaks instead of one, a phenomenon seen in some of the warmer growing regions, such as California.
Jones said four factors influence when a codling moth comes out of diapause: the temperature they experience when they’re going into diapause, the temperature after they go into diapause but before chilling, the length of the chilling period, and the conditions they experience in the spring. A change in any of those conditions, particularly the period after they go into diapause and before chilling, could have a great impact on when they emerge in the spring.
Higher temperatures could also lead to pests moving into the area that would not normally be able to survive the winter.
Jones said there is little that growers can do to address these issues at the orchard level. It will take a coordinated national effort to address global warming.
Other countries are also concerned about the potential for new pests. New Zealand is beefing up the capability of its biosecurity border services partly because of potential climate change, according to a report in the June issue of The Orchardist. A warmer climate could increase the chances of exotic pests establishing themselves there.
Dr. Gary Grove, plant pathologist with WSU in Prosser, said higher average temperatures might make some of the major tree fruit diseases, such as cherry powdery mildew, less severe, because when the temperature exceeds 80°F for an extended time, growth of the disease organisms slows down.
However, grape mildew thrives in hot weather and could probably tolerate temperatures a couple of degrees higher, as illustrated by the fact that the disease is extremely prevalent in California’s Central Joaquin Valley.
And a warming trend might lead to new diseases, Grove warned. "I imagine we might see some diseases we haven’t seen before."
For example, it might expand the range of Pierce’s disease by creating a more suitable environment for the vectors of the disease, which are leafhoppers known as sharpshooters.
Dr. Greg Jones, climatologist at Southern Oregon University, said about half the state of California has a climate conducive to Pierce’s disease. It’s been calculated that if the temperature continues to rise during the next 25 years at the same rate as it has in the last 25 years, the entire state would have potential for the disease. Some of the warmer areas in western Oregon and Washington could have the same potential if the temperature rose, but Jones thinks cold winters in eastern Washington might prevent survival.
As far as preparing for climate change, Grove said growers could be thinking ahead and planting warmer-climate grape varieties, but the temperature changes will be gradual, and it’s the next generation of growers who will face the hotter climate.
"The people who are in our schools right now are the ones who are going to have to deal with it," he said. "You would almost have to embed the information in a rap song."