This Winter's Weather

SUMMARY: The harsh winter weather this year will help decrease problems with Phytophthora root rot and Cinara aphids, but probably not other pests. And even these pests aren't eliminated by the cold.

Doug Hundley sent me this picture of his trees during the Christmas Day ice storm.

I'm sure a lot of folks are wondering what the weather this winter will mean to pests of Fraser fir Christmas trees.

Of course last winter we had even colder temperatures, though they didn't last as long. I have pictures of the creek near my house taken January 2009 with ice over about half of it, but so farthis year, little ice has shown up on the creek .

I was hoping last year that the cold temperatures would reduce elongate hemlock scale, but they didn't seem too. Of course, when you examine the scales under the microscope, there are always a lot of dead indivduals present. Last spring after a cold winter, I think the percentage of dead scales was higher than I usually observe -- sometimes as high as 45%. But the scale still spread successfully. At one farm, there were scales on 32% more trees the summer of 2009 than there had been in 2008. So cold weather may slow scales down some, but not enough to get excited about.

Cinara aphids are one pest that do appear to be less common after a harsh winter. There weren't many fields that had Cinaras in 2009. Those I did see didn't have aphids until fall. Even so, I still had close to 10 calls about Cinaras before Christmas 2009. So though they aren't prevalent, they can still be a problem.

One pest that cold winter weather does help with is root rot. Research has clearly demonstrated that Phytophthora cinnamomi resting spores will slowly die as soil temperatures remain below freezing. The longer soils are frozen, the more spores will die. This can be a big benefit to Christmas tree growers this year. But again, there will always be a few spores that survive, and given the right conditions of warm weather and flooding, those few spores can still cause quite a problem.

Other pests are well adapted to cold mountain temperatures. That includes about everything else -- woollies, twig aphids, spider mites and rust mites. The only time these pests seem to be affected by cold temperatures is if they start to become active in the spring, and then they are caught by a winter storm. For instance, during the blizzard of 1993 which occurred in March, rust mites on hemlocks were killed out and ended up not being much of a problem that year. However on the white pines that caught and held the snow, the mites were insulated and ended up surviving.

The same can happen in the fall -- if it's warm and pests remain active late in the season and then freezing temperatures suddenly occur, they can be killed out. That didn't happen this fall. We'll have to wait and see what the spring brings!

I plan on going out this week to see how elongate hemlock scales are fairing if I can make it to the field I'm monitoring. I'll report on what I find later this week. I'm especially interested to see what stages are surviving this weather, and if any crawlers or many eggs are found.