Spring will come. I know it doesn't seem like it now, but it will! And one thing that Christmas tree growers need to think about is what to do for insect and mite control in the spring.
What each grower has to do depends on two things: 1. what they treated with for insect control last year and 2. what pests you have in your trees.
What you did last year sets you up for this year. Many growers are treating their market trees in the fall to control Cinara aphids, woollies, and other pests. This is a good way to set up a more problem-free spring. If you did treat last fall, this post is for you! Unfortunately, that doesn't mean you get to be lazy this spring! You still need to scout.
Hopefully if you treated in the fall with either Talstar or Asana, you should have controlled your woollies and Cinaras, your twig aphids for this spring, and not created any problems with rust mites The key word though -- I use these kinds of qualifiers a lot! -- is "SHOULD." Control of none of these pests is guaranteed. Bugs don't read books after all. That's why you need scout come spring.
It doesn't have to be an extensive scout, just a quick look-see. Check on any old woolly trees and see if you can find any live woollies. Beat a few trees to see if any twig aphids are falling out (be use to use your hand lens to make sure you can find the littlest ones). Also pull some shoots and look for rust mites.
You should wait to do this first checking until the twig aphids are close to being fully hatched. Sometime the 1st or 2nd week in April, make your evaluations.
If you don't find anything -- GREAT! Come back again right before bud break and checkagain, just to make sure that twig aphids have been controlled. Remember that these aphids multiply rapidly, so their numbers can quickly build up.
If you found many aphids, you will probably need to treat for twig aphids again -- that is, if they are go-to-market trees that you are concerned about needle curl. Why didn't the fall chemicals work? It might have been an issue with coverage, or not a full enough rate. In any case, it's time to haul out the spray equipment and get busy before bud break.
If you are't finding twig aphids, but you are other pests -- say a few rust mites or spider mites -- the question of treating because a bit harder. These pests have the potential to build in numbers and cause damage -- but the numbers can also crash, resulting in little to no damage. My suggestion would be to keep scouting to see if numbers are increasing or not. If they have reached treatment threshold, a good miticide such as Envidor should give season-long control. If you are seeing these early enough in April, you can also get good control using a 2% horticultural oil solution. Remember that coverage doesn't have to be as good to control rust mites, but it is more of an issue with spider mites.
If you are finding elongate hemlock scale, you can treat in the spring, or you can wait until summer. If all you have to control is scale, I would probably wait until summer. If you are spraying for other pests though, go ahead and add the materials to control scale as well. Control for scale is somewhat reduced in the spring, but some have gotten decent control. I would wait as close to bud break as possible to allow the scales to produce as many crawlers as possible.
Just one scouting trip wouldn't be enough to give me peace of mind. It's important to check again right before bud break. Now if only 1-3 aphids are found, you can feel a bit more safe that twig aphids won't cause much damage. However, if more than a couple of trees have aphids, and you can beat out several, you might need to treat quickly to avoid damage. Once bud break has started, Dimethoate is a good choice to get control.
Scouting takes time, but it's worth it to identify fields where you can avoid making another insecticide application.