The Value of Christmas Trees

"...there is no reason why the joy associated with the Christmas evergreen may not be a means of arousing in the minds of children an appreciation of the beauty and usefulness of trees; and keen appreciation of the beauty and usefulness of trees is a long stop toward the will to plant and care for them (Arthur Sowder, US Forest Service, 1949)."

Monday, January 9, 2012

How Do You Know When Scale Control Is Working?

It's not always easy knowing if elongate hemlock scale (EHS) controls are working. On Friday, Meghan Baker and I helped a grower evaluate some Safari and Talstar treatments he'd made in July and August. I collected  shoots to look at under the microscope to determine if the scales were alive or dead. That is the only way to know for sure what kind of control you've gotten. Unfortunately, it takes a lot of work, and few people have access to a microscope.

So how can you tell how well your scale controls are working? Here are some things you can look for several months after treatment. Remember, you won't be able to tell within a few weeks. In fact, all of our treatments can take several months to work.
  1. It's a lot harder to find the scales after treatment. The grower we were working with really knews his fields. He went to several trees that he knew had a lot of scale on them, and had a hard timing finding any. That's a good sign that controls are working. One suggestion is to tag trees with lots of scale prior to treating so you know where to go back to!
  2. There isn't white cotton from the males scales. We were looking on a beautiful day in January. Though the weather was warm, you wouldn't expect to see the white cotton from the male scales yet. That's more of a summer thing. But, if you treated in the spring and were scouting in the summer and didn't see white on the trees, that's a good indication that there has been some control. But, I've seen fields where the control was only marginal where the male production was definitely suppressed. So in my opinion, this is not the best indicator of control.
  3. There aren't crawlers. Crawlers and immature scales are yellow. If you scout several months after application and don't see yellow individuals, then it's a good indication that the treatments have worked. 
  4. There aren't scales on the newest growth. If you treated last year or in the spring, and you don't see any scales forming on the new growth in June or July, that's a really good indication that controls are working . The scales will remain on older growth even if they are dead. Of course some do fall off, but some will remain. So always look at the newest growth to see if there are any scales forming. This is one of the best indicators of control, but you have to wait for it!
  5. The scales look dried up and are falling off. When controls are really good, there is a different appearance to the scales. The males all look dried up, and the females do too. You don't see any yellow immatures. And you can sometimes see on the needle that there used to be a scale present, and there isn't any more. This is a little tricky, because the females scales are brown, and might appear dead. But if you've seen enough treatments, you can develop an eye for what is really dead and what isn't. It helps to train your eye by looking at some of these samples under the microscope.
  6. Looking under the microscope. When I evaluate scale control, I pull 2 or 3 needles off of 6 or 7 different shoots. I try to get some older and some younger needles. I take anything sharp like a pin, and gently prick the very top of the scale. The goal is not to stab the scale and the needle, but to pry off the top of the scales's outer covering to find if the scale is still alive inside. Below are some photos of what to look for.
Obviously, this scale is still very active.
A crawler or any small yellow scales indicates controls didn't work.

Is this female alive or dead?
It is dead because it is empty and dried up inside.
This female is alive, and there are eggs present.
The same photo with everything labeled.
Scales on new growth in June show that treatments didn't work.
The Safari and Talstar treatments made in August gave 99% control. What does that mean? That means that out of 100 or more scales that I examined, only one female was still alive. In actuality, the control is probably even greater because there weren't as many scales as there were prior to treatment.

The grower had some other Safari + Talstar treatments where he used the low rate of Safari (4 ounces per 100 gallons vs. 8 ounces/100 gallons). These were giving 94% control. That means I found 2 or 3 live scales. So should he treat again?

I think I would wait until June to see if scales were showing up on the new growth. If they aren't, I would wait until 2013 to treat again. If they are, he would still have a couple of months to make a treatment in 2012.

This grower also had another scale -- pine needle scale. This scale is much easier to control, and all his treatments worked against it. In the photo below, note some of the scales have holes in the middle, indicating that parasitic wasps have attacked them.

These are all pine needle scale and not elongate hemlock scale.
Assessing EHS control isn't easy. If you need help, contact your local county extension agent and I'll be glad to come out and see. Being patient and waiting until June is probably the easiest way to determine control results.

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