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)."

Thursday, June 23, 2011

Elongate Hemlock Scale Lifecycle

In talking with folks, I find that people are confused about the lifecycle of the elongate hemlock scale.

It is a bit confusing. The same insect can be white (for the males), brown (for the females) and yellow (for the immatures). It's hard to know if they are alive or dead. So here's a bit on the lifecycle that will hopefully make things clearer.

Life for an elongate hemlock scale starts out as an egg which is found under the protective scale of the mother. Here is a photo of the mother flipped over and broken into to show the eggs and the same photo labeled.

These eggs will hatch and the crawler will move out from under the mother scale and find a place to settle down. I actually got by accident a photo of a crawler leaving the mother. It is below. See the crawler at the bottom of the picture? Once again, everything is labeled in the second shot.

The crawler moves around until it finds a place to settle down and start moving. There it will molt, and start creating the scale covering for itself. If it is male, eventually it will fly away  so it can mate. That only happens when it is mature. If it is female, it will never move again.

The crawler is the small yellow thing.
The nymphs settle down, and then they produce the adult scale behind them. Here is a photo of one female scale. The yellow portion is the nymph part of the female, and then the brown part is produced.There aren't two scales, just one. It is all white around it because the scale has grown under the waxy cuticle of the needle. It is also almost impossible to tell if it is dead or alive. Under a microscope you can poke it with a pin and it will be moist. This one was alive when I took the photo.

The males are white. They develop into a winged insect that flies off to mate with the stationary females. You can almost think of the white scales as ultimately being a cocoon in which the male is developing. So often when you see the white scales, they are empty because the adult male has already flown off. The adult male is pictured below. This isn't my photo, but it's all I've got of the males.

Now let's put time into the picture. When does all this happen? The following are observations made by Paris Lambdin, a researcher in Tennessee.

          “EHS has two complete overlapping generations per year at sites within the southern Appalachians.”  My comment -- That means that you can find all life stages virtually any time through the year.
          “The spring peak for crawler emergence occurred in June while the fall peak occurred in late October into November.”  
          “Fall peak emergence for males occurred in August, coincident to the highest number of adult females.”  My comment -- That's why we see a second "wave" of white coming onto the trees in August. That's the second generation of males being produced.
          “Gravid adult females were most numerous in late May and October—November.”  My comment -- Gravid means they have eggs.
          “Females have three stages of development while males have additional prepupal and pupal stages.”  My comment -- That refers to the number of molts each sex goes through before maturity. For females, they are mature when they can lay eggs. For males they are mature when they emerge and fly away.
          “Each female produced 12-16 eggs which hatched over time.”
          “Males do not feed and live only 24 to 72 hours upon emergence. Although capable of flight, males tend to walk across the needles seeking out females for mating.”  My comment -- These are the adult males with wings that have emerged from the "cocoon." Some folks were finding the adult males when they were beating the foliage in the spring looking for twig aphids.

Other observations from scientific literature include:

          All life stages found any time of year (Davidson and McComb 1958) – eggs laid throughout growing season
          Mature females may often live for more than one year. My comment -- Isn't that great!
          After a month, the eggs hatch and the first instar nymphs ("crawlers") emerge and migrate to the underside of new needles. 

No wonder people were having a hard time figuring out the life cycle. It's complicated! If anyone has any questions, please let me know.

1 comment:

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