Aerial surveys are the backbone of most public and private forest protection organizations.

Bark Beetles

Because of the characteristic foliage coloration of trees killed by bark beetles, they are highly visible from the air (Slide). Aerial monitoring of beetle-caused mortality can provide valuable clues for predicting beetle outbreaks. For example, we can estimate the trend index by dividing the number of trees (or groups of trees) killed by bark beetles in one year by the number killed in the previous year. Even more important may be the change in spatial distribution of dead trees from one year to the next. For example, the observation that trees killed in one year are, on the average, far away from those killed in the previous year, indicates that susceptible trees are rare and widely scattered. On the other hand, if trees killed in one year are close to those killed in previous years, then it probably means that most of the trees in the stand are susceptible.

EXAMPLES

1. Southern pine beetles kill trees in tight groups called "spots" which are easily detected from the air (Slide). Notice that the age of the infestation can be inferred from the color of the foliage. For more information on aerial surveys for the southern pine beetle, go to Virginia Tech by clicking here. Remember that the tight groups of killed trees result from the continuous growth of the multivoltine southern pine beetle (many generations per year). Spot growth may slow or even stop in the cooler periods of the year.
2. Although trees killed by univoltine bark beetles like the mountain pine beetle and Douglas-fir beetle are often more scattered (Slide), large single-aged groups of dead trees may also be seen with these bark beetles (Slide).
3. Sometimes infrared photography can provide a more accurate picture of mortality caused by bark beetles, particularly under hazy conditions (Slide).
4. For accurate assessment of tree mortality caused by bark beetles, stereoscopic aerial photographs are taken at random points and fixed heights along a predetermined flight path (Slide). Dead trees are then counted on a light table.

Defoliators

Aerial surveys are also used to detect and map defoliator outbreaks. However, detection from the air is only possible under outbreak conditions because light defoliation is not visible from the air.

EXAMPLES

1. Stands heavily defoliated by the Douglas-fir tussock moth (Slide).
2. Infrared photograph of defoliation by the gypsy moth (Slide).

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