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Evaluating Potential Spot Expansion


Figure 3.-SPB spot expanding in several directions.
Figure 3.-SPB spot expanding in several directions.

The major purpose of SPB control programs during the summer months is to reduce timber losses by locating and treating expanding infestations. Spots which are no longer expanding are soon abandoned by the beetles and have little need for control. To assist ground crews, you need to distinguish SPB spots showing visible symptoms of expansion from other spots likely to become inactive.

But how can you tell from the air if a spot will expand? Yellow crowns are the most useful clue. An expanding spot will have at least some yellow crowns. Rapidly expanding spots contain as many yellow-crowned trees as red ones. The location of the yellow-crowned trees within the spot marks the path of its spread, which may be in one direction (see fig. 1), or in several (fig. 3). When a spot becomes inactive, it will no longer have such trees (fig. 4). For control purposes, you do not need to report spots without yellow crowns.

During SPB outbreaks, spots may range in size from one to several hundred trees. As a general rule, the larger a SPB spot appears from the air, the more likely it is to grow. You can greatly aid control operations by reporting only spots with a total of five or more red- and yellow-crowned trees. Spots with fewer than five trees are not likely to expand and will often become inactive during the summer (fig. 5). This minimum will also eliminate recording many small Ips and black turpentine beetle spots, which are less prone to cause economic losses. True, you may overlook a few expanding SPB spots by using this practice, but these will be recorded later if they exceed five trees in size. During severe beetle outbreaks, forest managers may make the minimum reporting size larger than five trees if workloads of ground check crews become too great.

You should estimate the size of each spot reported. Two methods exist for such estimates. one, recording total number of dead and dying trees, which is indicative of the amount of salvageable timber; or two, noting only the number of red- and yellow-crowned trees, but not those having lost most or all of their foliage. This second method provides a better measure of trees that still contain beetles. Before selecting one method over the other, check the survey policy of your organization. And be sure that ground crews know which estimating system you use.

Figure 4. - At left, expanding spot; at right, nonexpanding spot.
Figure 4. - At left, expanding spot; at right, nonexpanding spot.

Figure 5. - Small spot with low beetle activity.
Figure 5. - Small spot with low beetle activity.

 
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