In the fall, when hardwood leaves change color, it is difficult to detect
new SPB spots in mixed pine-hardwood forests. The yellow or red foliage
of cypress trees in the fall also may be mistaken for beetle-infested
pines. As a result, survey flights are usually discontinued until hardwoods
have completely dropped their leaves.
Figure 14.-Winter spots in Virginia (Virginia Division
Figure 15.-Winter SPB spot in Texas.
The purpose of winter surveys is to locate overwintering SPB populations
so that control can be applied before beetles disperse in the spring.
Ground check priorities during winter surveys can be based on just two
factors: number of affected trees and accessibility for control. Remember,
SPB broods develop more slowly during the winter, often remaining within
trees until after the foliage drops. Red crowns in a SPB spot during the
winter, unlike those in summer, indicate that beetle broods are present.
The effectiveness of winter surveys for SPB varies among different geographic
regions. Their activity restricted by cold temperatures, beetles in the
Piedmont and Atlantic Coast States tend to remain throughout the winter
in the same multiple-tree spots they occupied during the late fall. These
spots become detectable when aerial surveys are resumed in December or
January (fig. 14).
In the Gulf Coast, however, beetles in the fall often leave large spots
to infest single trees scattered throughout the forest, and, in mild winters,
may continue to infest new trees. Pines infested in winter maintain green
crowns for 2-4 months, thus escaping observation during aerial surveys.
In winter, even multiple- tree spots started in the fall . are easy to
overlook on the Gulf Coast because the foliage color of infested trees
does not change dramatically. Between December and April, infestations
seldom show the yellow crowns
that clearly mark SPB spots in summer. At normal survey altitudes (1000-2000
ft), you may be able to see only the larger groups of bare and red-crowned
trees (fig. 15). Most of the red-topped pines still contain SPB. But much
of the beetle population in winter and early spring occurs in scattered
single trees which elude detection. By flying slowly with a helicopter
at low altitudes (100-500 ft), you have a better chance of seeing beetle-infested
pines in winter along the Gulf Coast. At these low altitudes you may see
off-color crowns of beetle-infested pines (fig. 16) that at higher altitudes
would not be distinguishable from uninfested trees. Also, trees killed
by SPB often have bark stripped by woodpeckers, which leaves them with
highly visible white boles. Although effective, surveys by helicopter
are costly and generally applicable only to high value stands.
Figure 16.-Helicopter view of scattered infested pines
Trees killed by beetles in late winter on the Gulf Coast can be seen
during March and April. Even though early emerging beetles may kill large
numbers of trees, new spots in spring seldom persist or expand. Temperatures
in the spring are still too cool for continuous spot growth. This results
in many scattered, short-lived spots in which groups of infested trees
show the same foliage color.
Locating new SPB spots in early spring is difficult in mixed pine-hardwood
stands: new foliage on hardwood trees makes them resemble SPB-killed pines
with fading foliage. As conditions improve for long distance dispersal,
however, beetles leave scattered brood trees to concentrate in expanding
spots. These multiple-tree spots become easily seen by early summer in
the Gulf Coast or by midsummer or later in the remainder of the South.
Because of these seasonal limitations, SPB observation surveys along
the Gulf Coast are most effective from May to October. For States along
the Atlantic Coast and in the Piedmont region, SPB surveys are practical
during midwinter as well as throughout the summer.