Woodpeckers and the Southern Pine Beetle

James C. Kroll - Associate Professor of Forest Wildlife, Stephen F. Austin State University, Nacogdoches, Tex.
Richard N. Connor - Research Wildlife Biologist, Forest Service, U.S. Department of Agriculture, Southern Forest Experiment Station, Wildlife Habitat and Silviculture Labratory, Nacogdoches.
Robert R. Fleet - Research Associate in Forest Wildlife, Stephen F. Austin State University.

U.S.D.A. Combined Forest Pest Research and Development Program Agriculture Handbook No. 564

In 1974 the U.S. Department of Agriculture initiated the Combined Forest Pest Research and Development Program, an interagency effort that concentrated on the Douglas-fir tussock moth in the West, on the southern pine beetle in the South, and on the gypsy moth in the Northeast. The work reported in this publication was funded in part by the Program. This handbook is one in a series on the southern pine beetle.

Impact of Woodpeckers on SPB Populations

As we mentioned, initial attack by SPB occurs at about 10-15 ft and proceeds toward the crown and base of the tree. The first sign of woodpecker predation also appears at the initial SPB attack height, where birds strip away bark and expose immature beetles (Fig. 6). The area of bark stripping expands as woodpeckers seek more SPB above and below the original point of attack.

Figure 6. - Initial woodpecker feeding at or near midbole (left), and later feeding above and below this region as the beetles mature (right).

Predation by woodpeckers is greater in trees containing more advanced beetle life stages, such as pupal and emerging adult stages. Although the birds prey on all life stages, they prefer larger forms of the insect, like pupae and adults. These stages are easier to locate and excavate from the bark than eggs and larvae. Over all seasons, woodpeckers cause an average SPB mortality ranging from 3.5 percent for eggs to 63.5 percent for emerging adults in East Texas. Winter mortality for emerging adults averages 23 percent.

Table 2. - Comparison of winter population densities of southern pine beetle and insect associates by standardized tree height, for screened and unscreened trees.

Standerdized tree heights	SPB life stage or associate	Mean density (per dm²)
		Screened trees	Unscreened trees
Lower bole	Larvae	36.1	28.9
	Pupae	13.1	1.1
	Brood adults	9.8	11.4
	Predators	0.0	0.9*
	Parasites	0.1	0.2
Middle bole	Larvae	16.1	11.8
	Pupae	24.6	0.8*
	Brood adults	16.6	14.1*
	Predators	0.5	0.4
	Parasites	0.1	0.3
Upper bole	Larvae	19.5	24.8
	Pupae	7.6	1.0
	Brood adults	16.3	9.6
	Predators	0.0	0.4*
	Parasites	0.4	0.4

* Significantly different at the 0.05 level. (From Kroll and Fleet 1979.)

East Texas studies, in which woodpeckers were excluded from portions of SPB-infested trees by screens (Fig. 7), indicated that significant beetle impact occurred only at midbole (Table 2). Differences in predation were not significantly different between screened and unscreened portions of lower and upper boles, respectively.

Figure 7. - Partial screening of beetle infested bole to exclude woodpecker predation on SPB.	Figure 8. - Heavier bark removal from shortleaf pine (left) than from loblolly pine (right).
Woodpecker predation on SPB is controlled by several variables, including season and temperature, species of pine attacked, and species of woodpecker feeding in the area of beetle attack. In East Texas, for example, woodpecker populations vary seasonally, the highest numbers occurring in midsummer and the lowest in late winter. But the birds remove more beetles in winter and fewer in summer. In winter, because of cooler temperatures, beetles develop more slowly than in summer, so woodpeckers have more time to prey on a single generation. In the absence of other food sources, the birds probably concentrate on beetle infested trees, too.
	Figure 9. - Piles of beetle-infested bark debris around the base of infested tree following woodpecker foraging.

Researchers in Texas and Arkansas found that SPB have lower survival in shortleaf than in loblolly pine. This is probably a result of many factors, one of which seems to be the thinner bark on shortleaf pine. Studies also revealed that woodpeckers remove about twice as much bark from shortleaf pines as from loblolly pines (Fig. 8).

Woodpeckers do not actually eat all the SPB they remove from trees. Some of the larvae and pupae wriggle free from the bark and fall to the ground in dislodged bark chips (Fig. 9). Survival in dislodged bark is low for all seasons, ranging from 5 percent in winter to 23 percent in spring.

Woodpecker feeding or scaling of bark also indirectly increases SPB mortality. Thinning of bark exposes various life stages to adverse environmental conditions like excessive heat or low humidity. As a result, larvae escaping woodpecker predation may later die from dehydration or heat stress. Exposure of the inner bark surface also permits the early establishment of fungi and bacteria, some of which reduce bark beetle survival.

Woodpecker foraging favors predation and parasitism by such insects as clerid beetles and wasps. Clerid beetles, both as adults on the bark surface and larvae inside the bark, are well-known predators of SPB. Density of this predator increases in the bark remaining after woodpecker feeding. As the birds strip more and more bark, surviving clerid larvae seem to concentrate in the remaining bark. This increases the probability that an SPB will be consumed. In addition, bark thinning by woodpeckers makes SPB larvae more accessible to parasitic wasps. In East Texas, average within-tree insect predator densities and parasite densities were 38 to 87 percent more abundant, respectively, in bark remaining on trees after woodpecker foraging.

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