The Southern Pine Beetle (Dendroctonus frontalis) in Central America

How to Recognize, Prevent and Control Outbreaks

Publication 0605/15000
Texas Forest Service
2005

Contents

• Introduction
• What is the pine bark beetle?
• How to recognize pine bark beetle attacks.
• Which forests are most susceptible to bark beetle attack?
• How to prevent outbreaks.
• How to apply methods of direct control.
• How to restore affected areas.
• Literature cited
• Acknowledgements

Cover - Aerial view of a bark beetle infestation in a pine forest in the Olancho Forest Region, Honduras.

Adult southern pine beetle, Dendroctonus frontalis (magnified photograph); average size = 3-4 mm long.

How to Recognize, Prevent and Control Outbreaks

by
Ronald F. Billings
Texas Forest Service
and
J. Vicente Espino Mendoza
Honduran Forestry Development Corporation

Introduction

Pine bark beetles of the genera Dendroctonus and Ips are insect pests of major economic importance in conifer forests and their range extends from Canada and the United States to Nicaragua. Of the various species found in Central America, the southern pine beetle (SPB), Dendroctonus frontalis, is one of the most damaging pests. Losses provoked by this bark beetle can be reduced by means of monitoring, detection, evaluation and direct control programs (see Billings 1990, 1996 a, b). However, the most recommended method to prevent outbreaks is good management of forest stands that are potentially susceptible to bark beetles before outbreaks appear.

This field guide describes how to recognize attacks by Dendroctonus frontalis and distinguish them from attacks by secondary bark beetles of the genus Ips. Furthermore, the guide describes those pine forests most susceptible to bark beetle attacks and how to apply methods of prevention and direct control. These recommendations are based on the authors' many years of experience in management of pine bark beetle outbreaks in Honduras, Nicaragua, Guatemala, and the state of Texas.

What is the pine beetle?

The pine beetle is a bark beetle that attacks pine forests. Among the species of pine bark beetles in Central America, Dendroctonus frontalis is one of the most destructive. Adults (see photo on the cover page) are small beetles (3-4 mm long, about half the size of a grain of rice). They initiate their attacks on pines weakened by lightning, fires, high stand density or other causes.

Once 20-30 pines are attacked, southern pine beetle infestations (spots) are capable of growing rapidly if no control is applied (see photo on the cover page). Under these conditions of outbreak, the bark beetles can kill even healthy pines in sparse stands. The bark beetle brood (eggs, larvae, pupae, and new adults) develop within the bark of infested pines (Fig. 1), completing their life cycle in 4 to 6 weeks. Upon emerging from the tree, the new adults fly in search of a new host and the adult beetles only survive a few days outside the host tree.

Figure 1: The developmental stages of the southern pine beetle consist of egg, larva, pupa, and adult; the life cycle lasts 4-6 weeks within the tree and a few days outside.

How to recognize southern pine beetle attacks.

At first sight, the symptom that a tree has been attacked by bark beetles is the discoloration of foliage. The needles change color from green to yellow and then to red or maroon (Fig. 2). Upon close inspection, one can see small accumulations of resin or "pitch tubes" in the bark crevices that indicate the entrance points of attacking parent beetles (Fig. 3). Upon removing the bark from an infested tree with a yellow or red crown, "S"- shaped galleries can be seen that are filled with boring dust (Fig. 4), indicating attacks by Dendroctonus frontalis.

If the galleries are in the form of a "Y" or "H" and empty of boring dust, these signify attacks by secondary bark beetles of the genus Ips (Fig. 5).

In general, Ips bark beetles are found in the same trees attacked by the southern pine beetle, occupying the upper region of the stem. In the case of fallen pines or logs, the presence of brown sawdust on the surface of the bark is another sign of attacks by Ips (Fig. 6). It is important to recognize that pine bark beetles of the genus Dendroctonus only attack standing trees. In contrast, Ips bark beetles prefer to colonize fallen pines or standing trees weakened by drought, resin extraction, fires or other causes and rarely produce expanding infestations.

Three categories of infested trees have been proposed, reflecting the different stages of attack (Fig. 2). The number of trees in each stage of development and where they are located in the spot is the key for determining if a spot is active and whether it will expand. It is essential to recognize these three stages in order to correctly determine the need for control (for more details, see Billings et al. 1990).

Figure 2: In expanding infestations of Dendroctonus frontalis, there stages of attack can be distinguished: stage 1 with green foliage, soft pitch tubes in the bark crevices and with the wood surface having a white color (without bark beetle galleries); stage 2 with fading or yellow crown, serpentine galleries under the bark, and the surface of the wood being brown in color; stage 3 with a red or maroon crown (or without foliage) and with small, circular exit holes in the bark.

Stage 1: Pines with fresh attacks (lasts 5-10 days).

Stage 2: Pines with SPB broods (lasts 25-35 days).

Stage 3: Dead pines abandoned by beetle broods.

A pine under attack by Dendroctonus frontalis (stage 1) can be recognized by the green crown and fresh pitch tubes in bark crevices. An infested pine with bark beetle broods (stage 2) will have a yellow or fading crown with pitch tubes that are more dry and hard. In turn, a pine that has been vacated by beetle broods will have a red or maroon crown (or with no foliage) and the loose bark will have many small exit holes. An expanding SPB spot will have more than 20 infested pines with some trees in each stage of development.

Figure 3: Pitch tubes indicating attacks of adult SPB on the trunk of a pine tree.	Figure 4: Galleries under the bark in the form of "S", filled with boring dust, characterize attacks by the SPB, Dendroctonus frontalis.

Figure 5: Galleries under the bark in the form of a "Y" or "H" that are empty of boring dust indicate attacks by bark beetles of the genus Ips. In this case, the boring dust-filled galleries that extend from the principal galleries were made by the larvae of Ips.	Figure 6: On fallen pines or on pine logs, the accumulations of red or brown sawdust on the surface of the bark are another sign of attacks by secondary bark beetles (Ips spp.).

Which forests are most susceptible to attack?

According to its name, the southern pine beetle only attacks pines and not hardwoods. Among the pines found in Central America, Pinus oocarpa and Pinus caribaea are the most susceptible to bark beetle attack. The forests that are most susceptible to bark beetle attack are characterized by a high density (Fig. 7), a reduction in radial growth, weakened by fires (Fig. 8) or resin extraction operations (Fig. 9a) and/or located on poor soils. Infestations of Dendroctonus frontalis in Central America commonly start on ridge tops and grow downwards (see photo on the cover). Extreme droughts or floods increase the probability of bark beetle outbreaks, especially those of Ips.

How to prevent bark beetle outbreaks.

Without doubt, the best method to reduce losses in forests due to bark beetles is to continually apply good forest management, constantly monitor stand conditions, and control spots as soon as they are detected. The following preventive measures are recommended to maintain pine forests in good health:

• Elimination of high risk pines: During the first years of stand establishment, sanitary thinnings are recommended which consist of eliminating poorly formed trees and those that are forked, suppressed, with broken crowns or crooked stems. Also, trees severely infected with mistletoes, diseases or other pests should be eliminated. Such weakened trees can attract both Dendroctonus and Ips bark beetles.
• Density regulation: In young stands, thinnings should be conducted every 5-10 years beginning at the age of 8-12 years to reduce competition among trees and to maintain rapid growth (Fig. 10). Pine stands that have been thinned periodically are more capable of resisting initial attack as well the growth of established spots. An ideal stand density would be between 18-20 square meters per hectare (80-90 square feet per acre). Suppressed or intermediate trees with sparse crowns should be cut, leaving standing the dominant and co-dominant trees with characteristics of good growth (live and vigorous crown).
• Prevent forest fires: Forests weakened frequently by fires are more susceptible to bark beetle attack (Fig. 8). In contrast, in stands of 10 years of age or older, prescribed burns every 3-5 years can reduce competition among trees and eliminate suppressed trees in dense stands as well as plants in the understory.
• Reduce damage during forest harvest operations: During thinning operations and other opportunities to harvest trees from a stand, precautions should be taken to minimize injuries on live pines. At the same time, pines damaged by lightning strikes, abandoned resin extraction operations, mistletoes, diseases, or other causes of poor condition should be eliminated from the stand. Those pines that have bark removed from the trunks due to harvest equipment also should be removed in thinning operations.
• Regeneration of overmature stands: Older, overmature pine stands (more than 60 years) are less resistant to bark beetle attacks. In commercial forests, overmature stands should be harvested and the sites regenerated with pines.
• Plant pines on adequate sites: Pines do not grow well if not planted on adequate sites. Pines located on very poor or shallow soils have a high risk of being affected by insects or diseases.
• Favor stands of diverse ages and of more resistant species: Pine forests suffer less damage from bark beetle outbreaks if they consist of a diversity of ages and species. For example, in Hondurus, Pinus oocarpa and Pinus caribaea are more susceptible to bark beetle attacks in comparison to Pinus pseudostrobus. En Guatemala, Pinus ayacahuite is known to be less susceptible. Pine forests mixed with hardwoods are less susceptible to bark beetle attack than are pure pine forests.

In summary, good forest management is very important to assure healthy and productive forests. A forest management plan should be formulated and carried out during all stages of stand development. High hazard pine stands can be identified and treated to reduce their susceptibility and potential for attacks by insects and diseases. Pine stands and forests that are highly resistant to attacks by bark beetles and other pests should be the first objective of management; preventive silvicultural practices offer the most practical and long lasting form of achieving this objective.

In a few words, " Good forest management is good pest management."

Figure 7: The forests that are most susceptible to bark beetle attack are characterized by a high density.

Figure 8: Forests weakened frequently by fires are more susceptible to bark beetle attack.

Figure 9: Pines utilized for resin production and burned frequently (a) as well as pines damaged by harvesting equipment (b) attract bark beetles and should be eliminated from the stand.

Figure 10: Pine stands that have been thinned are more resistant to initial bark beetle attack and also to the expansion of spots once they become established.

How to apply methods of direct control.

In Central America, bark beetle outbreaks are cyclic, occurring generally every 10-20 years and last 2-5 years. During outbreak periods, if no control is applied, even healthy forests can be attacked by bark beetles once their populations increase to high levels. Thus, it is very important that the forest landowner and/or forest technician concerns himself with prompt detection and timely control of existing spots. For more details, see the field guides on aerial detection (Billings 1996a), ground evaluation (Billings et. al 1990) and direct control (Billings 1996b).

Expanding spots (Fig. 11) should be controlled while they are small using the cut-and-leave method (Fig. 12) in order to reduce the economic and ecological losses. To halt the advance of very large infestations (more than 10 hectares), a buffer strip should be applied, felling all stage 1 trees and a number of uninfested trees (20-50 m wide) around the active front of the spot (Fig. 13). Once the spot is controlled, control crews should continue felling stage 2 trees and, finally, harvest and utilized the felled trees (Fig. 14). In addition, in order to not transfer the beetles to uninfested areas, it is recommended to debark all infested logs before transport (Fig. 15). This treatment also generates local employment in the communities near the infested areas.

Figure 11: An expanding Dendroctonus frontalis spot contains pines in the three stages of attack and warrants a high priority for control.

Figure 12: Cut-and-leave is the most recommended method to control expanding spots of small to medium size (less than 10 hectares). Cut-and-leave should be applied as soon as possible after the spot is detected and, preferably, before it affects more than one hectare.

Figure 13: To control large infestations of more than 10 hectares in size, a buffer strip should be applied, felling all stage 1 pines, plus a preventive strip of uninfested pines (20-50 m wide in both stages, fresh attacks and uninfested) around the active front.

How to restore affected areas.

Unfortunately, many pine forests affected by bark beetles suffer a change in land use after an outbreak. To prevent the permanent loss of forests, the landowner or forest technician should restore the affected area, assuring new forests for the future.

Dead pines should be eliminated and the affected area cleaned to favor natural pine regeneration. If there is not a sufficient number of pine seed trees available, seedlings from a nursery should be used. It will be important to protect the pine seedlings and saplings from fires for a minimum of five years to establish a new plantation.

Figure 14: Once the beetle spot is controlled, felled and standing dead trees should be harvested and utilized to reduce economic losses, as seen in these salvage operations in Nicaragua (a) and Honduras (b).

Figure 15: Before transporting infested logs from the site, it is recommended that the logs be debarked, to avoid transporting the beetles to other areas.

Literature Cited

Billings, R. F., H. A. Pase III, and Jaime Flores L. 1990. Los escarabajos descortezadores del pino, con énfasis en Dendroctonus frontalis: Guía de campo para la inspección terrestre. Texas Forest Service Publication 146. 19 p.

Billings, R. F., Jaime Flores L., and R. S. Cameron 1996a. Los escarabajos descortezadores del pino, con énfasis en Dendroctonus frontalis: Guía para la detección aérea. Texas Forest Service Publication 149. 27 p.

Billings, R. F., Jaime Flores L., and R. S. Cameron 1996b. Los escarabajos descortezadores del pino, con énfasis en Dendroctonus frontalis: Métodos de control directo. Texas Forest Service Publication 149. 19 p.

Visit the Internet site http://www.barkbeetles.org/spb.html to see these publications and other information on pine bark beetles.

Acknowledgements

The authors extend their gratitude to the following persons and organizations for their contributions to this publication:

To the US Agency for International Development for providing the funds to prepare and publish this field guide. In particular, to Shiela A. Young, Subdirector of the Office of the Environment, Agriculture, and Commerce, USAID/Honduras, for her support and assistance.

To the USDA Forest Service, International Forestry Program, for financing and supporting the trips of Dr. Ronald Billings to Honduras to evaluate the outbreak situation existent in this country.

To the Honduran Forestry Development Corporation (Corporación Hondureña de Desarrollo Forestal) for the logistical support that this organization has provided to Dr. Billings and his counterpart forester J. Vicente Espino M. during their 23 years of collaboration.

To the Texas Forest Service (TFS) for offering the services of Dr. Billings and other employees that helped in the preparation of this publication.

To Dr. John Foltz, University of Florida, for use of the photograph of adult Dendroctonus frontalis presented on the cover page, and to Richard Kliefoth, Boyce Thompson Institute, for providing the illustration in Figure 1. The other photographs utilized in this field guide were taken by Dr. Billings.

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