Ips: The Other Pine Bark Beetles
Compared to the southern pine beetle (SPB; Dendroctonus frontalis Zimmermann), which is a primary killer of healthy pine trees, less attention is given to the three species of Ips bark beetles that occur in the Southeast: the six-spined engraver, Ips calligraphus (Germar); the eastern five-spined engraver, Ips grandicollis (Eichhoff); and the small southern pine engraver, Ips avulses (Eichhoff). Ips species derive their common names from and can be identified by the number of spines along a depression along the rear of the abdomen. Adult Ips beetles range in color as they mature from light brown (freshly emerged) to black. The beetles are small, ranging between one-eighth inch (Ips avulses) and one-fifth inch (Ips calligraphus). Ips beetles also can be identified by the size and pattern of galleries they leave behind under the bark of trees they infest (Figure 1). Modern intensive forest management practices create ideal conditions for Ips (Nebeker, 2003), which is likely the reason Ips-infested trees are common in most Mississippi pine forests.
Attacks on pine trees or logging slash are usually initiated by male Ips, which bore entrance tunnels through the outer bark and excavate chambers within the inner bark. Afterward, females enter, mate, and lay eggs in galleries. Females begin construction of egg galleries that often form an H or Y shape. Individual galleries range from 4 to 7 inches in length, with eggs being laid in niches on either side of the gallery. After hatching, larvae make individual feeding galleries in the inner bark (Figure 1). Larvae pupate at the end of feeding galleries, and new adults mature and bore out through the outer bark to repeat the life cycle. Ips beetle populations increase rapidly under warm weather conditions but develop slowly when temperatures drop below 59 degrees Fahrenheit (Connor and Wilkinson, 1998). Depending on weather conditions and host availability, Ips species can produce between six and ten generations per year (Eickwort et al., 2006).
Characteristics of Ips Infestations
Ips and southern pine beetles create many similar signs and symptoms on infested trees, so landowners often confuse Ips and SPB infestations (Coulson and Klepzig, 2011). Ips beetles often work alongside SPB; however, Ips are not typically known for causing large-scale mortality like SPB. Therefore, correctly identifying the bark beetle involved is critical for prescribing proper management strategies to lessen further damage to remaining trees.
Ips infestations typically do not appear in defined “spots,” as SPB infestations typically do. Ips tend to cause scattered mortality of only the weakest trees throughout stands. Consequently, Ips damage is generally distributed in a much more scattered pattern throughout a pine stand compared to that of the SPB. In many cases, the damage a stand suffers from an Ips infestation is limited to only one or a few trees (Nebeker, 2004). If Ips do persist, they are likely to create a checkerboard pattern of fading and healthy trees (Stone et al., 2007; Figure 2). This pattern is dramatically different from SPB infestations, which can continuously spread from the initially attacked tree unless environmental conditions or management activities halt beetle activity.
Ips populations are most visible to landowners when obvious symptoms of their infestation are present, such as fading treetops. However, this is not the only time Ips are present in a timber stand. Ips are present throughout most pine stands of the South and will take advantage of disturbances, such as thinning operations, to colonize logging slash or, worse, to colonize trees wounded during harvest. Fresh slash material from harvest operations or weather events provides breeding material that sustains all three southern Ips species. Minimizing wounding of residual trees and proper slash management following harvest operations are key to reducing subsequent Ips problems.
Large numbers of Ips may accumulate in areas where natural disturbances, such as lightning storms, ice storms, tornadoes, wildfires, hurricanes, and droughts result in large numbers of damaged pines suitable for colonization (Thatcher et al., 1980; Connor and Wilkinson, 1998). In 2005, Hurricane Katrina produced large amounts of suitable habitat in the form of slash, which greatly increased Ips populations in south Mississippi (Londo et al., 2009). Ips populations may also increase following forestry activities such as prescribed burns that are too intense, as well as clearcutting or thinning operations that wound trees and leave large amounts of slash for breeding sites (Connor and Wilkinson, 1998; Mayfield et al., 2006).
Signs and Symptoms of an Ips Attack
Symptoms of an Ips infestation are the tree’s response to attack. Fading crowns, dead trees, pitch tubes, and sloughing bark are all symptoms of an Ips attack. Signs of an attack that relate to the beetle directly include galleries, emergence holes, boring dust, frass (beetle excrement and sawdust), and Ips beetles or larvae themselves. Symptoms of an Ips infestation are easier to recognize than signs. Symptoms of bark beetle infestations are usually the first thing observed in an infestation, but, through closer inspection, signs allow you to identify the type of bark beetle present. One of the signature symptoms of an SPB infestation is the popcorn-shaped pitch tubes along the stem (Thatcher et al., 1980; Figure 3). While Ips species also can cause pitch tubes, they are often lacking in trees under attack by Ips beetles, because Ips typically attack trees too weak to produce enough resin pressure to form pitch tubes. In contrast, SPB can attack trees with high vigor that have sufficient resin pressure to produce the characteristic pitch tubes.
The first visible sign is usually boring dust, which can collect around the root collar (Stone et al., 2007). Additionally, small emergence holes can be observed along the bole of the tree. When inspecting slash material, a pitch tube will likely not be present because of the lack of resin flow. Boring dust and frass will still be present, as it must be removed from the galleries.
Management Practices to Prevent Ips Infestations
Knowing what species of bark beetle is responsible for the damage in question will determine the optimal type of control method to use. Southern pine beetle infestations can grow continuously and eventually kill thousands of acres of pine forest if left unchecked. Cut-and-remove or cut-and-leave operations (along with adequate buffer strips in either case) are often recommended for suppressing SPB infestations (Thatcher et al., 1980; Clark and Nowak, 2009). However, Ips beetle infestations are often randomly distributed throughout a stand, making these operations less feasible. Additionally, Ips rarely kill healthy pines and do not generally cause large-scale tree mortality over large acreages (Coyle et al., 2016). For these reasons, proper Ips management is typically different from SPB management. In fact, more often than not, letting individual Ips infestations run their course may be the most economical option.
Management practices for control of Ips have not been as extensively researched as those for SPB, but several techniques should lessen both frequency and severity of damage. Thinning is useful in preventing bark beetle attacks, but it should not be performed in drought conditions. It is during these periods that pines are the most susceptible to attack from Ips. Logging slash should not be left in concentrated quantities on loading decks or along skid trails. Slash should be distributed over as wide an area as possible so that it will be broken up by equipment traffic and dry out. Trees wounded during harvest should be removed, and any trees actually infested by Ips also should be removed post-harvest to ensure low populations of Ips moving forward.
A few insecticides are labeled for Ips; however, these chemicals must be applied repeatedly, and the entire tree, including the upper portions of the crown, must be sprayed by a certified applicator (Londo et al., 2009). These treatments are not cost-effective for most nonindustrial private landowners for stand-level treatments. However, they may be cost-effective for highly valued trees in residential areas (Figure 4). Additionally, they are not very effective at killing Ips once trees are actually infested. Most importantly, a heavily infested tree will likely die anyway and cannot be saved by insecticides. Insecticides should, therefore, only be used as a preventive measure to control Ips damage in weakened, wounded, or stressed high-value trees before infestation occurs. Supplemental watering of pine trees growing in residential areas during periods of drought is also useful in preventing Ips attacks.
Stand Management Practices
When it comes to stand-level prevention of Ips, sound management practices are key. Thinnings should be scheduled so that trees remain vigorous. Thinning too early can decrease the value of future sawtimber, while thinning too late decreases tree vigor and can increase susceptibility to bark beetle infestations (Traugott and Dicke, 2006). Consult a forester before making a pine thinning decision. Foresters may be found on the Mississippi Board of Registered Foresters website: https://www.borf.ms.gov/.
To help alleviate Ips problems after harvest, slash should be redistributed throughout the stand. The more times slash is run over by equipment, the more it is broken down and the less suitable it becomes for infestation. Slash distribution over a wide area exposes it to moisture, light, wind, and temperature variability. All of these factors help lessen the suitability of slash material for Ips beetles.
Slash distribution is feasible for logging crews. After delivering a load to the deck, a skidder picks up slash material and deposits it along skid trails and thinning rows/corridors while retrieving the next load (Figure 5). Spreading potential Ips beetle habitat throughout your residual trees may seem strange, but logging equipment passing back and forth and exposure to the elements will make most redistributed slash less suitable for Ips. Slash distribution may also provide some protection against soil compaction from logging equipment. If slash redistribution is not possible or desired, residual slash piles can be burned. However, if burning is employed, take care to ensure residual trees are not scorched.
Ips infestations are often misidentified as SPB infestations. However, proper identification of the species of bark beetle responsible for damage will greatly influence the recommended control method. Unlike SPB, Ips may infest slash left by natural disturbances or harvesting activity. Consideration should be given to proper handling of this potential resource for Ips. Not only is Ips habitat created in the form of slash material, but damage to residual trees and rutting/compacting soils during pine harvest is a possibility. However, if properly conducted, thinning can be a preventive measure that will help reduce future bark beetle damage. Proactive management practices are the best course of action to minimize future damage from both SPB and Ips bark beetles.
Clark, S. R. & J. T. Nowak. 2009. Southern Pine Beetle. U.S. Forest Service, Washington, DC, Forest and Disease Leaflet 49.
Connor, M. D. & R. C. Wilkinson. 1998. Ips bark beetles in the south: Forest Insect and Disease Leaflet 129, U.S. Department of Agriculture Forest Service.
Coulson, R. N. & K. D. Klepzig. 2011. The Southern Pine Beetle II. Gen. Tech. Rep. SRS140. Asheville, NC: U.S. Department of Agriculture Forest Service, Southern Research Station.
Coyle, D. R., A. B. Self, J. D. Floyd & J. J. Riggins. 2016. Ips bark beetles in the southeastern U.S., Southern Regional Extension Forestry Publications. SREF-FH-002. Online at http://southernforesthealth.net/insects/ips-bark-beetle/ips-bark-beetles-in-the-southeastern-u.s
Eickwort, M. J., E. A. Mayfield III & L. J. Foltz. 2006. Ips Engraver Beetles (Coleoptera: Curculionidae: Scolytinae). Florida Dept. Agriculture & Con. Serv. Division of Plant Industry, Entomology Circular No. 417.
Nebeker, N. 2009. Forest Health. In: Publication 2470 Managing the Family Forest in Mississippi. Mississippi Forestry Commission and Mississippi State University Extension. pp. 95-98.
Mayfield, A. E., III, J. Nowak & G. C. Moses. 2006. Southern Pine Beetle Prevention in Florida: Assessing Landowner Awareness, Attitudes, and Actions. Journal of Forestry, 104:241–247.
Nebeker, T. E. 2004. Advances in the control and management of the southern pine beetles. General Technical Report SRS-75, U.S. Department of Agriculture, Forest Service. Southern Research Station. pp. 155–160.
Nebeker, T. E. 2003. Integrated forest pest management. Integrated Pest Management: Current and Future Strategies, Council for Agriculture Science and Technology (CAST), Task Force Report 140. pp. 11–116.
Stone, D., T. E. Nebeker, & A. J. Londo. 2007. Publication 2448 Identifying and Controlling the Southern Pine Bark Beetles. Mississippi Forestry Commission and Mississippi State Extension.
Thatcher, R. C., J. L. Searcy, J. E. Coster & G. D. Hertel (eds.). 1980. The southern pine beetle. U.S. Forest Service, Technical Bulletin 1631.
Traugott, T. A. & S. Dicke. 2006. Publication 2260 Are My Pine Trees Ready To Thin? Mississippi State Extension.
Publication 2876 (POD-06-21)
By A. Brady Self, PhD, Associate Extension Professor, Forestry; James D. Floyd, Forestry Instructor, Holmes Community College; and John J. Riggins, PhD, Associate Professor, Biochemistry, Molecular Biology, Entomology, and Plant Pathology.