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# GTBOP Webinar Archive Summary

## Understanding Tree Pests: Disease Interactions, Invasive Threats, and Management Strategies

Webinar Date: January 15, 2026 Speaker: Dr. Ignazio Graziosi, Assistant Professor, Warnell School of Forestry and Natural Resources, University of Georgia Moderator: Dr. Bodie Pennisi, UGA Horticulturist Duration: 52:11 Series: Green & Commercial CEU Categories: TBD

## NARRATIVE SUMMARY

Dr. Ignazio Graziosi, an assistant professor in the Warnell School of Forestry and Natural Resources at the University of Georgia, presented a framework for understanding tree pest damage through ecological interactions. He emphasized that damage results from the interplay of three components — the pest, the host tree, and the environment — known as the disease triangle. He extended this concept through the spiral of tree decline, illustrating how predisposing factors like soil compaction, inciting factors like drought, and contributing factors including wood-boring insects and fungi push trees toward death, particularly in urban environments.

Dr. Graziosi applied this framework to three case studies. The first examined the emerald ash borer (EAB), a non-native pest devastating native ash across North America. He reviewed the EAB life cycle, the role of firewood movement in spreading the invasion, and the diversity of ash species at risk in Georgia, including the white fringetree (family Oleaceae) as an alternate host and population reservoir. He explained how EAB populations remain low in Asia due to co-evolved resistant trees and specialist parasitoid wasps, which USDA has introduced to North America through importation biological control. Chemical protection via tree injection, bark spray, and soil drench remains critical for high-value landscape trees.

The second case study addressed crapemyrtle bark scale (CMBS), a non-native pest attacking crapemyrtle (Lagerstroemia indica) across the South. Dr. Graziosi described overlapping generations that complicate control timing, an expanding host range including American beautyberry and St. John's wort, and urban heat island effects that stress trees while accelerating insect development. Chemical options include soil drench, soil injection, and foliar sprays, though trunk injection is ineffective in crapemyrtle. Native predators such as lady beetles and green lacewings provide some control but lack the specialization needed for sustained suppression.

The third case study examined the orange-striped oakworm moth, a native pest of native oaks. Dr. Graziosi explained why clonal nursery stock in urban landscapes creates genetically uniform stands with low resistance, compounded by urban heat effects. He outlined a practical decision framework: managers should distinguish aesthetic damage from actual harm using a 25% defoliation threshold and recognize that late-season defoliation is less damaging. Btk (Bacillus thuringiensis kurstaki) as a foliar spray is effective against young larvae with minimal impact on natural enemies.

## YOUTUBE TIMESTAMPS

0:00 Introduction and speaker credentials 1:28 The disease triangle: pest, tree, and environment 3:02 The spiral of tree decline 6:07 Native vs. non-native pest and tree interactions 7:22 Example 1: Emerald ash borer (EAB) — overview 8:23 EAB life cycle and damage symptoms 10:48 EAB generation time and temperature effects 11:57 EAB spread across North America and firewood 13:47 EAB in Georgia and native ash species diversity 15:26 White fringetree as alternate EAB host 16:43 Global trade and non-native species introductions 18:20 The invasion curve: detection, eradication, and control 21:32 Why EAB is not a pest in Asia 22:34 Chemical control methods for EAB 23:57 Importation biological control: parasitoid wasps 28:01 Native natural enemies and the goal of balance 29:42 Example 2: Crapemyrtle bark scale (CMBS) — overview 31:47 CMBS biology, life cycle, and overlapping generations 33:47 CMBS invasion timeline and early detection 35:02 CMBS and the invasion curve in Georgia 36:06 CMBS host range expansion in North America 36:42 Urban heat island effects on scales 37:28 Chemical control options for CMBS 38:31 Natural enemies: lady beetles and lacewings 42:04 Balancing chemical and biological control for CMBS 42:25 Example 3: Orange-striped oakworm moth — overview 43:53 Oakworm life cycle and seasonal timing 44:51 Why urban and clonal trees are vulnerable 46:35 Natural enemies of the oakworm 47:56 Control decisions: damage thresholds and Btk 49:19 Presentation wrap-up and key takeaways 50:01 Moderator comments on CMBS expansion in Georgia

## QUESTIONS & ANSWERS

Q: What is the disease triangle, and why is it important for understanding tree pest damage? A: The disease triangle describes how damage to a tree results from the interaction of three components: the pest, the host tree, and the environment. Damage is not caused by the pest alone — a susceptible tree in a stressful environment amplifies the impact. Dr. Graziosi emphasized that this framework helps professionals identify which factors they can influence when managing pest problems, particularly in urban settings where environmental stress is high.

Q: What is the spiral of tree decline, and how does it relate to tree death in urban environments? A: The spiral of tree decline is a diagram with three levels of stressors — predisposing factors (such as soil compaction and genetic potential), inciting factors (such as defoliating insects and drought), and contributing factors (such as wood-boring insects, nematodes, and Armillaria). Each level can independently bring a tree to death, and urban environments intensify these stressors. The key takeaway is that tree death typically results from multiple interacting factors rather than a single cause.

Q: How does firewood movement contribute to the spread of the emerald ash borer? A: The movement of infested firewood by campers and travelers was the primary pathway for EAB's rapid spread across North America. Researchers were able to connect the pattern of EAB spread closely with the highway and freeway system. This led to the "Don't Move Firewood" campaign, which also helps contain other invasive insects such as the Asian longhorned beetle.

Q: Why is the emerald ash borer not considered a pest in its native range in Asia? A: Two main reasons explain this. First, native Asian ash species co-evolved with the EAB and are resistant or less susceptible — only very weak, stressed trees are attacked there. Second, a community of specialist natural enemies, including parasitoid wasps, keeps EAB populations low in Asia. This understanding directly informed both the chemical protection approach (making North American trees artificially resistant) and the importation biological control program.

Q: What are the chemical control options for protecting ash trees from the emerald ash borer? A: Three main application methods are available: tree injection, bark spray, and soil drench. Some products can be applied by homeowners while others require a professional applicator. Timing is important — for example, soil drench needs to be applied in spring before leaves emerge. Dr. Graziosi noted that chemical protection remains critical for saving individual high-value trees, since biological control has not yet been fully successful for EAB in North America.

Q: Why is trunk injection not effective for controlling crapemyrtle bark scale? A: Crapemyrtle does not absorb systemic insecticides well through trunk injection — the chemical moves very slowly through the plant, making it an ineffective delivery method. Instead, soil drench, soil injection, and foliar sprays are the recommended chemical control approaches for CMBS. Soaps can also be used to target crawlers, the young mobile nymphs.

Q: What native predators help control crapemyrtle bark scale, and what are their limitations? A: Three lady beetle species — the twice-stabbed lady beetle, the bigeminal lady beetle, and the non-native Harlequin lady beetle — are active predators of CMBS, feeding on both nymphs and adults. Green lacewings, particularly the red-lipped green lacewing, also prey on CMBS nymphs and eggs and are commercially available. However, these predators are generalists that often arrive late in the season and do not build sustained populations on infested trees, limiting their effectiveness as standalone control agents.

Q: How should a landscape professional decide whether to treat for orange-striped oakworm? A: Dr. Graziosi outlined a two-part decision framework. First, distinguish between aesthetic damage and actual harm to the tree — the threshold is approximately 25% defoliation. Second, assess the season: late-season defoliation, even if substantial, is less harmful because the tree has already stored its nutrients. Treatment is most warranted for young or small trees experiencing significant early-season defoliation.

Q: Why are urban landscape trees particularly vulnerable to the orange-striped oakworm? A: Urban landscape trees are often clonal nursery stock with very low genetic variability. Dr. Graziosi used the example of Nuttall oaks on the UGA Athens campus — beautiful, high-quality trees that are essentially genetically identical, meaning the susceptibility of one tree is the susceptibility of all. Combined with urban heat island effects that accelerate insect development, this creates conditions favoring pest outbreaks.

Q: What is importation biological control, and how has it been applied to the emerald ash borer? A: Importation biological control involves studying the natural enemies that effectively control a pest in its native range and introducing them to the invaded region. USDA conducted risk assessments and introduced three specialist parasitoid wasp species from Asia to target EAB. Two species attack EAB larvae under the bark using ovipositors to locate them through vibrational cues, while the third parasitizes EAB eggs. These wasps are specialists that only attack EAB, ensuring they won't harm other insects.

Q: What is the significance of the white fringetree for emerald ash borer management in Georgia? A: The white fringetree, which belongs to the same family (Oleaceae) as ash, was discovered in 2014 to be an alternate host for the EAB. This is significant not only because it means another native plant species is at risk, but more importantly because white fringetree can serve as a population reservoir, allowing EAB to persist in an area even after all ash trees are gone. This has direct implications for long-term management strategies in Georgia.

## ADDITIONAL RESOURCES

- Dr. Graziosi referenced QR codes linking to UGA publications on EAB status and control in Georgia, the full insecticide protocol for EAB, and CMBS biology and management.
- The iTree tool suite (mentioned for calculating ecosystem services and benefits of trees, and for right-tree-right-place selection) is available at itreetools.org.
- Sign-in sheets for CEU credit should be submitted to gtbop@uga.edu or mailed to the address on the sign-in sheet.
- The next GTBOP webinar was announced for March 2026.

Processed for UGA Center for Urban Agriculture / GTBOP Archives