Fossil pollen records forecast response of forests to hemlock woolly adelgid invasion

Forecasting the impact of invasive species can provide valuable insights to land managers and scientists. In this study we examined the threat posed to hemlock Tsuga canadensis forests by an invasive insect, the hemlock woolly adelgid Adelges tsugae (HWA). Our main objective was to determine if paleo-ecological pollen records, showing the response of tree genera to a previous Tsuga decline, could predict contemporary responses to the HWA. To assess this, we matched fossil pollen sites from the Global Pollen Database to sites in the northeastern US that were invaded by the HWA approximately two decades ago. Our analyses showed that several of the same genera responded positively to Tsuga declines at both fossil pollen and contemporary sites. We then applied this methodology to an area currently threatened by the HWA, Great Smoky Mountains National Park (GSMNP), where invasions began just seven years ago. Our analysis suggests that the canopy genera mostly likely to respond positively to declining hemlock abundances in GSMNP are Acer, Betula, Fagus, and Quercus . Our findings provide land managers in GSMNP with the first predictions of likely shifts in canopy composition. We expect that this methodology will have application at other areas threatened by the HWA. Finally, we believe that studies on the impacts of contemporary invasions have underutilized relevant fossil data and that greater efforts should be extended to doing so in the future.     

A geographically variable model of hemlock woolly adelgid spread

The hemlock woolly adelgid (HWA) (Adelges tsugae) has spread through the range of eastern hemlock (Tsuga canadensis) leaving dead hemlock trees in its wake. We combined county and previously unavailable township level infestation records with geographic variables including plant hardiness zone, elevation, forest cover type, urbanization, precipitation, temperature, and hemlock range in a geographic information system to build models linking HWA spread to the structure and characteristics of the environment. We explored the connections between site variables and spread rate using quantile regression, Tobit regression, a parametric model for heterogeneous error variance, and classification and regression tree modeling. The results show that since 1990 HWA has spread at a rate closer to 12.5 km/yr rather than the 20–30 km/yr previously estimated, however spread rates vary significantly with environmental variables. County-based data show a faster rate of range expansion in the south, 15.6 km/yr, and very slow spread in the northern part of the current range of HWA. The rate of spread based on township records for Pennsylvania and north is 8.13 km/yr. Our study suggests that HWA spreads more slowly in colder areas, with a mean minimum temperature of  − 26.1°C (plant hardiness zone 5B) delineating areas of slower range expansion. HWA also spreads more slowly during cold years, those with lower than average mean January temperatures. Our analysis demonstrates the value of quantile regression, Tobit regression, and classification and regression tree models for understanding the spread of invasive, exotic insects.     

Changes in the regional abundance of hemlock associated with the invasion of hemlock woolly adelgid (Adelges tsugae Annand)

Since its introduction, the non-native hemlock woolly adelgid (Adelges tsugae) has spread to infest hemlock (Tsuga spp.) in at least 18 states in the eastern USA. Previous studies have documented highly variable rates of hemlock mortality among infested stands making it difficult to estimate regional impacts. Here data from the US Forest Service Forest Inventory and Analysis program collected from 432 eastern U.S. counties reveals several surprising and conflicting regional patterns. First, median live and dead hemlock basal area has generally increased over the last two decades across the eastern U.S. This has generally been the case in both infested and uninfested counties. Second, the median percentage of hemlock which is alive has decreased over the past ~20 years, again in both infested and uninfested counties. Third, the ages of infestations are negatively correlated with the percentage of live hemlock, as might be expected given the known impact adelgids can have on a stand through time; however this relationship depends on the exclusion of uninfested counties, as counties infested >12 years and uninfested counties have similar percentages of live hemlock. Combined, these data suggest increasing tree density associated with the past century of reforestation and succession in the eastern U.S. may currently be overwhelming the negative impacts of the adelgid at the regional scale, however, the long-term stability of this situation is not known, and data from long-infested counties suggest the landscape may be at a “tipping point”.     

Using dendrochronology to model hemlock woolly adelgid effects on eastern hemlock growth and vulnerability

This study examined the relationship between eastern hemlock (Tsuga canadensis (L.) Carr.) crown condition and changes in radial growth associated with infestation by hemlock woolly adelgid Adelges tsugae (Homoptera: Adelgidae) (HWA). Tree-ring chronologies of eastern hemlock were used to develop a binomial decline index based on three consecutive years of below average growth. Radial growth decline was modeled, using logistic regression, as a function of an extensive array of tree, crown, and site variables that were collected over an 11 year period in Delaware Water Gap National Recreation Area. Some site-related variables such as site-location and aspect were significantly related to decline probabilities when considered individually. However, the total proportion of response variance accounted for was low, and the only site variable included in the final model was mean plot-level HWA infestation level. For every 1% increase in mean percent HWA infestation per plot, there was an 8% increase in the likelihood that a tree would be classified as being in decline. Tree crown variables such as live crown ratio, crown density, and the modified ZB_adj index, a combination of foliage transparency and branch dieback, had the most explanatory power, both individually and in the final model. These crown variables were relatively accurate predictors of the degree of hemlock growth decline during HWA infestation.     

Spatiotemporal distribution of the hemlock woolly adelgid predator Laricobius nigrinus after release in eastern hemlock forests

• 1 Post‐release distributions of Laricobius nigrinus, a biological control predator of hemlock woolly adelgid Adelges tsugae Annand, were evaluated at eight hemlock forests in the eastern U.S.A.
• 2 Vertical dispersal of F₁ and F₂L. nigrinus were assessed from within three crown strata (<7, 7–15 and >15 m) at four release sites.
• 3 Horizontal distributions of L. nigrinus within the forest surrounding central release areas were observed in two separate studies, which included (i) release and monitor to capture parent and F₁ movement by sampling the immature life stages of the offspring, and (ii) assessment of F₃ to F₆ generations where beetles were previously determined to be established.
• 4 Laricobius nigrinus, released on lower crown branches, oviposited within the upper crown stratum and were slow to disperse from release trees. Monitoring L. nigrinus only from the lower crown would likely underestimate its presence because 86% of the F₂ generation were detected above 15 m.
• 5 By the fifth generation, the frequency distributions of larvae increased at increasing distance from release areas; larvae were recovered at a maximum distance of approximately 400 m and the spread rate was approximately 39 m/year.
• 6 Slow dispersal of L. nigrinus and uninterrupted recovery of six generations in the presence of fluctuating prey density support its continued release as part of the A. tsugae biological control programme. These data contribute toward improved release strategies and monitoring for this biological control agent.

     

False ring formation in eastern hemlock branches: impacts of hemlock woolly adelgid and elongate hemlock scale

Herbivores can alter plant physiology through the induction of abnormal wood formation. Feeding by some insects induces the formation of false rings, a band of thick-walled latewood cells within the earlywood portion of the tree ring that reduces water transport. Hemlock woolly adelgid (Adelges tsugae Annand) and elongate hemlock scale (Fiorinia externa Ferris) are invasive insects that both feed on eastern hemlock [Tsuga canadensis (L.) Carrière]. Adelges tsugae has a greater effect on tree health than F. externa, but the mechanism underlying their differential effect is unknown. We explored the effects of these herbivores by assessing growth ring formation in branches of trees that had been experimentally infested for 4 yr with A. tsugae, F. externa, or neither insect. We measured false ring density, ring growth, and earlywood: latewood ratios in the two most recently deposited growth rings. Branches from A. tsugae-infested trees had 30% more false rings than branches from F. externa-infested trees and 50% more than branches from uninfested trees. In contrast, branches from F. externa-infested trees and control trees did not differ in false ring formation. Radial growth and earlywood: latewood ratios did not differ among treatments. Our results show that two invasive herbivores with piercing-sucking mouth parts have differing effects on false ring formation in eastern hemlock. These false rings may be the product of a systemic plant hypersensitive response to feeding by A. tsugae on hemlock stems. If false rings are responsible for or symptomatic of hemlock water stress, this may provide a potential explanation for the relatively large effect of A. tsugae infestations on tree health.     

Anisotropic spread of hemlock woolly adelgid in the eastern United States

Simple population models predict that the spread of an invading species through a homogenous habitat should be equal in all directions, but geographic variation in the habitat that affects either reproduction or movement could result in variable rates of spread. We analyse records of the historical range expansion of the hemlock woolly adelgid (HWA) (Adelges tsugae Annand) in the eastern United States from 1951 to 2006 to document that this species has spread in an anisotropic fashion. Furthermore, the magnitude and direction of this anisotropy has varied through time. We explore the extent to which this spatial and temporal variation in spread can be explained by geographical variation in climate and by the abundance of hosts, eastern hemlock (Tsuga canadensis L.) and Carolina hemlock (Tsuga caroliniana Engelm.). We found that a significant component of the spatial anisotropy in HWA spread rate can be explained by the geographical distribution of host trees. January temperatures were negatively associated with spread rates but this may be an artifact of the association between hemlock and cold climates. The current distribution of the adelgid in eastern N. America may be approaching the extent of its potential range to the south and west determined by availability of host hemlock and to the north determined by lethal cold winter temperatures.     

Volatile emissions of eastern hemlock,Tsuga canadensis,and the influence of hemlock woolly adelgid

The volatile emissions of eastern hemlock, Tsuga canadensis Carriere, were identified and quantified using standard and chiral gas chromatography and mass spectrometry. All of the identified compounds were monoterpenes, and included alpha-pinene, myrcene, tricyclene, camphene, alpha-phellandrene, beta-pinene, limonene, beta-phellandrene, terpinolene, and bornyl acetate. alpha-Pinene, myrcene, and camphene comprised greater than 75% by mass of the total release. Infestation by the exotic insect, hemlock woolly adelgid (HWA, Adelges tsugae Annand), resulted in an increased release rate of monoterpenes from branch tips. Release rate was negatively correlated to the amount of the branch tip sample that was new growth, suggesting that release rate is greater from previous-year foliage. Additionally the percent composition of the volatile profile is slightly altered by infestation, with alpha-pinene comprising 57% of volatiles from infested foliage and 66% from uninfested foliage.     

Simulating the dispersal of hemlock woolly adelgid in the temperate forest understory

The hemlock woolly adelgid (HWA), Adelges tsugae Annand (Hemiptera: Adelgidae), has spread rapidly across the eastern USA since its introduction from Japan 60 years ago, causing widespread mortality of both eastern hemlock [Tsuga canadensis (L.) Carrière] and Carolina hemlock [Tsuga caroliniana Engelm. (Pinaceae)]. Although HWA spread patterns have been repeatedly analyzed at regional scales, comparatively little is known about its dispersal potential within and between hemlock stands. As the small size and clonal nature of HWA make it nearly impossible to identify the source populations of dispersing individuals, we simulated intra‐stand HWA movement in the field by monitoring the movement of clumps of fluorescent powder that are slightly larger than HWA, but much easier to detect in the forest understory. Using three hemlock trees with three colors of fluorescent powder as source populations, we detected dispersal events at the farthest distances within our trapping array (400 m). However, more than 90% of dispersal events were <25 m. Dispersal patterns were similar from all three source trees and the distribution of dispersal distances in all cases could be described by lognormal probability density functions with mean dispersal distance of 12–14 m, suggesting that dispersal was relatively independent of location of source trees. In general, we documented tens of thousands of passive dispersal events in the forest understory despite the presence of a dense forest canopy. Thus, even under relatively light‐wind conditions, particles of similar dimensions to HWA are capable of intra‐stand movement, suggesting that a large population of HWA could rapidly infest other trees within several hundred meter radius, or beyond.     

斑翅肩花蝽对铁杉球蚜的捕食作用

室内用铁杉球Adelges tsugae Annand(Hemlock woolly adelgids)蚜卵、1～4龄若虫、成虫饲养斑翅肩花蝽Tetraphleps galchanoides Ghauri,研究斑翅肩花蝽对铁杉球蚜Adelges tsugae Annand(Hemlock woolly adelgids)的捕食效应。结果表明,斑翅肩花蝽对铁杉球蚜的捕食功能反应属于HollingⅡ型反应,对球蚜密度的捕食干扰反应可能存在种内的干扰作用。对于同一发育期同一密度猎物,斑翅肩花蝽成虫或若虫在高密度条件下的日均捕食量均显著高于低猎物密度下的日均捕食量;而对于不同发育期不同密度猎物,斑翅肩花蝽在低猎物密度时对测定的铁杉球蚜6个不同虫态无明显选择性;在中、高密度时斑翅肩花蝽对测定的铁杉球蚜的不同虫态表现出明显喜好性差异,若虫最喜好食铁杉球蚜卵和1龄若虫,成虫最喜好食铁杉球蚜4龄若虫和成虫。为评价和利用斑翅肩花蝽对铁杉球蚜防治作用进行了讨论。     

Facilitation between invasive herbivores: hemlock woolly adelgid increases gypsy moth preference for and performance on eastern hemlock

1. Interactions between invertebrate herbivores with different feeding modes are common on long-lived woody plants. In cases where one herbivore facilitates the success of another, the consequences for their shared host plant may be severe. Eastern hemlock (Tsuga canadensis), a canopy-dominant conifer native to the eastern U.S., is currently threatened with extirpation by the invasive stylet-feeding hemlock woolly adelgid (Adelges tsugae). The effect of adelgid on invasive hemlock-feeding folivores remains unknown. 2. This study evaluated the impact of feeding by hemlock woolly adelgid on gypsy moth (Lymantria dispar) larval preference for, and performance on, eastern hemlock. To assess preference, 245 field-grown hemlocks were surveyed for gypsy moth herbivory damage and laboratory paired-choice bioassays were conducted. To assess performance, gypsy moth larvae were reared to pupation on adelgid-infested or uninfested hemlock foliage, and pupal weight, proportional weight gain, and larval period were analysed. 3. Adelgid-infested hemlocks experienced more gypsy moth herbivory than did uninfested control trees, and laboratory tests confirmed that gypsy moth larvae preferentially feed on adelgid-infested hemlock foliage. Gypsy moth larvae reared to pupation on adelgid-infested foliage gained more weight than larvae reared on uninfested control foliage. 4. These results suggest that the synergistic effect of adelgid and gypsy moth poses an additional threat to eastern hemlock that may increase extirpation risk and ecological impact throughout most of its range.     

铁杉球蚜的生物学及空间分布

通过对铁杉(Tusga chinensis Pritz)-云杉(Picea retroflexac Mast)、铁杉-华山松(Pinus armardi Franch)、云南铁杉(Tusga domosa Eichler)-槭树(Acer mono Maxim)-桦木(Betula platyphylla Suk)四川主要铁杉林类型中铁杉球蚜Adelges tsugae Annand林间定株、种群随机抽样、室内饲养研究表明,铁杉球蚜在四川1年发生2代(越冬代和第1代),世代重叠,成虫营孤雌生殖。越冬代从4月上旬至第2年的4月下旬,产卵盛期在5月下旬,平均产卵量为15.58粒,1龄若虫具有滞育越夏习性;第1代从12月下旬至8月中旬,产卵盛期在3月下旬,平均产卵量为67.37粒,并可产生有翅成虫,但无转主危害现象。种群的发育与温度有相关性,温度高林分种群发育进度快于温度低林分。该虫的危害与生境有一定相关性,铁杉针阔混交林危害重于铁杉针叶林;同一树冠不同层次之间、不同方位之间危害程度差异不显著;当年受害严重的树株第2年受害不严重。     

Binomial sequential sampling plan for hemlock woolly adelgid (Hemiptera: Adelgidae) sistens infesting individual eastern hemlock trees

The hemlock woolly adelgid, Adelges tsugae Annand (Hemiptera: Adelgidae), is an exotic insect pest that is killing eastern hemlock, Tsuga canadensis (L.) Carrière, and Carolina hemlock, Tsuga caroliniana Engelmann, in the eastern United States. We used the sequential interval procedure to develop a binomial sequential sampling plan for A. tsugae sistens on individual eastern hemlock trees that uses nondestructive sampling of new shoots. The actual a (type I) and beta (type II) error rates were essentially 5 and 10%, respectively. Tallies of new shoots infested by at least one A. tsugae sistens were compared with stop values for thresholds of 10 and 30% of new hemlock shoots infested. Twenty to 80 new shoots had to be examined per tree to render a low, high, or indeterminate classification, which took < 2 min per tree regardless of the threshold used. This plan should be an efficient and cost-effective tool in the management of A. tsugae infestations on individual, high-value eastern hemlock trees.     

Factors affecting settlement rate of the hemlock woolly adelgid,Adelges tsugae,on eastern hemlock,Tsuga canadensis

1 We assessed the importance of several factors potentially affecting the settlement rate of the invasive hemlock woolly adelgid Adelges tsugae (Hemiptera: Adelgidae) on uninfested foliage of the eastern hemlock, Tsuga canadensis. We conducted our experiments in Massachusetts (U.S.A.) with overwintering sistens adelgids, and applied standard densities of infested foliage to uninfested branches in a planned multiple‐comparison design. 2 Settlement rates of progrediens crawlers produced by the overwintering sistens were highest when adelgid‐infested foliage was loosely attached to uninfested foliage and both branches were then enclosed in a mesh sleeve. 3 Early‐emerging crawlers settled at a higher rate than did late‐emerging crawlers. 4 Increasing the density of infested branches did not affect settlement rates. 5 We also tested whether less severe winter conditions improved settlement, and found that overwintering infested foliage in a refrigerator decreased settlement rate relative to foliage overwintered outdoors. 6 Our results suggest a protocol for adelgid inoculations that could substantially increase the success rate of experimental manipulations and encourage additional research on the population dynamics of this pest.     

Hemlock woolly adelgid invasion affects microhabitat characteristics and small mammal communities

Hemlock woolly adelgid (HWA) invasion and preemptive logging practices alter the habitat structure of New England forests and may indirectly affect associated small mammal communities. Microhabitat structure was measured and small mammals were censused in eight large experimental plots to quantify these effects. The Harvard Forest long-term ecological research experiment is a replicated two-block design that includes four ~ 0.81-ha canopy treatments: (1) hemlock control, (2) hardwood control, (3) girdled Treatment, in which hemlock trees were killed by girdling in 2005 and left standing to simulate HWA invasion, and (4) logged treatment, in which trees were removed to simulate preemptive logging management practices. Nine microhabitat characteristics were measured from plot photos revealing differences among ground and canopy microhabitat structure. Small mammals were censused during high (2012) and low (2013) abundant years. Populations of common species were estimated with mark-recapture analysis. Peromyscus spp. were not affected by treatment in either year, but southern red-backed vole populations were greatest in the girdled treatments in 2012 and rarely captured in 2013. Between 6 and 9 mammal species were recorded in all treatments and species composition varied slightly. Estimated species richness was greater in girdled treatments than hemlock controls, but did not differ between girdled and logged treatments, which suggests preemptive logging is as detrimental to some small mammal species as HWA invasion. Overall, there is little evidence of a major shift in small mammal community structure in response to HWA invasion, with only minor changes in relative abundance both years.     

Establishment and coexistence of two predators,Laricobius nigrinus and Sasajiscymnus tsugae,introduced against hemlock woolly adelgid on eastern hemlock

The coexistence of two introduced predatory species, Laricobius nigrinus Fender and Sasajiscymnus tsugae (Sasaji and McClure), and a native predator, L. rubidus LeConte, on eastern hemlock was documented for the first time. Details of their coexistence and implications to management of hemlock woolly adelgid, Adelges tsugae Annand, are discussed.     

Predators associated with the hemlock woolly adelgid (Hemiptera: Adelgidae) in the Pacific Northwest

The hemlock woolly adelgid, Adelges tsugae Annand (Hemiptera: Adelgidae), is causing widespread mortality of eastern hemlock, Tsuga canadensis L. Carrière, in the eastern United States. In western North America, feeding by A. tsugae results in negligible damage to western hemlock, Tsuga heterophylla (Raf.) Sargent. Host tolerance and presence of endemic predators may be contributing to the relatively low levels of injury to T. heterophylla caused by A. tsugae. Field surveys of the predator community associated with A. tsugae infestations on 116 T. heterophylla at 16 sites in Oregon and Washington were conducted every 4-6 wk from March 2005 through November 2006. Fourteen uninfested T. heterophylla were also surveyed across 5 of the 16 sites. Each sample tree was assigned an A. tsugae population score ranging from 0 to 3. Predators collected from A. tsugae-infested T. heterophylla represent 55 species in 14 families, listed in order of abundance: Derodontidae, Chamaemyiidae, Hemerobiidae, Coccinellidae, Cantharidae, Reduviidae, Miridae, Syrphidae, Chrysopidae, Coniopterygidae, Staphylinidae, Anthocoridae, Nabidae, and Raphidiidae. Laricobius nigrinus Fender (Coleoptera: Derodontidae), Leucopis argenticollis Zetterstedt (Diptera: Chamaemyiidae), and Leucopis atrifacies (Aldrich) (Chamaemyiidae) were the most abundant predators; together comprising 59% of predator specimens recovered. Relationships among predators and A. tsugae were determined through community structure analysis. The abundances of Laricobius spp. larvae, L. nigrinus adults, Leucopis spp. larvae, and L. argenticollis adults were found to be positively correlated to A. tsugae population score. Predators were most abundant when the two generations of A. tsugae eggs were present. L. argenticollis and L. atrifacies were reared on A. tsugae in the laboratory, and host records show them to feed exclusively on Adelgidae.     

The aftermath of an invasion: Structure and composition of Central Appalachian hemlock forests following establishment of the hemlock woolly adelgid, <Emphasis Type="Italic">Adelges tsugae</Emphasis>

As the highly invasive hemlock woolly adelgid, Adelges tsugae, continues to expand its distribution in eastern North America, affected forests will incur drastic changes in composition and structure. While these changes have been well-studied in dense hemlock forests in the Northeast, relatively little work is known about the effects of the adelgid at the western edge of the range of eastern hemlock, Tsuga canadensis. We evaluated the nature and extent of these changes using vegetation assessments coupled with growth simulations. The woody plant community was assessed in three strata (upper, mid- and lower) and was used to predict forest succession. Using the Forest Vegetation Simulator (FVS), we then projected the growth of hemlock forests 20 years into the future with and without the effects of the adelgid. In forest simulations lacking adelgid invasion, little change in composition or structure is forecast. In contrast, our projections predict a near complete loss of the hemlock forest type within 20 years of adelgid establishment, with widespread conversion to hardwood forest types, most notably white oak-red oak-hickory, chestnut oak-black oak-scarlet oak, and yellow poplar-white oak-red oak. Hemlock loss will result in denser deciduous forests with thinner canopies and multiple gaps, and significant alterations to terrestrial and aquatic wildlife habitat.     

Ecological factors influencing the beneficial endosymbionts of the hemlock woolly adelgid (Hemiptera: Adelgidae)

Bacterial endosymbionts of sap-sucking insects provide their host with a num- ber of beneficial qualities,including the supply of nutrition,defense against parasitoids, and protection from heat stress.Damage to these bacterial associates can therefore have a negative impact on the fitness of their insect host.We evaluated observational and experi- mental factors regarding the normative hemlock woolly adelgid (Adelges tsugae Annand) (Hemiptera:Adelgidae)to help understand the roles of its three recently identified symbionts,including under heat stress conditions.The prevalence of A.tsugae's facultafive symbiont (Serratia symbiotica)was examined at different spatial scales to determine how variable infection rates are for this symbiont.There was no significant difference found in infection rates between adelgids on a tree,within a plot,or within a state.However, significantly more adelgids in Georgia (95%)had S.symbiotica compared to those in New York (68%).Microsatellite genotyping of the adelgids found that this difference was most likely not the result of a second introduction ofA.tsugae into eastern North America.Comparison orS.symbiotica proportions between first and fourth instars showed that symbiont absence did not affect the ability ofA.tsugae to survive aestivation.Evaluations of sym- biont densities within each adelgid found that when S.symbiotica was absent,the density of obligate symbionts was significantly higher.Exposure to heat stress (32.5℃)was not consistently correlated with changes in symbiont densities over a 4-d period.Overall,we have shown that symbiont prevalence and densities vary within the broad population of A.tsugae in eastern North America,with potentially significant effects upon the ecology of this important pest.     

Recovery of Sasajiscymnus tsugae,released against hemlock woolly adelgid,Adelges tsugae,in the southern Appalachians

Eastern hemlock in the Great Smoky Mountains National Park is currently threatened by the hemlock woolly adelgid, Adelges tsugae Annand (Hemiptera: Adelgidae). As part of a management plan against this invasive insect pest, about 350,000 adults of the predatory beetle Sasajiscymnus tsugae (Sasaji and McClure) (Coleoptera: Coccinellidae) were released at ca. 150 sites in the Park from 2002 to 2007. Of these adult release sites, 33 were sampled in 2008 and 2009 using beat-sheet sampling for 4 man-hours. Sasajiscymnus tsugae adults (n=78) and/or larvae (n=145) were recovered from seven sites (21.2% of the release sites sampled). Recovery of S. tsugae was significantly associated with older release sites, with the most beetles recovered from 2002 release sites. These results indicate that S. tsugae may require more time (i.e., 5–7 years) than anticipated for population densities to reach readily detectable levels in some areas.