Comparison of numerical response and predation effects of two coccinellid species on hemlock woolly adelgid (Homoptera: Adelgidae)

The hemlock woolly adelgid, Adelges tsugae Annand, is an introduced pest in North America that is native to Asia, and is causing extensive damage to eastern hemlock (Tsuga canadensis Carriere) and Carolina hemlock (Tsuga caroliniana Englemann) in the eastern United States. We compared two coccinellids imported for biological control of the adelgid: Scymnus ningshanensis Yu et Yao from China and Pseudoscymnus tsugae Sasaji and McClure from Japan. In a laboratory study, we measured the numerical response of each beetle species to a range of prey densities, and in field studies we examined the reproductive success and ability of the coccinellids to reduce populations of the hemlock woolly adelgid. In the laboratory, S. ningshanensis showed a positive numerical response as hemlock woolly adelgid density increased, and P. tsugae showed a density-independent response. In field cages, the presence of S. ningshanensis resulted in negative hemlock woolly adelgid population growth, in contrast to positive growth in both control cages and cages containing P. tsugae. Both our laboratory and field experiments suggest that S. ningshanensis has good potential as a biological control agent of hemlock woolly adelgid.     

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.     

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.     

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.     

Assessment of the potential for hybridisation between Laricobius nigrinus (Coleoptera: Derodontidae) and Laricobius osakensis,predators of the hemlock woolly adelgid (Hemiptera: Adelgidae)

In 2003, Laricobius nigrinus Fender was introduced into the eastern United States as a biological control agent of the hemlock woolly adelgid (Adelges tsugae Annand). Following its release, it was discovered that L. nigrinus was hybridising and producing viable progeny with Laricobius rubidus LeConte, a species native to eastern North America. Recently, Laricobius osakensis Montgomery and Shiyake was imported from Japan into the USA as a potential biological control agent of hemlock woolly adelgid. Hybridisation between L. nigrinus and L. rubidus led to interest in the outcome of interactions between L. osakensis and the other two Laricobius spp. The purpose of this study was to determine if L. osakensis could mate with L. nigrinus, if they could produce hybrid progeny, and whether mating interferes with reproductive output. Laricobius spp. were observed mating directly following emergence and found to be capable of producing sterile eggs in the absence of a mating event. Laboratory and confined field studies found no evidence that L. osakensis and L. nigrinus could produce hybrid progeny and the interaction between the two species did not result in a lower reproduction associated with interspecific mating attempts. Interbreeding should therefore not have an impact on biological control using these species. Fecundity experiments showed that L. osakensis produced eggs earlier in the season and at a higher rate than L. nigrinus, suggesting that L. osakensis may have the potential to be an even more successful biological control agent than L. nigrinus.     

Competitive interactions among two specialist predators and a generalist predator of hemlock woolly adelgid, Adelges tsugae (Hemiptera: Adelgidae) in south-western Virginia

1 Competitive interactions among two specialist predators, Laricobius nigrinus and Sasajiscymnus (Pseudoscymnus) tsugae, and a generalist predator, Harmonia axyridis Pallas, of hemlock woolly adelgid, Adelges tsugae were evaluated in hemlock stands in south‐western Virginia. The two specialist predators are part of a biological control program for A. tsugae, and the potential for competition among these species and previously established generalist predators in the field is unknown. 2 Adult predators were evaluated in branch cages during spring and summer at two field sites infested with A. tsugae. Using females only in 2003 and sexual pairs in 2004, predator survival and net reproduction were examined, as well as their feeding and impact on A. tsugae when present alone and in conspecific and heterospecific groupings. 3 Predator survival was not affected by the presence of additional predators. Total feeding was greater for all species when placed in predator groupings, suggesting that interactions do not significantly interfere with feeding activity. Net reproduction per predator was negatively affected by conspecifics, but unaffected by heterospecifics, indicating that direct or indirect intraspecific interference may occur. In spring, L. nigrinus showed the greatest impact on A. tsugae, and H. axyridis had the greatest impact during summer. 4 These results suggest that it would be beneficial to utilize multiple predator species combinations over single species when implementing biological control for A. tsugae. Low‐density releases are also recommended to reduce intraspecific interference.     

Seasonal synchrony of life cycles between the exotic predator,Pseudoscymnus tsugae (Coleoptera: Coccinellidae) and its prey,the hemlock woolly adelgid Adelges tsugae (Homoptera:Adelgidae)

1 The seasonal synchrony between the exotic predator, Pseudoscymnus tsugae and its prey, the hemlock woolly adelgid, Adelges tsugae, was investigated in field cages and in the forest in Connecticut, U.S.A. from 1997–1999. 2 In early spring, egg to adult development took 45 d at 18.7 °C, 39.7 d at 20.2 °C and 31.5 d at 22.7 °C. Earliest emerging F1 adults mated and oviposited in the same year. whereas F1 and F2 females emerging later in the summer mated and reserved most of their egg complement for the following year. 3 A second generation of P. tsugae is possible in Connecticut but may be delayed by cool mid‐spring temperatures. Individuals of three generations of P. tsugae, including overwintering survivors, may coexist in July and August and adults can be found year‐round with A. tsugae in infested hemlock forests. 4 A linear regression model for development from egg to adult under field temperatures gave good agreement with results from constant temperature findings. The model predicted a lower development threshold of 9.5 °C and a sum of effective temperatures of 405 day °C. Development time of P. tsugae is shorter relative to its prey A. tsugae and generation time ratios of predator to prey was 0.16–0.5, with an advantage conferred on the coccinellid. 5 Overwintering ability and behaviour were determined in 1998–1999 and adults remained on infested hemlock branches throughout a mild winter, becoming reproductively active in mid‐April. Peak oviposition period extended from April to July, in synchrony with peak oviposition and developing stages of two generations of A. tsugae.     

Haemocytes of the hemlock woolly adelgid Adelges tsugae Annand (Hom., Adelgidae) and changes after exposure to low temperatures

The main cellular components of the haemolymph of Adelges tsugae Annand were identified from individuals collected in Massachusetts, USA in February and October 1997. The cell types present were formative cells, plasmatocytes, adipohaemocytes, vermiform cells, prohaemocytes and oenocytoids. Vermiform cells were the most abundant. Very few oenocytoids were found. They were significantly more cells of all types in the haemolymph of A. tsugae collected in October than in those collected in February. Adelges tsugae collected in February were exposed in the laboratory to -20, -25 and -30°C for 2, 4 and 8h. No intact cells were found in the haemolymph of A. tsugae exposed to -30°C. Haemocytes were damaged by exposure to -20 and -25°C. The effect of these temperatures on the different cell types is presented.     

Survival and reproduction of Laricobius nigrinus Fender (Coleoptera: Derodontidae), a predator of hemlock woolly adelgid,Adelges tsugae Annand (Homoptera: Adelgidae) in field cages

Laricobius nigrinus Fender (Coleoptera: Derodontidae) is being evaluated as a biological control agent for the hemlock woolly adelgid (HWA), Adelges tsugae Annand (Homoptera: Adelgidae). Predator exclusion studies on survival, reproduction, and impact on HWA populations were investigated over two years in Virginia, US. In year 1, branches were selected to receive one of three treatments: caged hemlock branches with predators; caged hemlock branches without predators; and uncaged hemlock branches. L. nigrinus adults survived from February to April, producing up to 41 progeny per female. Adelgid densities on branches exposed to L. nigrinus exhibited a significantly higher rate of decline than those on branches not exposed to predators. Additionally, the final density of sistens and progrediens was significantly lower on caged branches containing L. nigrinus than on caged and uncaged branches without predators. In year 2, L. nigrinus survival and predation was evaluated over two 10-week sample periods: (November–January and February–April). L. nigrinus survived throughout the 6-month test period, with 89% surviving through January and 55% through April. Between February and April, 38 progeny were produced per beetle. The decrease in adelgids, measured in both numbers of adelgids and percent reduction per branch, was significantly higher (p < 0.0001) on caged branches with L. nigrinus than on those without predators.     

The tree crown distribution of hemlock woolly adelgid, Adelges tsugae (Hem., Adelgidae) from randomized branch sampling

Abstract:  An exotic insect, the hemlock woolly adelgid, Adelges tsugae Annand (Hem., Adelgidae), is spreading through eastern North America, killing hemlock trees [ Tsuga canadensis (L.) Carrière], and thereby impoverishing ecosystems. Adelges tsugae , like many alien invasive insects, is difficult to monitor or sample in the forest. Monitoring of A. tsugae has been hampered by lack of information about its distribution within tree crowns. In order to assist future monitoring and biocontrol of A. tsugae , this study investigates the crown distribution of A. tsugae by sampling from the entire height of mature hemlock trees in a forest with an established infestation. In addition to A. tsugae , sampling includes scale insects, which are another group of important pests on hemlock trees. This study demonstrates the utility of a randomized branch sampling (RBS) plan for monitoring both invasive insects as well as native insects that are difficult to sample. Results from the RBS show that in trees with high populations of A. tsugae , branches from the lower crown have slightly higher densities of A. tsugae than upper crown branches. In trees with low A. tsugae populations, the upper crown may have higher A. tsugae densities than the lower crown. North pointing branches also have higher densities of A. tsugae than branches pointing in other cardinal directions. Future sampling efforts for A. tsugae can take advantage of higher densities in certain portions of the crown to increase accuracy.     

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.     

Activity and residues of imidacloprid applied to soil and tree trunks to control hemlock woolly adelgid (Hemiptera: Adelgidae) in forests

We studied imidacloprid application methods and timing to control the hemlock woolly adelgid, Adelges tsugae Annand (Hemiptera: Adelgidae), in forests. The methods compared were 1) soil injection near the trunk; 2) soil injection dispersed throughout the area under the canopy; 3) soil drench near the base of the trunk; and trunk injection with the 4) Arborjet, 5) Wedgle, and 6) Mauget systems. The applications were made in the fall and the following spring. Adelgid populations on the hemlocks (Tsuga spp.) were assessed in the fall of two successive years after the treatments. Relative to the untreated control trees, all the soil applications resulted in population reductions, but none of the trunk injections resulted in reductions. Fall and spring treatment efficacy did not differ. Reductions by the soil treatments were between 50 and 100% (avg 80%) by the first fall and 83-100% (avg 98.5%) by the second fall. Analysis of imidacloprid residues using enzyme-linked immunosorbent assay found residues in sap, needles, and twigs 1 mo to 3-yr after application. A laboratory dose-response bioassay using excised, adelgid-infested hemlock branches with cut ends immersed in serial dilutions of imidacloprid determined the LC50 value to be 300 ppb, based on an exposure of 20 d. A high degree of suppression of the adelgid on forest trees was associated with residues in hemlock tissue > 120 ppb 2 yr after soil treatment. Although precise relationships between residues and efficacy are elusive, it is clear that soil application of imidacloprid resulted in chronic residues of imidacloprid in tissues and suppression of adelgid populations for > 2 yr.     

Prey suitability and phenology of Leucopis spp. (Diptera: Chamaemyiidae) associated with hemlock woolly adelgid (Hemiptera: Adelgidae) in the Pacific Northwest

Leucopis spp. (Diptera: Chamaemyiidae) from the Pacific Northwest previously were identified as potential biological control agents for the hemlock woolly adelgid, Adelges tsugae Annand (Hemiptera: Adelgidae), in the eastern United States. We collected Leucopis spp. larvae from A. tsugae infested western hemlocks in Oregon and Washington and reared them on an unidentified Pineus spp., Pineus strobi (Hartig), Adelges cooleyi (Gillette), Adelges piceae (Ratzeburg), and A. tsugae in three no-choice tests. Leucopis spp. survival on A. tsugae was significantly higher than on A. piceae during the 2010 progrediens generation test and significantly higher than on P. strobi and A. cooleyi during the 2010 sistens generation test. However, across all three tests, some larvae completed development to adult on all four of the alternative adelgid species. Larvae that survived to the adult stage were identified as Leucopis argenticollis Zetterstedt and Leucopis piniperda Malloch. These results suggest that populations of L. argenticollis and L. piniperda in the Pacific Northwest may not be specific to A. tsugae. We also studied the phenology of Leucopis spp. on fourteen A. tsugae infested western hemlock trees in Oregon and Washington over a period of 14 mo. Leucopis spp. larvae were collected year-round, but highest densities coincided with the presence of progrediens and sistens eggs and adults of A. tsugae. There was a positive correlation between Leucopis spp. and A. tsugae abundance.     

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.     

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”.     

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.     

Effects of fertilization of four hemlock species on Adelges tsugae (Hemiptera: Adelgidae) growth and feeding preference of predators

Understanding how fertilization affects host resistance to hemlock woolly adelgid, Adelges tsugae Annand (Hemiptera: Adelgidae), is important because fertilizers are often used to grow resistant selections to a suitable size for testing. We evaluated four hemlock species (Tsuga) under three different fertilizer regimes to assess whether fertility affected resistance to the adelgid and to determine whether it affected feeding preferences of the adelgid predators Laricobius nigrinus Fender and Sasajiscymnus tsugae (Sasaji & McClure). Treatments were long-term fertilization (from June 2008 to June 2009), short-term fertilization (from March to June 2009), and no fertilizer. Fertilizer was applied biweekly with 240 ppm N by using water-soluble fertilizer (N-P-K, 20:20:20). Plants (>1 yr old) were artificially infested with adelgids on 31 March 2009. Among unfertilized hemlocks (n=10 per species), foliar N was highest in Tsuga mertensiana (Bong.) CarriBre and lowest in T. chinensis (Franch.) E. Pritz. Significantly more progredien ovisacs or sisten eggs were present on T. mertensiana than on the other hemlock species with none on unfertilized T. chinensis. A. tsugae adults on T. heterophylla (Raf.) Sarg. were unaffected by fertility, but densities of developing A. tsugae nymphs were higher on unfertilized T. heterophylla plants than on fertilized T. heterophylla plants regardless of fertilizer treatment. Both L. nigrinus and S. tsugae consumed more adelgid eggs that developed on fertilized T. canadensis than from unfertilized plants. The predators did not exhibit this preference for adelgid eggs from females that developed on T. heterophylla or T. mertensiana.     

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.