Distribution and abundance of Adelges tsugae (Hemiptera: Adelgidae) within hemlock trees

We studied the distribution of hemlock woolly adelgid, Adelges tsugae Annand (Hemiptera: Adelgidae), within hemlock trees for three summer (progrediens) and two winter (sistens) generations in northern Georgia. Eastern hemlock, Tsuga canadensis (L.) Carrière, trees were treated with 0, 10, or 25% of 1.5 g of imidacloprid per 2.5 cm of tree diameter at breast height and fertilized or not in a factorial design. Adelgid ovisacs per centimeter of branch were more abundant from June 2007 to June 2008 in the upper tree crown of insecticide untreated trees and when all trees were combined and that was the general trend for most comparisons. However, ovisacs were more abundant in the lower crown of insecticide treated trees in June 2008. More sistens nymphs settled on the upper crown branches than on the lower branches in summers 2007 and 2008. Higher eggs per ovisac were observed in the upper crown in February 2008 and in both the winter and summer 2009. In contrast, adelgids were more fecund in the lower crown in June 2008. On fertilized trees, eggs laid per adult were higher in the upper crown in February 2008. In summer 2008, eggs per ovisac were higher in the lower crown, but this reversed again to the upper crown by summer 2009. New growth of branches also varied among sample dates. These data demonstrate the variable distribution of adelgid and hemlock growth within trees over time and suggest that sampling only one crown area will not provide accurate estimates of adelgid densities.     

Effects of fertilizer and low rates of imidacloprid on Adelges tsugae (Hemiptera: Adelgidae)

Healthy hemlock trees, Tsuga canadensis (L.) Carrière, and hemlock woolly adelgid, Adelges tsugae Annand (Hemiptera: Adelgidae), populations should favor retention and population growth of adelgid predators such as Laricobius nigrinus Fender (Coleoptera: Derodontidae) and Sasajiscymnus tsugae (Sasaji & McClure) (Coleoptera: Coccinellidae). Eastern hemlock trees between 15 and 38 cm diameter at breast height (dbh) were treated with 0, 10, or 25% of 1.5 g imidacloprid (Merit 75 WP) per 2.5 cm dbh and were either fertilized or not, in a 3 by 2 factorial design. After 2 yr, imidacloprid reduced the numbers of ovisacs and eggs found on trees in a dosage-dependent manner, while enhancing tree growth parameters such as new shoots or needles and the length of new shoots. Fertilized trees had greater adelgid fecundity, which was positively correlated with total foliar N in both winter generations. In February 2009 (27 mo after imidacloprid treatment), higher imidacloprid dosages to unfertilized trees resulted in reduced adelgid fecundity. Concentrations of N, P, and K were higher in the foliage of trees treated with insecticide, whereas foliar aluminum concentrations were consistently lower in trees with higher insecticide dosages. Trees treated with low rates of imidacloprid were healthier than untreated trees, but only trees treated with the 0.1 x dosage had sufficient adelgids to possibly sustain predators over extended periods.     

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.     

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.     

Feeding preference of three lady beetle predators of the hemlock woolly adelgid (Homoptera: Adelgidae)

In a laboratory study, we tested the feeding preferences of three coccinellid predators of hemlock woolly adelgid, Adelges tsugae Annand, an introduced pest of hemlock in the eastern United States. The species tested were Sasajiscymnus tsugae Sasaji & McClure (formerly Pseudoscymnus tsugae) from Japan, Scymnus ningshanensis Yu & Yao from China, and Harmonia axyridis (Pallas), a generalist species introduced from Asia that is currently widespread in eastern hemlock, Tsuga canadensis Carriere, forests. We measured the feeding preference of each beetle species when given the choice of A. tsugae and either 1) Pineus strobi (Hartig) on Pinus strobus L.; 2) Adelges laricis Vallot on Larix decidua Mill.; 3) Adelges cooleyi (Gillette) on Pseudotsuga menziesii (Mirb.) Franco; or 3) Paraprociphilus tessellatus (Fitch) on Alnus serrulata (Ait.) Willd. We evaluated beetle preference for adults, nymphs, and eggs of each prey species. Generally, when adult or nymphal prey stages were compared, S. tsugae preferred A. tsugae adults to P. strobi, A. cooleyi, A. laricis, and P. tessellatus. S. ningshanensis showed less preference between adelgid species, but it did not prefer P. tessellatus nymphs. When preferences for adelgid eggs were assayed, S. tsugae and S. ningshanensis showed no preference between A. tsugae and A. cooleyi or P. strobi, but S. tsugae did prefer A. tsugae to A. laricis. Larvae of S. tsugae were unable to survive on P. tessellatus nymphs. H. axyridis adults readily consumed both A. tsugae and P. tessellatus, but H. axyridis larvae did not complete their life cycle on A. tsugae. Our host range tests suggest that S. ningshanensis and S. tsugae may feed on several species of Adelgidae and that A. tsugae is often preferred.     

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.     

Bacteriocyte-associated gammaproteobacterial symbionts of the Adelges nordmannianae/piceae complex (Hemiptera: Adelgidae)

Adelgids (Insecta: Hemiptera: Adelgidae) are known as severe pests of various conifers in North America, Canada, Europe and Asia. Here, we present the first molecular identification of bacteriocyte-associated symbionts in these plant sap-sucking insects. Three geographically distant populations of members of the Adelges nordmannianae/piceae complex, identified based on coI and ef1alpha gene sequences, were investigated. Electron and light microscopy revealed two morphologically different endosymbionts, coccoid or polymorphic, which are located in distinct bacteriocytes. Phylogenetic analyses of their 16S and 23S rRNA gene sequences assigned both symbionts to novel lineages within the Gammaproteobacteria sharing <92% 16S rRNA sequence similarity with each other and showing no close relationship with known symbionts of insects. Their identity and intracellular location were confirmed by fluorescence in situ hybridization, and the names ‘Candidatus Steffania adelgidicola'' and ‘Candidatus Ecksteinia adelgidicola'' are proposed for tentative classification. Both symbionts were present in all individuals of all investigated populations and in different adelgid life stages including eggs, suggesting vertical transmission from mother to offspring. An 85 kb genome fragment of ‘Candidatus S. adelgidicola'' was reconstructed based on a metagenomic library created from purified symbionts. Genomic features including the frequency of pseudogenes, the average length of intergenic regions and the presence of several genes which are absent in other long-term obligate symbionts, suggested that ‘Candidatus S. adelgidicola'' is an evolutionarily young bacteriocyte-associated symbiont, which has been acquired after diversification of adelgids from their aphid sister group.     

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.     

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.     

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。     

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.     

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.     

Endosymbiotic microorganisms in Adelges (Sacchiphantes) viridis (Insecta,Hemiptera, Adelgoidea: Adelgidae): Molecular characterization,ultrastructure and transovarial transmission

The aim of this paper was to identify endosymbiotic microorganisms living in the body cavity of a Polish population of an aphid, Adelges (Sacchiphantes) viridis, as well as to describe their ultrastructure and mode of transmission between generations. Molecular data (amplification and sequencing of 16S rRNA genes) indicated that endosymbionts of A. (S.) viridis are Betaproteobacteria of the species “Candidatus Vallotia virida”. Endosymbiotic bacteria are rod-shaped and localized in the cytoplasm of specific cells, termed bacteriocytes, of host insects. Endosymbionts sharing the same bacteriocytes differ in the density of their cytoplasm. There are two morphotypes of endosymbiotic bacteria: with electron-dense cytoplasm and electron-translucent cytoplasm. Since only bacteria containing electron-dense cytoplasm were observed in the binary fusion stage, differences in density of the cytoplasm are probably due to changes in the cytoskeleton of bacteria during division. Endosymbionts of A. (S.) viridis are transovarially (i.e. via oocytes) transmitted from the mother to the offspring.     

Reshaping the Ecology of Invading Populations of Hemlock Woolly Adelgid, Adelges tsugae (Homoptera: Adelgidae), in Eastern North America

Hemlock woolly adelgid, Adelges tsugae Annand (Homoptera: Adelgidae), is native to Japan where it is an innocuous inhabitant of Tsuga diversifolia Masters and T. sieboldii Carriere throughout their natural growing areas. Native adelgid populations are regulated by host resistance and natural enemies, in particular the oribatid mite, Diapterobates humeralis (Hermann) and the coccinellid beetle, Pseudoscymnus tsugae Sasaji and McClure. Invading populations of A. tsugae in western North America on T. heterophylla Sargent and T. mertensiana Carriere are mainly regulated by host resistance. However, invading populations in eastern North America attain damaging levels on T. canadensis (L.) Carriere and T. caroliniana Engelmann and are regulated mainly by weather and negative density-dependent feedback mechanisms related to host deterioration. Although A. tsugae is only passively dispersed by wind, birds, forest-dwelling mammals and humans, it is spreading at an alarming rate and is sufficiently cold hardy to threaten the existence of the two eastern hemlock species throughout their natural ranges. The current hope for suppressing invading populations of hemlock woolly adelgid in eastern North America lies with the exotic predator, P. tsugae. Extensive laboratory studies of the biology and predatory ability of P. tsugae revealed that it feeds on all life stages of its prey, that its multivoltine life cycle is well synchronized with that of the adelgid, and that it has great potential for biological control. We have reared and released nearly 130,000 adults of P. tsugae in forests in Connecticut, New Jersey and Virginia during the past four years. P. tsugae has reproduced, dispersed, overwintered and reduced densities of hemlock woolly adelgid by 47–88% in only five months on release branches at these sites. Current studies are investigating the long-term ability of P. tsugae to regulate invading populations of A. tsugae in eastern North America.     

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.     

When one is not necessarily a lonely number: initial colonization dynamics of Adelges tsugae on eastern hemlock, Tsuga canadensis

The ability to establish successfully in a new area can vary considerably among species. In addition to the well-recognized importance of propagule pressure in driving the rates of establishment of biological invaders, the life history strategy of a species can also affect establishment success, such as in the extent to which Allee effects (positive density-dependence), and environmental and demographic stochasticity manifest themselves. We quantified the establishment success of Adelges tsugae, a non-native insect currently invading North American hemlock. We inoculated eastern hemlock host trees with varying densities of this parthenogenetic insect, from 1 to >500 progrediens ovisacs. The number of settled sistens (the subsequent generation) was positively related to the initial density. More interesting, however, was that we recorded successful establishment from released progrediens ovisacs, and the subsequent initiation of the next generation (sistens), in ≈39 % of host trees inoculated with 1 ovisac. The observation that successful establishment can be accomplished by a single ovisac produced by a single individual has important implications in the invasion dynamics and management of A. tsugae.     

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.     

A holocyclic life cycle in a gall-forming adelgid, Adelges japonicus (Homoptera: Adelgidae)

While the family Adelgidae (Homoptera) is typically holocyclic and periodically host-alternating between a primary and secondary host, some anholocyclic species may persist exclusively on the primary or secondary host. In this study, we investigated the life cycle of Adelges japonicus , an anholocyclic species that utilizes either Picea jezoensis and Picea sitchensis (Pinaceae) as the primary hosts. Transfer experiments conducted in Hokkaido, the northern-most island of Japan, revealed that A. japonicus also includes holocyclic forms that can migrate to the secondary host, Larix kaempferi . The holocyclic forms differed from anholocyclic forms in the date of gall dehiscence, oviposition preference of gallicolae and development of wax gland plates of gallicola adults. However, we treated these two forms as the same species because of a lack of information on their phylogenetic relationships and genetic isolation. Sequence of generations and developmental stages of each generation in the holocyclic forms were reported. Morphology of wax gland plates of first-instar exulis was consistent with those of the subgenus Cholodkovskya , but different from those of the subgenus Adelges , to which A. japonicus has been assigned. Morphological comparison of gallicola adults suggests that the holocyclic forms were introduced to Hokkaido from central Japan.     

Effects of voltinism, interspecific competition and parasitism on the population dynamics of the hemlock scales, Fiorinia externa and Tsugaspidiotus tsugae (Homoptera: Diaspididae)

Abstract.

• 1 Life tables were constructed for solitary and coexisting populations of univoltine Fiorinia externa Ferris and bivoltine Tsugaspidiotus tsugae (Marlatt) (Homoptera: Diaspididae), two exotic scale pests of eastern hemlock, Tsuga canadensis Carriere, in the northeastern United States.
• 2 Solitary and coexisting populations of F. externa had similar survivorship and population growth rates resulting in an annual increase in density of 7–16%. Survivorship of solitary and coexisting populations of T. tsugae also was similar, but growth rates differed substantially. While solitary populations increased their density by 68% annually, populations coexisting with F. extema were reduced 74% each year.
• 3 The annual reduction in T. tsugae density where it coexists with F. externa was due in part to interspecific competition which resulted in higher mortality to nymphs from dispersal and starvation and in lower fecundity of adult females relative to solitary populations. A host shift by the parasitoid, Aspidiotiphagus citrinus (Crawford) (Hymenoptera: Aphenlinidae) from F. externa to T. tsugae in autumn also accounted for 71% of the annual decrease in T. tsugae numbers. Therefore, F. externa adversely affects the growth of T. tsugae populations not only because of its superior competitive ability but also because adult para-sitoids which emerge from it subsequently attack nymphs of T. tsugae in autumn.
• 4 Parasitism and starvation of nymphs resulting from competition were the key mortality factors in the population dynamics of these exotic hemlock scales.

     

四种粉虱种群超微形态特征多样性研究

运用扫描电子显微镜对烟粉虱Bemisia tabaci、非洲伯粉虱Bemisia afer、温室粉虱Trialeurodes vaporariorum和柑桔粉虱Dialeurodes citri4种粉虱4龄若虫的超微形态特征进行了初步研究。结果表明,扫描电子显微镜下的超微结构特征可很好地将4种粉虱区分开来,如烟粉虱圆锥状的舌状突,非洲伯粉虱尾沟两侧隆起上形成的横脊,温室粉虱形态典型的乳突和泌蜡孔,柑桔粉虱近圆形的皿状孔及其尾沟内的卵石状乳突等特异特征。并根据种间的超微结构特异特征,编制了不同种的简明检索表。超微形态特征为粉虱种群鉴定提供了可靠的理论参考。