Parasitism of Cameraria ohridella (Lepidoptera, Gracillariidae) in natural and artificial horse-chestnut stands in the Balkans

1 The horse‐chestnut leafminer, Cameraria ohridella Deschka & Dimic, was discovered during a mass outbreak on planted horse‐chestnut trees, Aesculus hippocastanum, near Lake Ohrid in Macedonia. However, several peculiarities in the leafminer's biology and ecology, such as its inefficient control through natural enemies, indicate that it may be of exotic origin. 2 In this study, the parasitism of C. ohridella was examined in natural and artificial horse‐chestnut stands in South‐eastern Europe. The size and composition of the parasitoid complex of natural stands was comparable with that known from artificial stands. No specialist parasitoid species were detected in any of the samples. The only parasitoid species found exclusively in natural stands during this investigation were the braconid Colastes braconius and the eulophid Chrysocharis phryne. Both are known to be polyphagous on many leafminers in Europe and C. braconius has been often recorded from C. ohridella in artificial stands in previous studies. Parasitism levels varied within the same range (3.6–21.0%) throughout the entire region, in both types of habitats. 3 These results suggest that the parasitoid complex of C. ohridella in natural and planted horse‐chestnut stands in Europe is typically that of an exotic, recently introduced leafminer. Consequently, C. ohridella is probably not indigenous in the Balkans, but has been introduced to the area of its discovery from another continent.     

Factors favouring the development and maintenance of outbreaks in an invasive leaf miner Cameraria ohridella (Lepidoptera: Gracillariidae): a life table study

1 Life tables were constructed for seven generations of the invasive horse‐chestnut leaf miner Cameraria ohridella in Switzerland, to assess the factors allowing the moth to build and maintain outbreak densities and to identify ‘gaps’ among their mortality factors that could be targeted by new control methods. The fecundity of females was calculated and the mortality factors affecting all developmental stages determined. 2 Females contained 106.6 eggs. In captivity, the mean number of eggs laid per female varied between 34 and 82 eggs, with a maximum of 184 eggs. Egg mortality was 18–75% and was mostly due to unknown factors. 3 The main mortality factors affecting larvae and pupae in the leaves were predation by birds and arthropods in all generation and leaf senescence in the autumn generation. Parasitoids killed less than 5% of the population. 4 Nearly 90% of overwintering pupae died in dead leaves, the majority of them being killed by earthworms and other leaf decomposers. As a result, the overwintering generations were the only ones showing a net rate of increase less than one. Non‐overwintering generations had net rates of increase between four and 11, allowing populations to build up from spring to autumn. 5 The persistence of high outbreak densities in Europe is explained by a combination of at least three factors: (i) high fecundity; (ii) multivoltinism; (iii) the unusually low impact of traditional natural enemies of leaf miners and particularly parasitoids. 6 Yearly variations in population densities could be at least partly explained by differences in larval mortality due to leaf senescence and intraspecific competition in the last generation in autumn.     

Visual, semi-quantitative assessments allow accurate estimates of leafminer population densities: an example comparing image processing and visual evaluation of damage by the horse chestnut leafminer Cameraria ohridella (Lep., Gracillariidae)

Abstract: Qualitative or semi-quantitative visual assessments are most often used for estimating population size of herbivorous insects. The precision of these estimates, however, is often difficult to establish. A 'simulation game' with the horse chestnut leafminer, Cameraria ohridella Deschka & Dimic (Lep., Gracillariidae) shows that visual, semi-quantitative assessments can provide accurate information. Damaged areas of 411 horse chestnut leaves collected in 100 sites were closely related to mine numbers despite some variability in mine and leaf size ( R ² = 0.915; n  = 411; P < 0.001). On the basis of this relationship, two methods of population assessment are compared: (i) digital image processing of leaf damage and (ii) visual assessment using a damage key reflecting the relative infested area on each leaf (0, 0%; 1, 0–2%; 2, 2–5%; 3, 5–10%; 4, 10–25%; 5, 25–50%; 6, 50–75%; 7, 75–100%). Both methods used to estimate damage presented a similar, close relationship to the 'real' numbers of mines ( R ² = 0.858; n  = 777; P < 0.001 for image processing and R ² = 0.905; n  = 777; P < 0.001 for visual assessment). The potential of using visual assessments as an accurate and fast method in situ at the tree scale is discussed.     

Impact of the leaf miner Cameraria ohridella (Lepidoptera: Gracillariidae) and bleeding canker disease on horse‐chestnut: direct effects and interaction

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Oviposition patterns and larval damage by the invasive horse‐chestnut leaf miner Cameraria ohridella on different species of Aesculus

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Spatial patterns and infestation processes in the horse chestnut leafminer Cameraria ohridella: a tale of two cities

Spatial patterns of the horse chestnut leafminer Cameraria ohridella Deschka & Dimic (Lepidoptera: Gracillariidae) population density was analysed in the cities of Bern and Brussels in order to explore its spatial population dynamics. The surveys were carried out in Bern in 1998 and 2000 and in Brussels in 2001 to assess population density in relation to local characteristics. In Brussels, population density was also measured using pheromone traps distributed over the city and collected twice per moth generation. A quantitative relationship was found between local population density (measured by pheromone traps and survey observations) and the amount of leaves left on the ground the previous fall. Several other factors were related to observed infestation levels: the occurrence of the pathogen Guignardia aesculi was inversely related to infestation by C. ohridella in Bern in 1998 and 2000, the number of horse chestnut trees within 800 m distance was positively related to infestation level in Bern in 2000, and the proportion of green areas within 100 m and the number of other horse chestnut trees within 2000 m were positively related to infestation levels in the 2001 Brussels survey. The pattern of infestation levels as a function of distance to potential population reservoirs suggested that C. ohridella re‐invades areas where overwintering leaves have been cleaned from refuge areas such as parks or urban forests. Our results indicate that the removal of leaves is a feasible first aid control measure to reduce moth population densities. However, leaf removal may not reduce moth densities when done improperly. In places where proper leaf removal is not feasible, other control measures are needed.     

Recruitment of native parasitoids by an exotic leaf miner, Cameraria ohridella: host-parasitoid synchronization and influence of the environment

Abstract

• 1 The horse‐chestnut leaf miner, Cameraria ohridella, is a moth of unknown origin that has recently invaded Europe and severely defoliates the European horse‐chestnut, an important ornamental tree.
• 2 Several indigenous parasitoids have colonized this new host, but parasitism remains low. One of the hypotheses suggested to explain the low parasitism is that candidate parasitoids emerge too early in spring to attack the first host generation and, thus, need early‐occurring leaf miners as alternate hosts. This hypothesis was tested by observing the synchronization between the phenology of the moth and that of its main parasitoids, and by comparing parasitism rates and parasitoid richness in different environments with various levels of biological diversity.
• 3 In spring, the bulk of the parasitoids emerge at least 5 weeks before the occurrence of the first suitable larvae of C. ohridella whereas most parasitoid adults reared outdoors die within 5 weeks after emergence.
• 4 Parasitism rates and parasitoid richness do not increase with biological diversity, suggesting that most parasitoids attacking the first generation of C. ohridella do not come from alternate hosts. Parasitism does not increase later in the year in the subsequent generations, when host‐parasitoid synchronization becomes less critical.
• 5 We conclude that, although the spring emergence of parasitoids is not synchronized with the phenology of C. ohridella, the parasitoids attacking the first generation are probably old or late‐emerging adults of the overwintering generation. The lack of synchronization is probably not the only reason for the poor recruitment of native parasitoids by C. ohridella.

     

A 3 year field survey of the natural enemies of the horse-chestnut leaf miner <Emphasis Type="Italic">Cameraria ohridella</Emphasis> in Lombardy,Italy

A survey to inventory the natural enemies of the horse-chestnut leafmining moth, Cameraria ohridella Deschka and Dimic (Lepidoptera, Gracillariidae) and to evaluate their relative abundance, was conducted from 2001 through 2003 in Lombardy, northern Italy. Two predators and 10 parasitoids were found during the 3-year survey. An increasing number of parasitoid species associated with the leafminer was detected (4 species in 2001, 5 in the 2002 and 10 in 2003). Eulophidae (Hymenoptera: Chalcidoidea) were most represented: Minotetrastichus frontalis Nees, Closterocerus trifasciatus Westwood, Pnigalio pectinicornis L., Pnigalio agraules Walker, Pediobius saulius Walker, Chrysocharis pentheus Walker, Cirrospilus talitzkii Bouek, Sympiesis sericeicornis Nees, Baryscapus nigroviolaceus (Nees). Only one Ichneumonidae (Hymenoptera: Ichneumonoidea) was found during 2003: Itoplectis alternans (Grav.).     

The effects of host plant phenology on the demography and population dynamics of the leaf-mining moth, Cameraria hamadryadella (Lepidoptera: Gracillariidae)

Abstract.

• 1 We examined the effects of variation in the timing of spring leaf production and autumn leaf fall on the survival, mortality and abundance of Cameraria hamadryadella on Quercus alba and Q.macrocarpa.
• 2 We monitored and manipulated the timing of foliation on field and potted Q.alba trees and observed the abundance of C.hamadryadella on those trees. We also monitored and manipulated the timing of leaf fall on Q.alba and Q.macrocarpa trees in the field and observed its effects on survival, mortality and abundance of C.hamadryadella.
• 3 Variation in the timing of spring leaf production has no effect on C. hamudryadella abundance. However, a warm winter and spring in 1991 led to accelerated development and the imposition of a facultative third generation in one out of ten years of observation.
• 4 In 1989, leaves fell relatively early and leaf fall in the autumn accounted for more than 50% of the mortality of C.hamudryadella. in 1990 and 1991 leaves fell relatively late and leaf fall induced mortality was substantially reduced and overwinter survival was markedly increased.
• 5 The abundance of C.hamadryadella remained constant in the spring and summer of 1990 following the previous autumn's relatively early leaf fall, but increased by 10-fold in the spring of 1991 following the relatively late leaf fall of autumn 1990. The abundance of C.hamadryadella also increased 4-fold between the summer of 1991 and the spring of 1992 after another autumn of relatively late leaf fall. We attribute these increases in abundance in part to reduced mortality because of later leaf fall.
• 6 Variation in the timing of autumn leaf fall may be responsible for initiating outbreaks of C.hamadryadella.

     

A brief note on sexual differences in pupae of the horse-chestnut leaf miner,Cameraria ohridellaDeschka & Dimic (1986) (Lep., Gracillariidae), a new pest in Central Europe on Aesculus hippocastanum

The horse-chestnut leaf miner, Cameraria ohridella Deschka & Dimic (1986) , is a new pest of horse-chestnut in Central and Eastern Europe. To combat it, for example, by pheromones, experiments which require a differentiation of sexes are necessary. Here an easy method to distinguish the sexes by morphological characters of the pupae is described.     

Native parasitoids and their potential to control the invasive leafminer, Cameraria ohridella DESCH. & DIM. (Lep.: Gracillariidae)

In spite of the fact that since the end of the eighties, the horse chestnut leafminer, Cameraria ohridella, has established itself throughout Europe, native predators such as ants and birds are not attuned to this neozoic species. In contrast, several parasitic wasp species already started to exploit the invasive horse chestnut leafminer, but until now parasitation rates are quite low, mainly because of asynchrony in the lifecycles of parasitoids and host. Only the removal of leaf litter, in which pupae hibernate, is at the moment a strategy to reduce the infestation level in the next year. Unfortunately, not only hibernating horse chestnut leafminers but also parasitoids are removed, and important resources for biocontrol are unused. In the current study, we investigated the potential efficiency of the horse chestnut leafminer parasitoid complex extracted from leaf litter in defined environments. Parasitoids were released at different densities to investigate density dependence in parasitation rates.Although seven different species were released in our experiments, only Pnigalio agraules turned out to be responsible for biocontrol of C. ohridella. We recorded parasitation rates of up to 35%. Overall, parasitation rates were independent of the leafminer density but increased fourfold if ten times more parasitoid individuals were released. Unfortunately, none of the parasitoid species could be established in the experimental units in the long run. Results are compared to other parasitoid-leafminer systems, and promotion of horse chestnut leafminer parasitoids to support natural selection and biological control of the horse chestnut leafminer is discussed.     

Mitochondrial haplotypes and the New Zealand origin of clonal European Potamopyrgus, an invasive aquatic snail

The small aquatic snail Potamopyrgus antipodarum is an important invading species in Europe, Australia and North America. European populations are generally believed to derive from accidental introductions from New Zealand, probably dating back to the mid-19th century. We have employed mitochondrial DNA sequences to test the proposed New Zealand origin of European Potamopyrgus, and to learn more about its genealogical history. Using a 481-bp region of the 16S ribosomal RNA gene, we identified 17 distinct haplotypes among 65 snails from New Zealand. In marked contrast, only two haplotypes were found across all European samples, which cover a large geographical area. Importantly, these two haplotypes are shared with snails from the North Island of New Zealand. Due to sampling limitations we cannot rule out a South Island origin for one of the haplotypes, but our results clearly demonstrate the New Zealand origin of European populations. The marked divergence among the two European haplotypes implies the successful colonization by two distinct mitochondrial lineages, which is consistent with previous data based on nuclear markers.     

Isolation and characterization of twelve polymorphic microsatellite markers for the invasive ascidian Styela clava (Tunicata)

The invasive solitary ascidian Styela clava has spread extensively along European coasts since its first occurrence in the early 1950s. In order to characterize spatial and temporal patterns of genetic change during its establishment and subsequent spread, we developed 12 species‐specific loci from an enriched microsatellite library. Polymorphism was explored in one native and two introduced populations (N = 31 ± 1). Number of alleles per locus varied from two to 13 (mean = 7). The average expected heterozygosity within populations ranged from 0.539 to 0.580.     

Population genetic structure of Chilo suppressalis (Walker) (Lepidoptera: Crambidae): strong subdivision in China inferred from microsatellite markers and mtDNA gene sequences

Chilo suppressalis (Walker) displays significant geographical differences in ecological preference that may be congruent with patterns of molecular variation. To test this, we collected and analysed 381 individuals of this species from cultivated rice at 18 localities in China during the rice-growing season of 2005–2006. We used four microsatellite DNA markers and four mitochondrial DNA gene fragments. We found that this species is highly differentiated, coupled with an estimated population expansion date of at least 60 000  bp . Phylogenetic analyses, Bayesian clustering, and phylogeographical analyses of statistical parsimony haplotype network consistently divided the populations into three clades: a central China (CC) clade, a northern plus northeastern China (NN) clade and a southwestern China (SW) clade. Analysis of molecular variance indicated a high level of geographical differentiation at different hierarchical levels [ F _ST for microsatellite markers, COI, COII, 16S and ND1 is 0.06004 ( P  < 0.0001), 0.27607 ( P  < 0.0001), 0.22949 ( P  < 0.0001), 0.19485 ( P  < 0.0001) and 0.29285 ( P  < 0.0001), respectively]. Isolation by distance appeared among the samples from within China ( r  = 0.404, P  = 0.0002); N_em values estimated using a coalescent-based method were small (< 2 migrants per generation), suggesting that the observed levels of differentiation are a result of migration–drift equilibrium. Our results imply that the genetic differentiation of this borer, which is approximately in accordance with its observed number of generations per year in different Chinese geographical regions, is probably attributed to climatic and/or geological events (e.g. the last glacial maximum) and subsequently strengthened by the domestication of rice.     

Mitochondrial DNA genome evidence for the existence of a third divergent lineage in the western Atlantic Ocean for the bull shark (Carcharhinus leucas)

We report for the first time a highly divergent lineage in the Caribbean Sea for the bull shark (Carcharhinus leucas) based on the analysis of 51 mitochondrial DNA genomes of individuals collected in the western North Atlantic. When comparing the mtDNA control region obtained from the mitogenomes to sequences reported previously for Brazil, the Caribbean lineage remained highly divergent. These results support the existence of a discrete population in Central America due to a phylogeographic break separating the Caribbean Sea from the western North Atlantic, Gulf of Mexico and South America.     

Characterization of eight polymorphic microsatellite DNA markers for the greenside darter,Etheostoma blennioides (Percidae)

The greenside darter, Etheostoma blennioides is a small benthic fish found in fast‐flowing streams in eastern North America. In Canada, this species is native to three, and introduced into one, Great Lakes tributaries in southwestern Ontario. It is currently listed as a species of Special Concern. To characterize population genetic structure and diversity in the Canadian populations of greenside darter, eight polymorphic microsatellite markers were developed for the species. The polymerase chain reaction primers were tested between 32 and 60 individuals from the Sydenham River and yielded a high number of alleles (four to 42 per locus), and observed heterozygosities ranging from 0.14 to 0.82.     

Characteristics of 13 polymorphic microsatellite markers in the corn earworm,Helicoverpa zea (Lepidoptera: Noctuidae)

Thirteen polymorphic microsatellite loci suitable for population genetic studies of Helicoverpa zea were discovered by screening partial genomic libraries enriched for microsatellite sequences. Insects collected (N = 96) in Stoneville, Mississippi were used to characterize these markers. The observed and effective number of alleles per locus ranged from two to nine (average 4.46) and from 1.07 to 2.45 (average 1.81), respectively. Fisher exact tests detected significant deviations from Hardy–Weinberg equilibrium at three loci, probably due to inbreeding, null alleles, or Wahlund's effect. Significant genotypic disequilibrium was not detected between any pair of loci.     

Postglacial recolonization patterns and genetic relationships among whitefish (Coregonus sp.) populations in Denmark, inferred from mitochondrial DNA and microsatellite markers

The genetic relationships among morphologically and geographically divergent populations of whitefish (genus: Coregonus ) from Denmark and the Baltic Sea region were studied by analysis of microsatellites and polymerase chain reaction–restriction fragment length polymorphism (PCR–RFLP) analysis of mitochondrial DNA (mtDNA) segments. The endangered North Sea houting (classified as C. oxyrhynchus ) differs morphologically and physiologically from other Danish whitefish ( C. lavaretus ). However, limited divergence of North Sea houting was observed both at the level of mtDNA and microsatellites. The implications of these results for the conservation status of North Sea houting are discussed in the light of current definitions of evolutionary significant units. Both mtDNA and microsatellite data indicated that postglacial recolonization by C. lavaretus in Denmark was less likely to have taken place from the Baltic Sea. Instead, the data suggested a recent common origin of all Danish whitefish populations, including North Sea houting, probably by recolonization via the postglacial Elbe River system. Estimates of genetic differentiation among populations based on mtDNA and microsatellites were qualitatively different. In addition, for both classes of markers analyses of genetic differentiation yielded different results, depending on whether molecular distances between alleles or haplotypes were included.