Possibilities to control the horse chestnut leaf miner (Cameraria ohridella) in urban environments

The continuous epidemic infestation of horse chestnuts by C. ohridella is a serious aesthetic and economic problem in urban environments. The aim of the presented studies was therefore to develop control measures which meet the special needs of urban plant protection. Removal of infested leaves in autumn and appropriate disposal of the leaflitter is the only practicable measure to reduce infestation levels so far. Another two methods, which are currently in an experimental stage, had an impact on the leafminer populations. A gelformulated combination of the C. ohridella-pheromone with a contact insecticide as attract-and-kill-system proved to be efficient at low population levels. Easily applicable and safe banks of systemic insecticides turned out to be effective in killing preimaginal stages of the leafminer on young trees. The complex of parasitoids attacking C. ohridella is comparable to that of other leafminers. Nevertheless, parasitism rates are very low at present and far away from having any controlling effect on the pest insect.     

Predator complex of the horse chestnut leafminer Cameraria ohridella: identification and impact assessment

Abstract:  The control of Cameraria ohridella Deschka and Dimic (Lepidoptera, Gracillariidae) by natural enemies in Europe is poor. Thus, in the past 15 years epidemic population densities of the moth regularly caused a continuous pre-mature defoliation of horse chestnut trees, Aesculus hippocastanum L.. Whereas several studies investigated the parasitoid complex of the leafminer and revealed its inefficiency, only little is known about the predators of C. ohridella . The aim of this study was to identify the predator complex in European horse chestnut stands by visual observations and exposure experiments and to assess the impact of different predator guilds on C. ohridella populations by an exclusion experiment. Of all potential predators observed, only blue tits, Parus caeruleus L., great tits, Parus major L., marsh tits, Parus palustris L. and the southern oak bushcricket Meconema meridionale (Costa) were found to prey on the pre-imaginal leafminer stages. Bushcrickets have to our knowledge never before been recorded preying on leafminers; moreover M. meridionale showed a measurable negative impact on the C. ohridella population. However, birds seem to be of greater importance as predators of C. ohridella than arthropods. Predation rates of birds ranged from 2 to 4% of leafminer populations, which is of the same order of magnitude as published parasitism rates. We conclude that the augmentation of tit densities, for example via nesting boxes in chestnut trees, should be considered as a component of an integrated control strategy against the horse chestnut leafminer.     

Sequential sampling plan for Cameraria ohridella (Lepidoptera: Gracillariidae) on horse chestnut tree

A fixed precision sequential sampling plan for estimating the density of the horse chestnut, Aesculus hippocastanum L., leafminer Cameraria ohridella Deschka & Dimic (Lepidoptera: Gracillariidae) was developed. Data were collected from 2002 to 2004 in Turin, northwestern Italy, with the aim of developing a sampling strategy for estimating populations of C. ohridella mines. Taylor's power law was used as a regression model. Sampling parameters were estimated from 216 data sets, and an additional 110 independent data sets were used to validate the fixed precision sequential sampling plan with resampling software. Covariance analysis indicated that there were not significant differences in the coefficient of Taylor's power law between heights of the foliage, months, and years. Dispersion patterns of C. ohridella were determined to be aggregated. The parameters of the Taylor's power law were used to calculate minimum sample sizes and sampling stop lines for different precision levels. Considering a mean density value of five mines per leaf, an average sample number of only 49 leaves was necessary to achieve a desired precision level of 0.25. As the precision level was increased to 0.10, the average sample size increased to 303 leaves. The sequential sampling plan should provide an effective management of C. ohridella in the urban areas, minimizing sampling time and cost, and at the same should be an effective tool to reduce insecticide applications and prevent the esthetic damage.     

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.     

Morphology of female sex pheromone gland in the horse chestnut leafminer Cameraria ohridella (Lep., Gracillariidae)

Abstract: The internal and external morphology of the female sex pheromone gland in Cameraria ohridella Deschka & Dimic, an European pest on Aesculus hippocastanum L., has been investigated by histological and electron microscopic techniques. The gland consists of a single layer of modified epidermal cells in the dorsal part of the intersegmental membrane between the eighth and ninth abdominal segments and laterally extends to the posterior apophyses. The epithelium contains large columnar- and cone-shaped cells with basally situated nuclei. The cuticle, which covers the glandular region, has a wrapped appearance and is divided into a hyalin and thickened endocuticle and a thin outer epicuticle: it considerably expands when the gland is protruded and provides a sufficiently large surface for evaporation of the pheromone. The cuticle does not show any orifices of pore channels. In the retracted position, the gland is folded within the body cavity of the seventh and eighth abdominal segments but is exposed to the environment by extension of the abdominal tip along with female calling. In virgin females, pheromone glands are well developed at least within the first days after eclosion; if copulation occurs, glandular epithelia degenerate soon. According to the current classification, the glandular type of C. ohridella most easily is consistent with eversible dorsal scent folds that are widely distributed amongst diverse taxa of Lepidoptera. However, this is the first report on the morphology of pheromone glands in the Gracillariidae.     

Occurrence and phenology of parasitic Chalcidoidea on the horse chestnut leafminer,Cameraria ohridella Deschka & Dimic (Lep., Gracillariidae)

Abstract: The parasitoid spectrum of the horse chestnut leafminer, Cameraria ohridella, was examined for its adaptation to a newly introduced host. A total of 15 parasitic species belonging to the suprafamilies Ichneumonoidea and Chalcidoidea was reared, all of them are polyphagous and common on other leafminers in Europe. The abundance of the moth and its natural enemies were studied during one entire vegetation period. The phenology of some of its major parasitic species was investigated and compared with the occurrence of the leafminer with special regard to its possible use in biological pest control.     

How to safely compost Cameraria ohridella-infested horse chestnut leaf litter on private compost heaps

Abstract:  The horse chestnut leafminer Cameraria ohridella (Lep., Gracillariidae) is a recently introduced invasive pest, which frequently defoliates horse chestnut trees Aesculus hippocastanum already in summer. Currently, the removal of leaf litter, in which pupae of C. ohridella hibernate, is the only effective short-term control measure available. In the past the deposition of pest-infested litter on private compost heaps was dissuaded because of the risk of leafminer emergence in the following spring. Thus, the aim of this study was to test safe ways to compost pest-infested horse chestnut leaf litter on small heaps so that private gardeners can save the costs for leaf disposal. The number of C. ohridella adults emerging from heaps covered with either soil or uninfested foliage was reduced by 96% compared with controls. Only single leafminers emerged from the top covered heaps and there was no difference between the use of soil or uninfested foliage as cover. However, cover did not prevent the emergence of a few leafminers over the sides of the heaps. Overall, we conclude that it is relatively safe to compost C. ohridella -infested A. hippocastanum leaf litter on small compost heaps provided they are properly covered.     

Monitoring and dispersal of the invading Gracillariidae Cameraria ohridella

The horse‐chestnut leafminer Cameraria ohridella is an invasive lepidoptera (Gracillariidae) which has spread throughout Europe over the last two decades. The species can be detected easily because of its noticeable damage to horse‐chestnut trees leaves, which allows the easy collection of large‐scale monitoring data. It is therefore an interesting model species to study invasion processes, and this review summarizes recent progresses in that regard. The date reported here result from monitoring of C. ohridella population based on pheromone traps and visual observation of damage carried out in Belgium, France, Germany, Switzerland and the UK. The patterns of spread measured using these different monitoring method, and the key factors associated with short‐distance dispersal (within cities), and large‐scale dispersal (across landscapes) are presented and discussed.     

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

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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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Cameraria ohridella: 10 years of sex pheromone and kairomone research

This review traces the identification and use of sex pheromone of the horse chestnut leafminer Cameraria ohridella Deschka et Dimi? (Lepidoptera, Gracillariidae, Lithocolletinae) to monitor population density and discuss its potential for direct control. Host volatiles endogenously emitted from Aesculus hippocastanum L. leaves are described in terms of their ability to control conspecific females.     

Laboratory rearing and biological parameters of the eulophid Pnigalio agraules,a parasitoid of Cameraria ohridella

Predators as well as parasitoids native to Europe accept the exotic horse chestnut leafminer, Cameraria ohridella Deschka and Dimi? 1986 (Lepidoptera: Gracillariidae), either as prey or as host. However, the influence of these antagonists on the populations of the pest insect is so far very low. Therefore, efforts to develop an integrated pest management system against C. ohridella should include methods which foster the natural enemy complex. In the present study we developed a laboratory rearing method and investigated several biological parameters of Pnigalio agraules (Walker 1839) (Hymenoptera: Eulophidae), a dominant species in the horse chestnut leafminer’s parasitoid complex in many European regions. This native parasitoid satisfies three basic requirements for successful use as a biocontrol agent with regard to C. ohridella. The parasitoid’s fecundity, longevity and the speed of juvenile development by far exceeds that of the leafminer, enabling the parasitoid population to increase faster than that of the host. Furthermore, our results show that the impact of this species on C. ohridella populations has been previously underestimated, because non‐reproductive killing (i.e. host‐feeding and host‐stinging) of the hosts, resulting in considerable larval mortality of the leafminer, has not been quantitatively assessed. However, naturally occurring parasitoid populations have negligible impact on C. ohridella populations. Further studies, including experimental releases of P. agraules, are necessary to understand the constraints limiting the parasitoid’s performance in the field and to assess the potential benefits of releases for the control of C. ohridella.     

Approaching risk assessment of complex disease development in horse chestnut trees: a chemical ecologist's perspective

Abstract The chemo‐ecological predispositions were investigated for the development of a complex disease on the basis of an insect–fungus mutualism using the system of horse chestnuts (Aesculus hippocastanum and Aesculus x carnea), the horse chestnut leaf miner (Cameraria ohridella) and the biotrophic powdery mildew (Erysiphe flexuosa). Both C. ohridella and E. flexuosa can appear on the same horse chestnut leaf tissue simultaneously. The olfactory detection of fungal infection by the insect, its ability to discriminate the potentially mutualistic fungus from other fungi and the impact of fungal infection on insect oviposition were examined. Gas chromatography coupled with mass spectroscopic and electroantennographic detection by C. ohridella (GC‐MS/EAD) was used to assess the olfactory detection of fungal‐infected A. hippocastanum and A. x carnea leaves by C. ohridella. Infection‐related compounds, such as benzyl alcohol, dodecane, tridecane and methyl salicylate as well as fungus‐related C8 compounds, are perceived by C. ohridella. The discrimination of E. flexuosa from another phytopathogenic fungus, such as Guignardia aesculi, is based primarily on the differing pattern of C8 compounds of these fungi. Oviposition on fungal‐infected leaves of A. hippocastanum and leaves treated with fungal‐related compounds showed that C. ohridella is able to respond to the modifications in the leaf volatile profiles of horse chestnuts caused by the different fungal infections. Thus, from the perception point of view, the necessary predispositions for the development of a close insect–fungus relation between the biotrophic fungus E. flexuosa and the leaf‐mining insect C. ohridella are fulfilled. However, decreased oviposition on infected leaves does not enhance the selective contact between the species. As a consequence, an important predisposition for forming an insect–fungus mutualism is not fulfilled by these two species and, according to this approach, the risk of forming a complex disease can be assessed as low.     

Evaluation of the community of native eulophid parasitoids on Cameraria ohridella Deschka and Dimic in urban areas

The parasitoid complex associated with the exotic leafminer Cameraria ohridella Deschka and Dimic (Lepidoptera: Gracillariidae), which attacks horse chestnut (Aesculus hippocastanum L.), was studied in the urban environment of Turin (northern Italy). The studies were carried out over 5 yr after the first detection of the pest in our region in 1999. To evaluate parasitism, 438,029 leaf mines were examined over the 5-yr period, of which 29,033 were found to be parasitized (6.6%). Also, ornamental broadleaf trees attacked by other native gracillariid leafminers and located in the proximity of the target horse chestnut trees were sampled. A total of 11 parasitoid species (Hymenoptera: Eulophidae) were recorded on C. ohridella, and the most common species were Minotetrastichus frontalis (Nees), Closterocerus trifasciatus Westwood, and Pnigalio agraules (Walker). The first species accounted for >77.5% of all parasitoids collected. Cirrospilus talitzkii Boucek was found for the first time in 2005. The high population level of the pest and the low parasitism rate show that the parasitoid complex is currently inadequate to contain C. ohridella populations effectively. The most frequent parasitoids of the moth were also found on the most common broadleaf trees in the studied area, showing how native leafminer parasitoid species are able to switch to other hosts. These results show that both native and broadleaf plants species may potentially provide an important reservoir of parasitic wasps to help protect a simple biotope, such as the urban environment, from pests.     

Within and between seasonal changes of detoxifying capabilities of Cameraria ohridella (Lepidoptera : Gracillariidae) larvae

Cameraria ohridella is an invasive leaf miner, a severe pest of horse chestnut trees. Chemical control needs recognition of intrinsic metabolic capability to cope with external stressing factors. Our tasks were to check annual effects of generation, and host tree age on detoxifying abilities of the last larval stage. Activities of CAT, SOD, GSTPX, GST CarE and AChE were assayed in the midgut of larvae from two localities during 3 years and three generations, annually. Activities of GSTPX and GST were high, but CAT and SOD were low in relation to other lepidopteran larvae. In general, the second generation larvae had the highest activity, indicating an effective defence against allelochemicals produced by the host. Effects of host tree age were significant for SOD, GSTPX and CarE activities. Significant annual differences on enzyme profiles of CAT, SOD, CarE and AChE reflected differences of temperature and precipitation between consecutive years.