QLK5-2000-01684 CONTROCAM: Sustainable control of the horse chestnut leafminer, Cameraria ohridella (Lepidoptera, Gracillariidae), a new invasive pest of Aesculus hippocastanum in Europe

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Contract No:	QLK5-2000-01684
Source:	Progress Report - January 2002

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Progress Report - January 2002

Introduction

This is a multidisciplinary project aimed at the control of the horse- chestnut leaf miner, Cameraria ohridella, a moth of unknown origin, which has spread recently over vast areas of Europe, It is proposed to assess the past present and potential future impact of the moth on horse-chestnut trees, both in urban areas in Europe and in natural forests in the Balkans. Then, it will investigate the potential of various environmentally safe and sustainable control measures, including pheromone-based monitoring and control, biological control and cultural methods. The epidemiology and dispersal of the moth will be studied. Finally, the project aims to develop and disseminate the basic structures of an IPM system for C. ohridella that will be applicable across all affected areas of Europe and to use the project as a generic model for developing sound approaches to the study and control of exotic pests in Europe.

Activities

At the start of the project, both field and laboratory experiments have been carried out, surveys were made, host-screening and preference tests were conducted, based on the participants experiences in the area of biological control. Special attention was focussed on plant physiological responses to the damage caused by the leaf miner. Experiments were designed to prove a possible negative impact of the pest on the host tree and seed production. The rates of parasitism were assessed by extensive dissections of mines across Europe. The reliability of pheromone traps in the field was determined by comparing the dynamics of the catches in the field with the seasonal phenology indicated by observations and the results of regular dissections of horse-chestnut leaves.

Achievements

Since it was first reported Macedonia in 1985, this pest has spread fast throughout Central and Eastern Europe. At present it occurs in France as far west as in the region around Paris. In the North it is close to the Danish border. Although, some 15 species of parasitoids (mainly chalcids) feed on larvae and pupae of C. ohridella in Europe, rates of parasitism are still low at most locations investigated in 2001 - usually not exceeding more than 3 to10%. However, in some stands in Eastern Europe rates of parasitism are higher. It is proposed to investigate further possible indications of adaptation of these parasitoids to this new invasive pest. Predators, mainly tits, can contribute to the mortality of this pest with a similar impact to that inflicted by parasitoids. Pheromone traps are a suitable tool for detecting and monitoring the horse-chestnut leaf miner. However, for mass trapping purposes trap types others than sticky delta traps have to be developed. Although, C. ohridella has an effect on plant fitness, eg, on the number of produced fruits and seed size, no indications could be found that trees suffer a dieback risk.

Objectives

This report covers progress during the first year. The main objectives of the project for this period were:

• Set up of field experiments for impact studies of Cameraria ohridella on Aesculus hippocastanum.
• First tests regarding the host range of C. ohridella and the potential expansion of the host range.
• The improvement of the pheromone synthesis and pheromone trap design and test of different trap methods for monitoring and for biological control by mass trapping.
• Evaluation of the natural enemy complex of the moth in Europe at natural horse-chestnut locations, in plantations and in towns
• Location of the origin of C ohridella.
• Improvement in present cultural methods by optimising mechanical methods to reduce population densities of C. ohridella.
• Study of the distribution and dispersal of C. ohridella in France, Greece and North Germany.

Progress

The horse-chestnut leaf miner Cameratia ohridella (Gracillariidae, Lepidoptera) was first observed at locations near Lake Ohrid in 1985 during an outbreak on horse-chestnut tress (Aesculus hippocastanum). From then on the moth spread with incredible speed throughout many parts of Central, Western, and Eastern Europe. At the present, mines on horse-chestnut trees can be observed as far north as Kiel (North Germany) close to the Danish Border and in the west up to the region around Paris. Thus, it is only a question of time before the leaf miner also enters both England and Denmark. After establishment, population densities increase within two to three years. As a result, leaves of horse-chestnut trees are destroyed by the feeding activity of the mining larvae in summer, dry out and the discoloured brown leaves are lost by abscision in early summer.

Up to now it was not known whether C. ohridella might have a negative impact on its main host plant Aesculus hippocastanum. Recent investigations in Switzerland, Germany and Italy suggest that high infestation levels lead to reduced numbers of fruits and numbers of seeds per fruit. However, the plant physiological investigations of the Italian research team suggests that there is no direct risk of tree dieback.

Reasons for the high population densities can be explained by a lack of effective natural enemies. The project studied mortality rates caused by parasitoids for the first time on a European scale in a comparable way by using standardised protocols. This will enable any adaptation of these natural enemies to this new invasive pest that may occur in future to be detected. In total, about 14 species of parasitiods feeding on pupae and larvae of C. ohridella have been recorded. The parasitoid complex is dominated by two chalcid species, Minotetrastischus frontalis and Pnigalio agraules. In general, rates of parasitism are low, usually not above 5-10%. Interestingly some exceptions were found in the Balkans where Pedibius saulius is the most abandoned species and caused up to 20% mortality in pupae. This species is also known from Central and Western Europe but was reared there only in a few cases. In future studies, investigations will aim to show if this anomaly is due to the presence of a sibling species in the Balkans, to a genetic adaptation of the species to this new host, or to ecological/climatic incompatibilities. In addition it was found that, to a certain extent, predators, such as tits can contribute to the observed rates of mortality.

During the first reporting period a basis for future live table studies was established. The necessary factors, such as winter mortality of leaf miner and its main parasitoids, fecundity, and host feeding behaviour of parasitoids were determined. With the necessary basic data determined, first life tables can be constructed at selected horse-chestnut sites in Switzerland and Bulgaria in 2002.

First pilot experiments were started to investigate the possibility of an augmentation and enhancement of natural enemies by offering nectar food sources to parasitoids. Another possibility in a biological control strategy based on natural enemies would be the introduction of natural enemies from the geographical region where C. ohridella originated. One aim of the project is to study possible biological control candidates and to give recommendations for future releases. Basing on screening experiments there were indications that the pest might originate from Asia where several indigenous horse-chestnut trees occur. However, during first surveys organised in Japan and China the leaf miner could not be detected, neither by visual inspection of chestnut stands nor by pheromone trapping. Surveys in Asia will be continued, but the project will also focus on other countries, such as the USA.

If the two additional partners from Bulgaria and Hungary which were included in the proposal of February 2002 are approved of, a phylogenetic and phylogeographic analysis both of C. ohridella and of closely related species will be included into the scientific programme in order to narrow down the possible origin of the horse-chestnut leaf miner.

Considering biological control methods other than by natural enem es, there are a number of alternatives, e.g. control by mass trapping using pheromone traps or confusion techniques. Another technique, often proposed in the literature, is the removal and destruction of autumn foliage. Delta traps, containing pheromone dispensers and sticky inserts, as are often used in monitoring programs, were not effective in mass trapping. Usually numbers of emerging moth are too high, thus the sticky inserts are totally covered by adults and have to be exchanged nearly every day. While not being very effective as a tool to protect large areas with several horse-chestnut trees, they might be effective at locations with single isolated trees where in addition most autumn foliage has been removed. Hence, hey could be a tool for gardeners. For other site conditions the trap design has to be changed or other methods, such as chemical confusion techniques, have to be used.

A new confusion method was investigated. In this method males are attracted to stations containing a powder impregnated with pheromone. This powder sticks to the males attracted by the pheromone and lead to increased number of male-by-male-matings. However, up to now the interval for which attracted males remain in these stations, and might come in contact with the pheromone-impregnated powder is too short and the design of these stations probably has to be modified.

Studies in the Czech Republic, in France, Greece, Austria and Germany in 2001indicated that delta sticky traps baited with pheromones are suitable tools for monitoring and dispersal studies. The number of males caught correlated with the infestation levels estimated by visual inspections. The catch rates, with associated temperature data, reflected the population dynamics of the horse-chestnut leaf miner (e.g. number of generations).

During the first reporting period pheromone synthesis was modified resulting in a higher yield of pheromone. An additional isomer was identified in this product, however, it does not seem to have an influence on the attractiveness of the pheromone and therefore the purity of the product is sufficient. In addition, the delta traps were optimised: the minimal pheromone dose for a short distance attraction was determined and the material of the dispensers was modified. The sizes of the openings of the traps were reduced resulting in a longer retention of the attracted males inside the traps.

Leaf removal experiments, performed in South Moravia in 2001, had only a slight effect in reducing the population densities in spring during the first generation of C. ohridella. Due to the fecundity of the moth, infestation levels increased again during summer and then they were comparable to untreated control trees.

Another aim of the project is the study of the distribution and dispersal of C. ohridella as a means of improving the understanding of dispersal mechanisms of new invading pests. Surveys were organised in France, Greece, and North Germany. Standard protocols were used for a better comparison of data and to establish a distribution model. Only 11% of questionnaires sent to institutes and administrations in charge of urban and forest tree protection and to associations of parks and gardeners were returned. The poor response from the public to volunteer for information regarding the occurrence of the moth via a web page was disappointing. Possible explanations are:

• Difficulties in identifying the damage of a leaf minner due to the relatively low infestation levels in France in 2001.
• People are not concerned about new invading species, especially when the damage caused by the pest is less obvious.

These investigations have to be continued in the next few years before final conclusions regarding the suitability of polls for an assessment of distribution of a pest insect can be drawn. However, similar investigations made in Germany in 1998 led to comparable results, even though the damage caused by the leaf miner was obvious at that time.

Conclusions

After the first reporting period it is too early to draw firm conclusions. However, the methods investigated above leads to one conclusion, or a major problem in combating the moth: there probably will be no 'alround solution' other than the establishment of a natural enemy complex. All other methods have their advantages and disadvantages:

• Reducing the number of emerging leaf miner adults in spring by leaf litter removal in autumn of the previous year might only be effective at locations where leaves can be removed completely without problems and where the distance to trees with high infestation levels is large enough that the immigration of fertilised females is rare or unlikely. In addition, removal of leave litter has to be repeated every year.
• Confusion techniques might not work at locations with few or only with a single tree due to the fact that a pheromone cloud surrounding the tree is not stable. On the other hand these kinds of methods might be successful in controlling the moth e.g. in nurseries where dozens or hundreds of horse-chestnut trees are planted.

Despite the fact that most of these methods will not protect treated trees completely, all attempts to reduce the damage at least during the first generation of the leaf miner seem useful due to the plant physiological investigations, performed so far. No correlation could be found between tree ring thickness and infestation levels. On the other hand data obtained so far by experiments performed in Italy, Switzerland, and Germany show that C. ohridella has an impact at least on some plant fitness parameters, i.e. the leaf gas exchange and particularly the photosynthesis is proportional to the amount of damaged leaf area. There are indications that attacked trees alocate less resources to reproduction, number of fruits and number of seeds per fruit respectively were reduced at increased defoliation levels. While a reduction of reproduction efforts might not be a direct problem in towns, it could be of consequence in the long term for natural horse-chestnut stands. Therefore, more extensive field experiments were set up in the Balkans to study the impact of C. ohridella on its host plant in such natural stands in more detail.

As an intermediate conclusion, it seems that A. hippocastanum trees are not under any risk of mortality at present, at least in stands such as those in north-eastern Italy where Cameraria only started to be a problem in 1996.