Controlling the European earwig on the Falklands

Contributed by Norbert Maczey, CABI

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The European earwig, Forficula auricularia (Photo: Norbert Maczey, CABI)

The European Earwig, Forficula auricularia (order Dermaptera) was recently introduced to the Falkland Islands and has since become locally common in Port Stanley and a number of settlements in both East and West Falkland. Since its introduction this invasive species has caused considerable problems ranging from yield losses in horticulture to health and safety issues (eg. hiding behind rubber seals in oxygen masks or in asthma inhalers) and threats to the indigenous ecosystems. There are now worrying observations of earwigs away from settlements indicating a considerable threat to the composition of native invertebrate communities. The exact date when earwigs were first introduced is unknown but early records stem from as far back as 1997. Earwigs have become a real nuisance pest since the mid-2000s.

Earwigs have turned into a pest species on the Falklands largely because they arrived without their natural enemies which control their numbers in Europe. Classical biological control, which involves the deliberate release of specialist natural enemies from the invasive specie’s native range, offers an economical and sustainable way to address such imbalances. There was consensus during a workshop in Port Stanley in March 2012, on the feasibility of biological control of invasive non-native species on the South Atlantic UK Overseas Territories, that the European earwig is a well suited target for classical biological control on the Falklands. Experts working on invasive species on the Islands and also members of the general public saw an urgent need for a sustainable control of this species. Equally, the government of South Georgia is very supportive of any control efforts which would reduce the risk of future introductions of earwigs to South Georgia.

The Falklands Island Government therefore decided to commission a host range testing programme to assess the safety and suitability of two parasitoid flies believed to be host specific to the European earwig to introduce to the Falklands. The aim of this programme is to reduce the number of earwigs to a level where they cause very little damage. Numbers of the control agents themselves are inextricably linked to the population levels of earwigs, as they are their only host. As soon as earwig densities fall so will the population of controlling flies until equilibrium is reached. We expect this level to be below a threshold where earwigs are perceived as pests. In this context it is important to point out that biological control will not lead to the complete eradication of the targeted species.

The European earwig F. auricularia is a particularly promising target species for classical biological control on the Falkland Islands; by using parasitoid tachinid flies from Europe which have already been used for controlling earwigs in other parts of the world. One of these species Triarthria setipennis has established successfully in Newfoundland and British Columbia (Morris 1984). Studies on the establishment of T. setipennis in Newfoundland indicated a considerable reduction in earwig numbers, which was most likely due to high levels of parasitism in the mid-1970s (Morris 1984). However, since 1978, no further evaluation of parasitoid impact has been undertaken.

A second species of parasitoid, Ocytata pallipes, was introduced into Canada to control the European earwig during the 1990s but no monitoring took place and establishment is unknown (Kuhlman et al. 2001). Ocytata pallipes and T. setipennis have also been introduced into the USA and New Zealand. Introductions into the USA were done as early as the 1920s (Oregon) (Kuhlman et al. 2001). Again, little is known about the success of these releases.

Although both fly species belong to the same family of parasitoid flies (Tachinidae) and look superficially very similar, they have developed different strategies to find and attack their host. Earwigs use a species specific pheromone to attract other earwigs and which during the summer season leads to sometimes large congregations in their preferred hiding places such as cracks behind tree bark or gaps inside of compost heaps. Not only do the earwigs themselves smell of this particular pheromone but after a while the material and food items they frequently come in contact with do also.

Both fly species use this typical ‘earwig smell’ to find their host. Ocytata pallipes attaches microscopically small eggs to plant material, on which earwigs frequently feeding from, and which subsequently has acquired their smell. The next time earwigs continue feeding they inadvertently ingest some of these eggs and tiny larvae hatch inside them. These then continue to develop until a fully grown maggot emerges from the earwig roughly a month later killing its host in the process.

The second fly species, T. setipennis, has developed a somewhat different strategy and places a small, already hatched larvae onto various substrates that smell of earwig. As soon as earwigs approach, the fly larvae actively crawl onto them and start to bore into their bodies, preferably in between individual segments where the chitinuous exoskeleton is weakest. Again, the larvae then develop inside their doomed host and hatch immediately before they pupate.

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Newly hatched adult Triarthria setipennis

Although it was always assumed that both fly species can only develop in earwigs, scientific evidence for this was still lacking and this knowledge gap needed to be closed before a decision was made needed to release these agents on the Falkland Islands. No native earwig species inhabits the Falkland Islands, therefore tests were conducted during 2013 and 2014 checking whether the control agents can only develop inside earwigs. These tests were conducted on insects species (crickets and cockroaches) representing insects orders which are either closely related to earwigs or are present on the Falklands; there is one native species of cricket, the camel cricket (Parudenus falklandicus).

The results of these tests showed that there was no indication that either of the two assessed fly species (Ocytata pallipes and Triarthria setipennis) can develop or otherwise impact on the vitality of any of the test species. This was even the case when crickets and cockroaches were artificially forced to ingest parasitoid eggs or inoculated with fly larvae, which would rarely happen under natural conditions. We have also considered other potential side effects such as the introduced flies becoming an additional food source to native predators like beetles or birds or whether the flies could impact on seed setting of flowering plants, both native and invasive, by pollination. There is currently no evidence that any of the theoretically possible side effects are likely to materialize.

Altogether, these assessments confirmed our opinion that there would be no risks to non-target species if one or both of these highly earwig-specific tachinid fly species were released on the Falkland Islands. From a biosecurity point of view both fly species can now safely be recommended for release in Stanley the area of infestation on the Falklands. Testing of more closely related earwig species was not conducted, therefore both species cannot at this stage be recommended for release in countries with native Dermaptera, such as Chile and Argentina, despite earwigs causing similar problems in this geographical region.


Because this would be the first introduction of non-native species for the control of an invasive species on the Falkland Islands, a certain level of concern about biological control from residents, both expert stakeholders and the general public, was anticipated. Up until now acceptance for the introduction of a new species to the Falkland Islands had not been assessed and the Environmental Planning Department of the Falkland Island Government on behalf of the Environmental Committee decided to conduct a range of awareness raising activities to encourage residents to voice their concerns and engage in open discussion on the safety and scope of biocontrol of earwigs. CABI was commissioned to conduct these activities through public consultations and direct discussions with stakeholders during March 2015.

At the core of all consultations with stakeholders were two major assumptions:

  • The release of the control agents is safe and does not pose any risks for native species, human health or food production; in contrast to the current use of large quantities of a highly toxic pesticide.
  • Although we currently see no major hurdles for a successful establishment of both fly species, establishment can never be guaranteed and this can be a reason for failure. Equally, if successful establishment has taken place, the amount of control exerted by the released agents is difficult to predict. However, we believe the likelihood for a good control is high, particularly in the absence of hyperparasitoids, in this case tiny wasps, which in Europe develop inside the fly pupae, which could impact negatively on the fly populations.

The general feedback most people gave either in public discussions or during separate stakeholder meetings was that of cautious optimism and being in favour for biological control provided it is safe. It was also important for most people to have the assurance that biological control and does not lead to the introduction of a species, which could become problematic by itself. We believed that through in depth discussions regarding the ecology of both control agents and the earwigs themselves generally, any worries and concerns could be dispelled. There was never an opposition to this programme voiced openly.

In summary, people are prepared and willing to trial a release of both control agents hoping that this will provide the anticipated long term solution to the earwig problem, whilst being fully aware that there remains a certain risk of failure (flies either not establishing or not exerting the desired degree of control). One of the main drivers for an altogether wide support of the biological control project is currently not so much an unbearable level of earwigs but more increasing concerns in part of the society about risks and side-effects associated with the current use of a highly toxic pesticide. Most people are also aware that spraying is a short term solution with a need for indefinite continuation. Within the scientific community, concerns are widespread regarding the prospect of earwigs becoming increasingly resistant against Demand® CS the pesticide currently widely in use for their control in Stanley.

As a result from very positive feedback during the extensive stakeholder consultations both parasitoids have now been licensed by the Falkland Island Government for release and trials to establish the control agents inside Stanley, the area with the highest levels of earwig infestation, are currently underway. The release trials currently, which are currently underway, are funded by the Darwin Initiative and we hope to be able to report on initial results very soon.

References

Kuhlmann, U., Sarazin, M. J., O’Hara, J. E., Mason, P. G., Huber, J. T. (2001) Forficula auricularia L., European earwig (Dermaptera: Forficulidae). – Biological Control Programmes in Canada, 1981-2000, CABI Publishing, Wallingford, UK: 127-131.

Morris, R.F. (1984) Forficula auricularia, European earwig (Dermaptera: Forficulidae). In: Kelleher, J.S. & Hulme, M.A. (eds.) Biological Control Programmes Against Insects and Weeds in Canada 1969–1980. – Commonwealth Agriculture Bureaux, Slough, UK: 39–40.

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