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Carabids as biocontrol agents for slugs in Oregon and Ireland - a novel and interdisciplinary approach to determine key malacophagous species and beneficial management options

Periodic Reporting for period 2 - CaraSlug (Carabids as biocontrol agents for slugs in Oregon and Ireland - a novel and interdisciplinary approach to determine key malacophagous species and beneficial management options)

Reporting period: 2019-10-01 to 2020-09-30

Agricultural pests such as slugs are major contributors to financial losses in the farming sector and the use of pesticides, which is currently the mainstay of control, is an unsustainable management option due to their toxicity to non-target species and consumers but also because of their cost and variable efficacy. Pest control using natural enemies poses a more sustainable approach. Ground beetles are the most significant natural enemies of slugs in agroecosystems and several studies have shown that slugs are a major part of their diet. Most ground beetle species do not exclusively eat slugs, and the presence of alternative food sources is crucial for their survival during slug-free periods. However, alternative prey can also impact the consumption rate of the target pest species by being preferentially consumed. Additionally, factors such as field margin composition / structure or production practices such as tillage can have a substantial influence on the abundance and community composition of farmland invertebrates.
This project aims to assess the potential of ground beetles as biocontrol agents against slugs and other pests in grass seed fields in western Oregon and grass fields in Ireland and to evaluate the influence of tillage and/or field margin structure on the abundance of slugs and other agricultural pests, ground beetles and invertebrates which could serve as possible alternative prey for the beetles. The control potential is evaluated by examining the abundance of ground beetles in agricultural fields at times when slugs cause most damage and through molecular gut-content analysis using high-throughput sequencing (to assess the breadth of their diet) as well as real-time PCR (to screen specifically for slugs, caterpillars and crane fly larvae and to get an approximate quantification of their predation).
This research generates valuable knowledge aimed at working towards a more environmentally friendly approach to farming by reducing the use of toxic molluscicides.
WORK PERFORMED

1. Field study
10 grass fields, primarily used as pasture, were sampled bi-weekly between 28.1. & 10.3.2020. 5 fields were classified as extensively, 5 as intensively managed. 2 parallel rows with 5 trapping points each were set up in every field. Each trapping point consisted of a refuge trap for slugs and 2 pitfall traps - 1 dry trap for carabids for subsequent molecular gut content analysis (MGCA) and 1 trap filled with a vinegar/salt solution to increase the number of carabids and alternative invertebrate prey (AIP)
2,122 carabid beetles were trapped, 92% were Nebria brevicollis (NB; 98% larvae) and 5% Pterostichus rhaeticus (all adults). Slugs were abundant throughout the study, with 60% Deroceras reticulatum (DR) and 35% Arion spp. No slug eggs were found.
Due to the short study time and the early season, only few species of carabid beetles were captured, and it was not possible to establish a temporal overlap between different carabid beetles species and slugs. However, the number of captured specimens was large enough to show a clear spatial overlap of NB, slugs and cranefly larvae with a preference for relatively dry and intensively managed fields, while P. rhaeticus was exclusively found in the extensively managed and very wet fields.

2. MGCA
DNA was extracted from 60 digestive tracts of NB larvae and submitted for Next Generation Sequencing (NGS). While the full analysis is still outstanding, results showed no slugs were consumed but diet included earthworms, Collembola and Diptera.

RESULTS AND EXPLOITATION/DISSEMINATION

Oregon:
- 2,442 carabids belonging to 36 species were collected; the most abundant species were NB (36.0%), Calosoma cancellatum (13.3%), Loricera foveata (9.0%), Poecilus laetulus (8.8%) and Agonum muelleri (6%).
- 1,840 slugs were collected, mainly DR (86.3%), in addition, a total of 1,026 cranefly larvae and 675 caterpillars (Lepidoptera) were also collected; the most abundant caterpillar was Noctua pronuba (89%).
- Using qPCR, slug DNA was detected in 9.3% of NB 4% of P. laetulus and 2% of A. muelleri. Lepidoptera DNA was detected in 7.5% of NB, 2.3% of A. muelleri and 5.9% of C. cancellatum. Cranefly DNA was detected from 6.1% NB, 6.7% A. muelleri and 4.9% P. laetulus.
- Of the common carabid species captured in this study, only NB was active in large numbers in the fields during the critical period after crop planting (Sept.-Oct.) when cutworms as well as cranefly larvae start emerging from their eggs. It was also the only species positively associated with a vegetation margin. None of the common carabid species nor the pests showed a significant response to tillage.
- Feeding trials showed that starved NB did not consume live juvenile slugs or their eggs but predation on live cutworms and cranefly larvae has been observed outside feeding experiments.
The above findings have been published in Insects (10.3390/insects11110722)

- 16,913 individuals of AIP were caught and identified to order level or higher. 20 different orders were identified, the most speciose were Coleoptera, Diptera and Araneae.
- The NGS results showed that the main AIP consumed by NB were earthworms, Collembola and Diptera, with spiders, ants and aphids consumed occasionally. P. laetulus fed mainly on Collembola, aphids and lady beetles and A. muelleri and C. cancellatum on Diptera.
These results will be published in the near future.

A peer-reviewed key to the common ground beetles of grass seed crops in the Willamette Valley has been published through the Oregon State University Extension system.

The results from the 'Irish study' will result in the publication of 2 research papers, 1 about the diet of NB larvae and 1 about invertebrate communities in extensively vs intensively managed fields.

Results from the Oregon study were presented at the Entomological Society of America Conference ‘Entomology 2019’ and the results of the Irish study will be presented at the International Congress of Entomology in Helsinki in 2022. To raise awareness of the importance of conservation biological control, I have created a blog entry about carabid beetles plus a manual on how to create a ground beetle habitat for the garden on the NUIG Ryan Institute blog ‘Nature at home’ and organized a webinar on beneficial Irish invertebrates, a recording of which has been uploaded to Youtube.
There has been very little research on carabid larvae and this project contributes to furthering our knowledge on their ecology and dietary preferences. The expected results will provide data about the preferred food choices of the larvae and will correlate their abundance with farm management intensity and other habitat factors. The project will also generate data on other invertebrate communities which will show how farming intensity influences species diversity. This is particularly important in the light of global decline in insect numbers and species in agroecosystems. The project outcomes will provide guidance for farmers who are interested in biological control and enhancement of habitat for beneficial insects. The project promotes the integration of biological control into farming rather than relying on toxic and costly pesticide applications will not only lead to a more sustainable way of farming, but will also be more cost effective in the long term and provide for a heathier environment and cleaner food for the consumer.
Calosoma cancellatum and spiders caught in pitfall trap
Dissected Nebria brevicollis
Nebria brevicollis feeding on slug in feeding trial