Slug Density Research Articles

Can indigenous carabid beetles confer biotic resistance to invasion of native grassland remnants by alien slugs?

Invasive alien slugs alter plant community assemblages via selective herbivory, yet evidence of their impacts on indigenous plant species in threatened Australian native grasslands are scarce. Carabid beetles have been explored as biocontrol agents of pest slugs in agriculture, with native grassland habitat adjoining agricultural fields thought to be a potential source of beneficial carabid predators. The association between indigenous carabid beetles and invasive alien slugs within remnant native grassland vegetation patches, however, is unquantified. Studies were conducted to identify (i) The extent and determinants of slug invasion into remnant native grassland, and (ii) Whether carabid beetles can confer biotic resistance to native plant species in remnant grasslands. The interaction between alien slugs, carabid beetles and a rare grassland forb was explored in microcosm experiments to test the hypothesis that carabid beetles, by regulating slug herbivory, influence plant survival and growth. Slugs were found at 90% of the remnant native grasslands surveyed; carabid beetles were infrequent and their active abundance was low. There was little evidence that landscape or site-level parameters influence slug density, although species-specific patterns were apparent for the two most abundant slug species. This may reflect the importance of propagule pressure from source populations (the agricultural matrix) affecting slug invasion success, suggesting that native grassland habitat features are of secondary importance. The microcosm experiments provided evidence that slug herbivory may significantly impact the survival of a native forb at the seedling stage, but less so at a mature plant stage. We found that carabid beetles can markedly reduce the impact of alien slugs on plants, provided that encounters between the two taxa occurs. Interactions in the field are likely to be density-dependent such that beneficial impacts of carabid beetles are only apparent when their abundance is high. The high abundance of alien slugs in remnant grasslands, and lack of evidence for biotic resistance via carabid beetle predation, indicates that grassland conservation management may need to include slug baiting to mitigate the potential for negative impacts of slugs on seedling recruitment of threatened native plant species.

Barriers with Valve Mechanisms Are Predicted to Protect Crops from Slug Carriers of Rat Lungworm Disease.

Angiostrongyliasis (Rat Lungworm disease) is an emerging parasitic disease caused by the ingestion of gastropods infected with the neurotropic nematode Angiostrongylus cantonensis. The reduction of crop infestation with infected slug carriers may vary widely by protection method. We explored the application of barriers with valve mechanisms, whereby selective directional forces caused a greater number of slugs to exit than enter the protected plot, leading to decreased slug population densities at a steady state. Using field data, we constructed predictive models to estimate slug population densities at a steady state in protected plots with (1) no valve effect, (2) a valve effect, (3) no valve effect with a single breach of the barrier, (4) a valve effect with a single breach of the barrier, (5) a valve effect with a constant breach of the barrier, and (6) a repelling effect. For all scenarios, plots protected using a barrier with a valve effect had consistently lower slug densities at a steady state. Our findings support the use of barriers with valve mechanisms under different conditions, and potentially in combination with other interventions to reduce the contamination of crops by slug carriers of A. cantonensis. Improving barriers extends beyond disease mitigation to economic and cultural impacts on the local farmer and consumer communities.

Evaluation of the Effect of Elevation on Terrain-Induced Slugging in a Riser System

This work investigates the impact of elevation on terrain-induced slugging in a riser system. Fluid compositions were modeled with multiflash and exported to OLGA in a readable PVT Table File. Pipeline materials and properties with geometry of different elevation were modeled in OLGA to develop a flowline model. Two geometries (1 and 2) of different elevations were defined to establish their effect on accumulated liquid volume flow, surge liquid volume, slug length, number of slugs and slug density. The result shows that Geometry 1 with a total elevation(height)of 450 ft has a total liquid volume flow of 10.1058 ft3/s and Geometry 2 with a total elevation(height) of 300 ft has a total liquid volume flow of 0.197822 ft3/s for the same time of 2473.99 seconds. Also, Geometry 1 has a surge liquid volume of 543.4608484 ft3 at a drain rate of 0.20843960 ft3/s and Geometry 2 with a surge liquid volume of 210.7395 ft3 at a drain rate of 0.23892 ft3/s. There was no slug length and number of slugs in pipe of Geometry 1 while Geometry 2 has a slug length of 531.78 ft indicating severe slug in Geometry 2 system of different elevation. The slug density of Geometry 2 system was twice that of Geometry 1. Change in the elevation for the geometries has shown a significant effect on terrain–induced slugging in the pipe-riser system.

Evaluation of Slug Refuge Traps in a Soybean Reduced-Tillage Cover Crop System.

Simple SummarySlugs have become more frequent pests of field crops, including soybean. Monitoring slugs during the day is difficult because slugs are nocturnal, so trapping is often used to monitor populations. A variety of traps have been developed, though there are few direct comparisons of the different trap types. The objective of this study was to compare trapping efficiency of two types of slug refuge traps in soybeans. We tested a traditional shingle trap and a modified shingle trap with a water-filled pitfall trap beneath it. The modified shingle traps captured significantly more slugs than the traditional shingle trap, mainly due to the pitfall component (which was significantly cooler than the shingle component). As slug density decreased, this trend was most pronounced, suggesting that the modified shingle trap is a more sensitive sampling tool which may be useful in agronomic slug research.As more farmers adopt no- or reduced-tillage and/or cover crop land management practices, slugs have become more frequent pests of field crops, including soybean. Monitoring slugs visually is difficult because they are nocturnal, so several trapping methods have been developed, though comparisons of trap types are rare. The objective of this study was to compare trapping efficiency of two types of slug refuge traps in reduced-tillage soybeans following cover crop termination. We tested a traditional shingle trap and a modified shingle trap with a water-filled pitfall trap beneath it. Traps were deployed in 24 pairs in 2018 and 2019 in experimental soybean plots. We counted slug captures weekly over a 5-week time period each year. In 2018, we counted the total number of slugs under each trap type. In 2019, counts were categorized into specific trap components (shingle vs. in/on/under the pitfall). Temperature was also recorded in 2019. The modified shingle traps captured significantly more slugs than the traditional shingle traps, mainly due to the pitfall component. This trend was most pronounced as slug density decreased, suggesting that the modified shingle trap is a more sensitive sampling tool which may be particularly valuable when used for research purposes.

Stability of Patches of Higher Population Density within the Heterogenous Distribution of the Gray Field Slug Deroceras reticulatum in Arable Fields in the UK.

Simple SummaryThere is growing public and legislative pressure to reduce the use of pesticides in crop production. It is thought that the gray field slug is not uniformly distributed in arable fields, but that patches of higher slug densities occur, interspersed within areas of lower slug numbers. Targeting molluscicide applications only at these patches, leaving other areas untreated, would substantially reduce the amount of molluscicide used but relies on patches being a feature of slug populations in all susceptible crops, and always occurring in the same places during the periods slug control is needed. This study investigated these requirements in 22 commercial arable field crops from different regions of the UK, and in three different years. Despite a variable proportion of slug populations being found below the soil surface (where they could not be assessed) on different sampling dates, existence of higher density patches were confirmed in all fields, although it was noted that, when too few slugs were active on the soil surface, they could not be reliably detected. When they were detected, they occurred in the same locations in each field. The potential for using these patches in more sustainable slug control approaches is discussed.Exploitation of heterogenous distributions of Deroceras reticulatum, in arable fields by targeting molluscicide applications toward areas with higher slug densities, relies on these patches displaying sufficient spatio-temporal stability. Regular sampling of slug activity/distribution was undertaken using 1 ha rectangular grids of 100 refuge traps established in 22 commercial arable field crops. Activity varied significantly between the three years of the study, and the degree of aggregation (Taylor’s Power Law) was higher in fields with higher mean trap catches. Hot spot analysis detected statistically significant spatial clusters in all fields, and in 162 of the 167 individual assessment visits. The five assessment visits in which no clusters were detected coincided with low slug activity (≤0.07 per trap). Generalized Linear Models showed significant spatial stability of patches in 11 fields, with non-significant fields also characterized by low slug activity (≤1.2 per trap). Mantel’s permutation tests revealed a high degree of correlation between location of individual patches between sampling dates. It was concluded that patches of higher slug density were spatio-temporally stable, but detection using surface refuge traps (which rely on slug activity on the soil surface) was less reliable when adverse environmental conditions resulted in slugs retreating into the upper soil horizons.

Modelling of complex boron dilution transients in PWRs—Validation of CFD simulation with ANSYS CFX against the ROCOM E2.3 experiment

Abstract This paper compares CFD simulation results of a boron dilution scenario with experimental results. The simulation was carried out with ANSYS CFX using the SST turbulence scheme, experimental data were produced in the ROCOM test facility, experiment E2.3. The main features of the scenario are asymmetric, transient mass flow conditions in the affected loops 1 and 2 of a KONVOI-type reactor vessel and reduced density of the underborated coolant slugs fed into the reactor trough the cold legs of both loops. The CFD simulation was able to capture the density stratification in the loops affected by underboration and the mixing in the downcomer and in the lower plenum. Good agreement with the experiment was obtained for the temporal evolution of average boron concentration in several measuring sections of the reactor vessel and of the boron distribution in the core inlet. In general, minimum values of boron concentration were found to be lower in the simulation than in the experiment.

The study of the influence of slug density on the mixing performance in the reactor vessel, using PLIF experiment and FLUENT simulation

The study of the influence of different densities on the mixing behavior is very important in industries such as nuclear reactors. In this study a PLIF experiments and CFD simulation were performed to study the effect of slug densities on the mixing process in the vessel down-comer. Three different tests of different injected slug densities 1028, 1060 and 1090 kg/m 3 were performed in HEU facility. Mixing of injected slug of 0.014 mg/L concentration of Rhodmine-B and ambient water in the vessel investigated by the Laser induced fluorescence technique. To provide a qualitative analysis of the results, the average values of the mass fraction at specified locations (y = 0 cm and y = 5 cm below the inlet part) were calculated for each experiment test and contour plots were created. These contours represent snapshots of the concentration field captured for the three tests. It is observed from the results, the mixing behavior in the vessel down-comer was strongly depend on the slug density. The mixing was slower when the density difference between the slug and ambient water was high and conversely the mixing behavior was higher when the density difference was low. A selected experimental result was used to validate CFD. The CFD simulations were carried out with FLUENT code and the turbulent model that used in this simulation was Reynolds stress model (RSM). A good agreement was achieved between experimental data and CFD-Fluent simulation. • PLIF was used to study the influence of density on the mixing performance in the vessel down-comer. • The mixing behavior is observed intuitively in the vessel down-comer for different slug densities. • FLUENT code results agree with the experimental data. • Better mixing is achieved when the density of the slug is closest to that of the ambient water.

Cover crops support arthropod predator activity with variable effects on crop damage during transition to organic management

• Predatory arthropods increase with time in organic management. • Invertebrate pest density is negatively correlated with predator activity-density. • Predator activity-density positively correlates with predation of sentinel prey. • Cover crop termination date can affect damage to cash crops by invertebrates. Across the Mid-Atlantic United States, organic grain growers are interested in minimizing tillage to reduce management costs and detrimental effects on soil. Organic cover crop-based rotational no-till (OrgCCRT), in which cash crops are no-till planted into a mat of rolled cover crops, may meet grower objectives. However, growers may not adopt OrgCCRT if the system provides habitat for pests, especially during the transition to certified organic management when financial risk may be high. We studied the effects of OrgCCRT on predatory arthropod activity-density, diversity and community composition during the three-year transition to organic management. We also assessed biological control potential of the system, pest activity-density, and damage by invertebrate pests to two, no-till-planted cash crops, corn ( Zea mays L.) and soybean ( Glycine max (L.) Merr.). A mixture of hairy vetch ( Vicia villosa Roth) and triticale (x Triticosecale Wittmack), and cereal rye ( Secale cereale L.) preceded corn and soybean, respectively, as winter cover crops, which were terminated by rolling on three dates (early, middle, and late). Predatory arthropod activity-density, richness, and the relative predation index increased by the third year of organic management in rolled hairy vetch-triticale. Lower caterpillar densities in hairy vetch-triticale and lower slug density in both rolled cover crop treatments correlated with significantly greater predator activity-density, regardless of year or planting date. Delaying cash crop planting date resulted in greater cutting, chewing and slug damage in some crop years. Time in organic management augments predatory arthropod activity-density and diversity in OrgCCRT systems, thus contributing to suppression of invertebrate pests and crop damage.

Locomotor behaviour promotes stability of the patchy distribution of slugs in arable fields: Tracking the movement of individual Deroceras reticulatum.

The distribution of the grey field slug (Deroceras reticulatum Müller) in arable fields is characterised by patches containing higher slug densities dispersed within areas of lower densities. Behavioural responses that lead to the spatial/temporal stability of these patches are poorly understood, thus this study investigated behavioural mechanisms underpinning slug distribution using a new method for long-term tracking of individual slug movement in the field. A technique for implanting radio frequency identification (RFID) tags (each with a unique identification code) beneath the body wall of slugs was developed. Laboratory tests indicated no consistent detrimental effect on survival, feeding, egg laying or locomotor behaviour (velocity, distance travelled). Movement of individual slugs above and below the soil surface was recorded for >5 weeks (in spring and autumn) in winter wheat fields. Most (~80%) foraged within a limited area; and at the end of the observation period were located at a mean distance of 78.7 ± 33.7 cm (spring) or 101.9 ± 24.1 cm (autumn) from their release point. The maximum detected distance from the release point was 408.8 cm. The remaining slugs (~20%) moved further away and ultimately were lost. RFID tagging allowed continuous tracking of individual slugs, even below the soil surface. Localised movement of 80% of tracked slugs over 5 weeks offers a mechanism promoting stable slug patches in arable crops. Rapid dispersal of the remaining slugs facilitates exchange of individuals between patches. Precision targeting of pesticides at such stable slug patches may facilitate reduced usage. © 2020 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Modelling a targeted use of pesticide procedure for pest populations with heterogeneous spatial distributions

• A standard decision-making protocol is revisited to apply pesticide to selected spatial sub-domains in the agricultural field. • A protocol for targeted use of pesticides depends on the definition of a spatial density patch in heterogeneous spatial distribution and various definitions of a patch are modelled in the paper. • The benefits of the targeted use of pesticides are demonstrated. Commercial and environmental considerations have led to increased pressure to reduce pesticide use in agricultural crops resulting in a growing interest in development of pesticide application protocols that allow for their targeted use. In this paper, we revisit a standard decision-making protocol for pesticide application and introduce protocol modifications to apply pesticide to selected spatial sub-domains in the agricultural field. The baseline case we consider is the control of populations of the grey field slug ( Deroceras reticulatum ) in commercial fields. It is well known that slugs have strongly heterogeneous (patchy) spatial distribution and we show that targeting patches with higher slug density only, may offer significant potential for reducing the use of pesticides. An approach to incorporating targeted application of pesticide into a control protocol with treatment decisions based on a threshold population abundance will be discussed. The benefits of the targeted use of pesticides will be clearly demonstrated using data on slug abundance collected in commercial fields. We then argue that employing a single threshold for decision-making in the pesticide application protocol is not the most efficient way to assess risks associated with the population abundance when pesticide is applied selectively. It will be shown that a protocol for targeted use of pesticides depends heavily on the definition of a spatial density patch in heterogeneous spatial distribution, and a single decision-making parameter such as the population threshold cannot accommodate important information about the patch size. Hence an alternative is to introduce two controlling parameters into the protocol in order to quantify the pest abundance in each patch and patch size separately and we discuss this approach in the paper.

Terrestrial Gastropod Grazing on Macrolichens in a Northern Broadleaf–Conifer Forest

Herbivory by terrestrial gastropods, particularly Arion spp. (a slug), can alter epiphytic lichen communities; however, little is known about this interaction in forests of North America. We used 3 lines of evidence to explore this interaction: field grazing assessments on lichen thalli, a 10-y re-measure of gastropod abundance, and gastropod feeding trials in a montane forest at Hubbard Brook Experimental Forest (HBEF) in northern New Hampshire. Grazing damage by terrestrial gastropods was widespread, though few sites had severe grazing. Grazing damage was significantly higher on flatter terrain and on broadleaf trees. Slug densities were significantly lower in 2016 than in earlier surveys (1997–2006) on 4 of 6 plots. In feeding trials, 2 common lichens, Hypogymnia physodes and Platismatia glauca, were grazed more heavily by both native and non-native slugs than other lichen species. However, the Succineidae (amber) snails preferred Lobaria pulmonaria, a lichen that has been declining at HBEF in the last decade. Overall, lichen communities in the HBEF were moderately impacted by terrestrial gastropod grazing, but potential effects of the non-native slugs at higher elevations and impacts on lichen health of widespread, moderate grazing deserve further study.

Annual occurrence and algal preferences of the kleptoplastic sea slug, Elysia papillosa

Kleptoplastic, sacoglossan sea slugs are some of the best known examples of specialist marine herbivores. However, while their chloroplast symbiosis (kleptoplasty) has been studied in many species, little is known about sacoglossan life histories or their seasonal and temporal variation in natural populations. Most sacoglossan species are associated with one or a few closely related algal species that provide the chloroplast source for the kleptoplastic association. Elysia papillosa, a small sacoglossan with a distribution ranging throughout the Caribbean to Florida, primarily feeds on the alga, Penicillus capitatus, although it has also been found on several additional species of algae. A yearlong field survey of E. papillosa in a rhizophytic algal bed in Tarpon Springs, FL, USA revealed a low density of slugs throughout the year except during April, when mean density sharply peaked to 7.0 ± 5.9 slugs (0.25 m2)−1 . Slugs were not found on any algae other than P. capitatus, except occasionally on P. lamourouxii, suggesting that the latter alga serves as a secondary chloroplast source for E. papillosa. The presence of slugs throughout the year implies a broad tolerance of both salinity and temperature although the peak numbers in April may indicate a physiological preference for warming spring temperatures and, perhaps, day length.

Coupled discrete/continuum simulations of the impact of granular slugs with clamped beams: Stand-off effects

Coupled discrete particle/continuum simulations of the normal (zero obliquity) impact of granular slugs against the centre of deformable, end-clamped beams are reported. The simulations analyse the experiments of Uth et al. (2015) enabling a detailed interpretation of their observations of temporal evolution of granular slug and a strong stand-off distance dependence of the structural response. The high velocity granular slugs were generated by the pushing action of a piston and develop a spatial velocity gradient due to elastic energy stored during the loading phase by the piston. The velocity gradient within the “stretching” slug is a strong function of the inter-particle contact stiffness and the time the piston takes to ramp up to its final velocity. Other inter-particle contact properties such as damping and friction are shown to have negligible effect on the evolution of the granular slug. The velocity gradients result in a slug density that decreases with increasing stand-off distance, and therefore the pressure imposed by the slug on the beams is reduced with increasing stand-off. This results in the stand-off dependence of the beam's deflection observed by Uth et al. (2015). The coupled simulations capture both the permanent deflections of the beams and their dynamic deformation modes with a high degree of fidelity. These simulations shed new light on the stand-off effect observed during the loading of structures by shallow-buried explosions.

Sand column impact onto a Kolsky pressure bar

Abstract A laboratory-based methodology to launch cylindrical sand slugs at high velocities is developed. The methodology generates well-characterised soil ejecta without the need for detonation of an explosive; this laboratory-based tool thereby allows for the experimental investigation of the soil-structure events. The experimental set-up comprises a launcher with a cylindrical cavity and a piston to push out the sand slug. The apparatus is used to launch both dry and water-saturated sand slugs. High speed photography is used to characterise the evolution of the sand slugs after launch. We find that the diameter of the slugs remains unchanged, and the sand particles possess only an axial component of velocity. However, the sand particles have a uniform spatial gradient of axial velocity and this results in lengthening of the slugs as they travel towards their target. Thus, the density of the sand slugs remains spatially homogenous but decreases with increasing time. The velocity gradient is typically higher in the dry sand slugs than that of the water-saturated slugs. The pressure exerted by the slugs on a rigid-stationary target is measured by impacting the slugs against a direct impact Kolsky bar. After an initial high transient pressure, the pressure reduces to a value of approximately ρv 2 where ρ is the density of the impacting sand slug and v is the particle velocity. This indicates that loading due to the sand is primarily inertial in nature. The momentum transmitted to the Kolsky bar was approximately equal to the incident momentum of the sand slugs, regardless of whether they are dry or water-saturated.

Slug responses to grassland cutting and fertilizer application under plant functional group removal

Abstract Current studies on trophic interactions in biodiversity experiments have largely relied on artificially sown gradients in plant diversity, but removal experiments with their more natural plant community composition are more realistic. Slugs are a major part of the invertebrate herbivore community, with some species being common pests in agriculture. We therefore investigated how strongly slugs are influenced by grassland management, plant biodiversity and composition. Here we analysed the effects of cutting frequency, fertilizer application and plant functional group composition on slug densities and their contribution to herbivory on Rumex acetosa in a removal experiment within a >100-year old grassland in Northern Germany. The experiment was laid out as a Latin rectangle with full factorial combinations of (i) plant functional group removal (3 levels) using herbicides, (ii) fertilizer application (2 levels) and (iii) cutting frequency (2 levels). The resulting 12 treatment combinations were replicated 6 times, resulting in 72 plots. We collected a total of 1020 individuals belonging to three species Arion distinctus (60.4% of individuals), Deroceras reticulatum (34.7%) and Arion lusitanicus (4.9%) using a cover board technique and additionally measured herbivore damage to R. acetosa . We found the highest slug abundance on plots with a low cutting frequency and high food resource availability (increased cover of forbs and taller vegetation). Fertilizer application had no significant effect on slug abundance, but caused higher herbivore damage to on R. acetosa , possibly as a result of increased tissue quality. The negative effect of higher cutting frequency on slug abundance was lowest in control plots with their naturally developed graminoid-forb communities (cutting reduced slug density by 6% in the control vs. 29% in herbicide plots). Our experiments therefore support the idea that more natural plant species compositions reduce the impact of disturbances (e.g. through cutting or grazing) on invertebrates.

Predation by carabid beetles on the invasive slugArion vulgarisin an agricultural semi-field experiment

Arion vulgaris Moquin-Tandon 1855 is one of the most important invasive species in Europe, affecting both biodiversity and agriculture. The species is spreading in many parts of Europe, inflicting severe damage to horticultural plants and cultivated crops partly due to a lack of satisfactory and effective management solutions. Molluscicides have traditionally been used to manage slug densities, although the effects are variable and some have severe side-effects on other biota. Thus, there is a need to explore potential alternatives such as biological control. The nematode Phasmarhabditis hermaphrodita is the only biological agent that has been applied commercially so far. However, other biological control agents such as carabid beetles have also been found to be promising. In addition, some carabid species have been shown to feed on A. vulgaris in the field as well as in the laboratory. Two species in particular have been found to be important predators of A. vulgaris, and these species are also common in agricultural environments: Pterostichus melanarius and Carabus nemoralis. This study is the first to use semi-field experiments in a strawberry field, manipulating densities, to investigate how P. melanarius and C. nemoralis affect densities of A. vulgaris eggs and juveniles, respectively. Gut contents of C. nemoralis were analysed using multiplex PCR methods to detect DNA of juvenile slugs. Results show that both P. melanarius and C. nemoralis significantly affect densities of slug eggs and juvenile slugs under semi-field conditions and that C. nemoralis seems to prefer slugs smaller than one gram. Carabus nemoralis seems to be especially promising in reducing densities of A. vulgaris, and future studies should investigate the potential of using this species as a biological control agent.

Ecological Effects of Invasive Slugs, Arion rufus, on Native Cascade Oregon Grape, Mahonia nervosa

Invasive species frequently affect urban ecosystems, but few studies investigate primary and secondary mechanisms to determine overall ecological effects. Previous work demonstrates slugs can negatively affect seedling recruitment and vegetation biomass, and positively affect litter decomposition, nutrient cycling, and litter invertebrate populations. This study aimed to assess components of how invasive European black slugs (Arion rufus) affect native Cascade Oregon grape (Mahonia nervosa), in Pacific Northwest forests. We hypothesized that slug herbivory would decrease leaf area, but excretion of leaf and litter matter would enhance soil and litter conditions. Field experiments aimed to alter slug densities to measure effects on leaf area, litter biomass, soil nutrients, and non-slug litter invertebrates. Leaf area was lower in areas with high slug densities along with slight increases in non-slug invertebrate abundance; whereas leaf area, number of leaves, and branches per plant were higher in areas with low slug densities. However, slugs did not affect soil nitrate or litter decomposition significantly. Laboratory experiments found slugs consumed fresh leaves faster than litter but there were no differences in assimilation. By testing potential primary and secondary ecological mechanisms, our results suggest the overall effect of invasive slugs on a common forest understory plant is negative, as expected, but secondary positive effects on the litter invertebrate community, which could mitigate cumulative impact, are subtle and detection may require longer-term study.

Molecular analysis of predation by carabid beetles (Carabidae) on the invasive Iberian slugArion lusitanicus

The invasive Iberian slug, Arion lusitanicus, is spreading through Europe and poses a major threat to horticulture and agriculture. Natural enemies, capable of killing A. lusitanicus, may be important to our understanding of its population dynamics in recently invaded regions. We used polymerase chain reaction (PCR) to study predation on A. lusitanicus by carabid beetles in the field. A first multiplex PCR was developed, incorporating species-specific primers, and optimised in order to amplify parts of the mitochondrial cytochrome c oxidase subunit 1 (cox1) gene of large Arion slugs, including A. lusitanicus from the gut contents of the predators. A second multiplex PCR, targeting 12S rRNA mtDNA, detected predation on smaller Arion species and the field slug Deroceras reticulatum. Feeding trials were conducted to measure the effects of digestion time on amplicon detectability. The median detection times (the time at which 50% of samples tested positive) for A. lusitanicus and D. reticulatum DNA in the foreguts of Carabus nemoralis were 22 h and 20 h, respectively. Beetle activity-densities were monitored using pitfall traps, and slug densities were estimated using quadrats. Predation rates on slugs in the field by C. nemoralis in spring ranged from 16-39% (beetles positive for slug DNA) and were density dependent, with numbers of beetles testing positive being positively correlated with densities of the respective slug species. Carabus nemoralis was shown to be a potentially important predator of the alien A. lusitanicus in spring and may contribute to conservation biological control.

Slug grazing effects on seedling and adult life stages of North American Prairie plants used in designed urban landscapes

Designed vegetation is a major contributor to ecosystem service provision in cities, and as such the study of how herbivory and other ecological factors determine its capacity to deliver such services, is long overdue. This study investigated the effect of slug grazing on the establishment and development of 26 species of North American prairie forbs and grasses used in sown or planted naturalistic communities in urban greenspace. The experiment was designed to provide slugs with the opportunity to choose between the plant species used, to mirror the situation that prevails in public greenspace. Slug density was manipulated through baiting with metaldehyde at different frequencies. Seedlings of prairie species were more palatable to slugs than adults. Seedling establishment was significantly reduced in most species by slug grazing, with only seven species showing no significant increase in establishment in response to baiting with metaldehyde. In many species successful establishment was based on moderate-high unpalatability and large or fast growing seedlings. Adult prairie plants were typically more able to withstand slug damage, and once their shoots reached a certain size, grazing declined. This was not true of the most palatable species, which even as adults were eventually eliminated by grazing in the absence of baiting. Phenology plays an important role in the survival of adults, with early emerging species potentially subject to severe damage due to the limited availability of alternate food plants. As a group, prairie forbs are typically palatable to slugs, and unlikely to be persistent in the most slug-rich urban situations.

Molecular identification of predation by carabid beetles on exotic and native slugs in a strawberry agroecosystem

Generalist predators have the capacity to exert significant pressure on prey populations. However, integrating them into biological control programs relies on a detailed understanding of their foraging behavior and the levels of trophic connectedness with pest species. Carabid beetles are important predators of slugs, pests of agricultural, floricultural and horticultural crops worldwide, but these interactions have been rarely studied outside the Western Palearctic ecozone. Diagnostic molecular gut-content analysis was used to examine the strength of trophic pathways between a community of carabid beetles and two slug species, the exotic Deroceras reticulatum and native Deroceras laeve, in strawberry agroecosystems. Strawberries were grown according to standard horticultural practices for central Kentucky, following traditional bare ground planting or with the addition of detrital subsidies, to quantify the impact of habitat management on the abundance of pests and the strength of these trophic pathways. Following laboratory characterization of species-specific molecular markers targeting both Deroceras species, carabid beetles collected from a strawberry agroecosystem were screened for slug DNA. Field collections revealed important food web pathways existed between Harpalus pensylvanicus and D. reticulatum, with 7.2% screening positive for these prey yet none screening positive for D. laeve. In contrast, Chlaenius tricolor was found to feed on both slug species in the field, with 16% screening positive for both Deroceras. Despite below normal rainfall limiting slug densities in the field, the results presented here reveal the potential importance of carabid beetles in slug population dynamics in the Nearctic.