Responses of slug numbers and slug damage to crops in a silvoarable agroforestry landscape

1. Alternative farming practices such as set-aside and agroforestry are likely to be of continuing interest to European agriculture but may have associated problems, such as increased populations of crop pests such as slugs.
2. A silvoarable agroforestry experiment has been in progress since 1987 at Leeds University Farms at Bramham, West Yorkshire, UK. It consists of four replicate blocks, each with rows of trees separating alleys of arable crops; all four blocks have their own arable control areas in adjacent fields.
3. Pitfall trap catches within the experiment indicate that the slug population increased over the period 1991–94. The increase was greatest, and most consistent, within the tree rows in the agroforestry blocks. The increase was slower and less consistent in the arable controls and the arable areas within the agroforestry blocks.
4. In spring 1994, the slugs in each of the treatments in the agroforestry experimental area were sampled using pipe traps, refuge traps and pitfall traps. The number and diversity of slugs were highest in the grassed understorey beneath the rows of trees and significantly higher in the alleys between the rows of trees than in the arable control areas.
5. The levels of slug damage to a pea crop were assessed by surveys that recorded the number of emerging plants and the number of damaged leaves per plant. There were significant correlations between the number of slugs caught and the damage to the crop by slugs. It is concluded that slugs have the potential to be important pests of some crops in silvoarable agroforestry landscapes and that this could influence the choice of crops for this type of farming.
6. Major conclusions are emboldened in the Discussion.     

A bioassay for screening materials influencing feeding in the field slug Deroceras reticulatum (Müller) (Mollusca: Pulmonata)

Test materials were incorporated in wheatflour pellets to measure their effect on slug feeding. The dry weights of individual pellets were measured before and after exposure for 24 h to starved slugs. The method gave consistent results, and can accommodate test materials with different physical properties. The phagostimulant effect of sucrose was confirmed. An extract of the herb tarragon (Artemisia dracunculus) was the most effective antifeedant of the materials used at the concentrations tested. The method is compared with other published techniques.     

Damage to oilseed rape seedlings by the field slug Deroceras reticulatum in relation to glucosinolate concentration

Seedlings of nine commercial cultivars of oilseed rape were exposed to the field slug Deroceras reticulatum immediately after sowing in compost in trays. There was a small reduction in seedling numbers in the presence of slugs which was not related to glucosinolate concentration in seeds or seedlings. However, the number and leaf-area of seedlings with damage symptoms were strongly and inversely related to the total concentration of glucosinolates in seeds and one wk-old seedlings. The presence of barley seedlings as alternative food did not significantly affect this relationship. The glucosinolate concentration of seeds was closely correlated with that of wk-old seedlings. Analysis of individual glucosinolates in four cultivars spanning the range of concentrations found, showed that the concentration of most components declined as total glucosinolate concentration decreased. However, 2-phenyl ethyl-glucosinolate (gluconasturtiin) concentration tended to increase in seeds and 3-indolyl methyl-glucosinolate (glucobrassicin) increased in seedlings as total glucosinolate concentration decreased. Damage by slugs was inversely related to the concentration of those glucosinolates which decreased and was positively correlated with the two compounds which increased as total glucosinolate concentration decreased. The results support the hypothesis that glucosinolates in brassicas protect them from polyphagous herbivores, and, in particular, that an important function of glucosinolates in rape seeds is to protect seedlings from slugs. As glucosinolate concentrations of oilseed rape cultivars continue to decline, so the risk of slug damage to seedlings may well increase.     

Tracking predator density dependence and subterranean predation by carabid larvae on slugs using monoclonal antibodies

1. Subterranean carabid larvae are more numerous than surface‐active adults, yet very little is known about their ecological significance, dietary preferences or ability to regulate populations of prey species, particularly pests. Part of the reason for this is that predator–prey interactions beneath the soil are almost impossible to observe. 2. Extensive field studies have shown that adult Pterostichus melanarius (Illiger) can affect the temporal and spatial dynamics of their slug prey. However, if larvae too are feeding on slugs, this could radically affect overall predator–prey dynamics. 3. We tested the hypotheses that P. melanarius larvae would kill and consume two slug species, Deroceras reticulatum Müller and Arion intermedius Normand, under laboratory and semi‐field conditions, and that there would be no significant difference in rates of predation on these slug species. 4. A new monoclonal antibody was developed that was capable of detecting the presence of slug proteins in the guts of P. melanarius larvae. 5. Pterostichus melanarius larvae killed both A. intermedius and D. reticulatum in the laboratory, feeding to a greater extent, and growing more rapidly, on the latter. The larvae were equally effective at reducing numbers of both slug species in a crop of wheat grown in semi‐field mini plots, but predation was affected by density‐dependent intra‐specific competition amongst the beetle larvae. 6. Future modelling of the dynamic interactions between carabids and slugs will need to take into account predation by larvae.     

Susceptibility of the eggs of the field slug Deroceras reticulatum to contact with pesticides and substances of biological origin on artificial soil

The toxicity of 14 substances, including a number of pesticides, to the eggs of the pest slug Deroceras reticulatum was determined in laboratory experiments. Eggs were kept in contact with a precisely defined artificial soil to which a range of concentrations of the test substances had been applied. Mortality of the eggs was assessed every 24 h and the median lethal doses (LD₅₀) were determined. The herbicides bromoxynil, ioxynil and pyridate + bromoxynil, the insecticides thiocyclam, diflubenzuron and azadirachtin, the molluscicides metaldehyde and methiocarb, and other compounds such as carvone, iron‐EDDHA, saponin, and an extract of Pongamia pinnata, killed the eggs after periods of exposure ranging from 2 to 14 days, depending on the compound and the dose. Only two compounds, the insecticides imidacloprid and teflubenzuron, failed to kill the eggs of D. reticulatum at any of the doses tested. Values of LD₅₀ below 0.01 mg a.i. cm^?2 were obtained for the herbicides bromoxynil, ioxynil and pyridate + bromoxynil, and for the biological pesticide azadirachtin. The feasibility of slug egg control in different contexts is discussed.     

The role of different slug species in damage to oilseed rape bordering on sown wildflower strips

Damage caused by different slug species on above and below ground parts of young oilseed rape bordering on sown wildfower strips was studied. Slug densities and damage were recorded in the field and damage rates for each slug species were measured in the laboratory. Severe slug damage was observed in rape areas 1 m from the wildflower strips. Damage rates of slug species feeding above ground were distinctly higher than those feeding below ground. The estimated damage caused by above-ground feeding in all four study fields ranged between 72% and 89 % and was always distinctly higher than damage caused by below-ground feeding (11% to 28 %). The estimated damage caused by Arion lusitanicus above ground was between 54% and 69 %, followed by Deroceras reticulatum (3% to 26 %). Thus A. lusitanicus was responsible for the majority of damage in the rape crops studied.     

Feeding behaviour and mortality of the slug, Deroceras reticulatum in relation to control with molluscicidal baits containing various combinations of metaldehyde with methiocarb

In terms of control of slug pests, the differing modes of action of the two molluscicides metaldehyde and methiocarb appear to provide some advantages for the former during warm, dry weather and for the latter during cool, wet weather. With a view to exploiting these different modes of action, the feeding behaviour of Deroceras reticulatum (Müller) presented with molluscicidal baits containing various combinations of metaldehyde with methiocarb was investigated. Voluntary feeding trials conducted in the laboratory with both candidate and standard treatments (containing metaldehyde or methiocarb alone) are described. By combining the two active ingredients in certain ratios the total concentration of active ingredients may be reduced without adversely affecting the efficacy of a bait formulation in terms of slug mortality.     

烟田龟纹瓢虫的空间分布型及抽样技术

对8块烟田的龟纹飘虫空间分布调查和分析结果表明,该虫空间分布符合负二项分布,均属聚集分布,聚集的原因主要是由环境因素所致,抽样方式以棋盘式抽样代表性最强。     

Soil sampling to detect potato cyst-nematodes {Globodera spp.)

Sampling agricultural land prior to planting potatoes usually forms the basis of estimating levels of potato cyst-nematode (PCN) populations for certification, advisory and research programmes. A series of investigations studied the design and efficiency of sampling equipment and the intensity and pattern of soil collection for PCN detection. A single 360 g sample in fields up to 2 ha is recommended as the minimum practical and economic sampling intensity for PCN detection for certification purposes. Additional rates are recommended for ‘high risk’ situations such as high grade seed and resampling with a view to descheduling land.     

草地贪夜蛾幼虫在苗期玉米田的空间分布格局及其抽样技术

草地贪夜蛾Spodoptera frugiperda(Smith)是2019年1月新入侵我国的重要害虫,研究并明确空间格局对确定该虫田间抽样技术、调查虫情具有指导意义。本研究调查获得了玉米苗期喇叭口初期、大喇叭口期草地贪夜蛾幼虫空间分布数据,应用多个聚集指标、Iwao m~*-m模型、Taylor幂法则等方法分析明确了该虫幼虫空间格局,发现苗期整体上幼虫个体群为聚集分布,喇叭口初期为聚集分布而大喇叭口期为均匀分布;苗期整体上及大喇叭口期幼虫聚集度对密度不具依赖性,而喇叭口初期幼虫聚集度对密度具依赖性;同一幼虫密度和误差条件下不同苗期抽样数量显著不同,玉米生育期越早则抽样数量越大。     

Impact of the field slug Deroceras reticulatum on establishment of ryegrass and white clover in mixed swards

The relationship between numbers of Deroceras reticulatum and the establishment of ryegrass cv. Melle and white clover cv. Milkanova was studied on a mini-plot experiment. Assessments 12 and 33 days after sowing showed that Deroceras reticulatum had a proportionally greater impact on survival and growth of clover than on ryegrass. Slugs had no apparent effect on numbers or growth of three species of broad-leaved weeds (Senecio vulgaris, Cardamine hirsuta and Epilobium sp.) which occurred in the field experiment. In pot experiments where seedlings of ryegrass together with one of three clover cultivars, with or without chickweed (Stellaria media), were exposed to Deroceras reticulatum, seedlings of white clover cv. Aran were less affected by slug grazing than cvs S184 or Milkanova. The presence of different clover cultivars did not significantly influence the impact of slugs on growth of ryegrass or chickweed, nor did the presence or absence of chickweed influence the effect of slugs on ryegrass and clover seedlings. Slugs had less impact on the growth of chickweed than on ryegrass or clover. Thus slug grazing would make clover less able to compete with chickweed as well as the other weeds found in the mini-plot experiment.     

Susceptibility of indigenous UK earthworms and an invasive pest flatworm to the slug parasitic nematode Phasmarhabditis hermaphrodita

We exposed five earthworm species, one flatworm species and the susceptible slug Deroceras reticulatum to the recommended field application rate and five times this rate of Phasmarhabditis hermaphrodita. P. hermaphrodita caused significant mortality to D. reticulatum at the recommended rate and five times the recommended rate. Survival of the earthworms and the flatworm was not different from the untreated controls. P. hermaphrodita is a safe biological control agent causing no harm to earthworms and could not be used to kill Arthurdendyus triangulatus.     

Spatial relationships between two Agriotes click-beetle species and wireworms in agricultural fields

1 The production of new insect pheromones for pest monitoring proceeds at a greater rate than their evaluation, with the consequential possibility of premature introduction. Fundamental to their successful deployment is the determination of a consistent relationship between adult male pheromone trap catches and pest damage. In the present study, adult pheromone traps and larval bait traps were used to examine spatial relationships between two species of Agriotes beetle and wireworms at the field scale. 2 The spatial distributions of adult male Agriotes lineatus and Agriotes obscurus in two fields were determined and compared with the distribution of their larvae. Data were assembled as spatially referenced trap counts, and analysed for evidence of aggregation and clustering using Spatial Analysis by Distance IndicEs (SADIE) methodology. Spatial stabilities of adult populations between sampling dates were tested using association tests. Spatial and quantitative linkages between adult and larval trap catches were also tested. Moreover, a new way of adapting SADIE methodologies is presented for situations where two datasets within an area do not share the same sampling points. 3 There was no significant difference in variance : mean relationships for the two species but there were differences in their spatial distributions, and this is a definitive example of the general argument stating that it is important to consider spatial as well as count data in ecological studies. The spatial distribution of A. lineatus varied between sampling occasions at both sites whereas A. obscurus had consistently significant SADIE indices over time at one site, and adult catches could also be linked to larval distributions and counts. It is proposed that observed differences between the two species can be explained by interference between traps and dissimilar movement rates. There was some evidence of an edge effect at the field boundaries. 4 The distance between pheromone traps is related to the time that elapses before adjacent traps interfere with trap captures and this limits the detection of statistically significant spatial patterns. It is shown that the current practice of adding trap counts for different Agriotes species and treating them as numerically equivalent is insufficiently robust to be recommended at this stage. 5 The implications for the use of sex pheromone traps in wireworm pest management are considered. It is concluded that pheromone traps, as currently used, will not reliably indicate where wireworms occur in a field, and that the complexity of interpreting adult male trap counts limits quantitative predictions of population size.     

A method to study slug predation on seedlings in the field

Dandelion is a common Asteraceae species that populates disturbed sites and gaps within swards where it becomes an important competitor of grasses. The natural control against dandelion includes seedling predation, with slugs, particularly Arion lusitanicus, being the most important in the Czech Republic. However, the study of slug seedling consumption is difficult because naturally established seedlings are not always available. Therefore, we developed a method of exposing laboratory‐grown seedlings as bait for slug predation. Dandelion seeds were sown in plastic cups containing a bottom layer of moist substrate. The emerged seedlings were thinned to 20 and displayed in an open area. During 2008–2010, the seedling baits were placed at 15 sites at 1 month intervals throughout the dandelion vegetative season, in parallel with plasticine baits that monitored slug feeding activity. For each 1 month interval, seedling survival was observed for a period of 8 days, and the estimated time to death was calculated; the percentage of surviving seedlings was then recorded. This method of seedling presentation demonstrated that local and temporal variation in seedling survival is correlated with slug feeding activity. The advantage of this technique is that the seedlings in baits may be presented at any place and time, as required by the experimental design; however, we found that the estimated time to seedling death was shorter for the exposed baits than for the naturally established seedlings. The method is suitable for the study of plant species other than dandelion, and also for aims other than the study of spatiotemporal trends in seedling consumption by slugs.     

Analysing the performance of a micro soil solution sampling device in a laboratory examination and a field experiment

The performance of a micro soil solution sampling device was tested in a laboratory examination and in a field experiment. The instrument allows detection of temporal and spatial changes in soil solution chemistry at a spatially high resolution. The flexible tube of the suction cell is made of a porous polymer with a diameter of 2.3 mm. To achieve more stability and to minimize disturbance of the instrument during field installation, the original device was modified by embedding the suction cell in a stainless steel and pressure absorbing corpus. During a laboratory test the new sampling system was compared to ceramic P-80 suction cells. Solution samples taken with the new device adapted more quickly to the given concentrations compared to the ceramic suction cells. In a field test, micro samplers were implanted in an existing soil solution monitoring plot, equipped with standard ceramic samplers. Bi-weekly sampling using the micro cells indicated high temporal and spatial variation, and in June 1995 it was possible, to identify a distinct nitrification. However, in a statistical comparison of the entire sampling period and respective sub-sampling areas the two sampler types indicated identical concentration ranges for nitrate. It is concluded that the new micro samplers can help to identify processes in soils which may cause short-term changes in the soil solution chemistry, whereas the standard sampling technique with ceramic cells seems to be still a suitable tool if long-term mean soil solution concentrations are to be measured.     

Assessing spatiotemporal variation in abundance: A flexible framework accounting for sampling bias with an application to common pochard (Aythya ferina)

Assessing trends in the relative abundance of populations is a key yet complex issue for management and conservation. This is a major aim of many large‐scale censusing schemes such as the International Waterbird Count (IWC). However, owing to the lack of sampling strategy and standardization, such schemes likely suffer from biases due to spatial heterogeneity in sampling effort. Despite huge improvements of the statistical tools that allow tackling these statistical issues (e.g., GLMM, Bayesian inference), many conservationists still prefer to rely on stand‐alone turn‐key statistical tools, often violating the prerequisites put forward by the developers of these tools. Here, we propose a straightforward and flexible approach to tackle the typical statistical issues one can encounter when analyzing count data of monitoring schemes such as the IWC. We rely on IWC counts of the declining common pochard populations of the Northwest European flyway as a case study (period 2002–2012). To standardize the size of sampling units and mitigate spatial autocorrelation, we grouped sampling sites using a 75 × 75 km grid cells overlaid over the flyway of interest. Then, we used a hierarchical modeling approach, assessing population trends with random effects at two spatial scales (grid cells, and sites within grid cells) in order to derive spatialized values and to compute the average population trend at the whole flyway scale. Our approach allowed to tackle many statistical issues inherent to this type of analysis but often neglected, including spatial autocorrelation. Concerning the case study, our main findings are that: (1) the northwestern population of common pochards experienced a steep decline (4.9% per year over the 2002–2012 period); (2) the decline was more pronounced at high than low latitude (11.6% and 0.5% per year at 60° and 46° of latitude, respectively); and, (3) the decline was independent of the initial number of individuals in a given site (random across sites). Beyond the case study of the common pochard, our study provides a conceptual statistical framework for estimating and assessing potential drivers of population trends at various spatial scales.     

柠条重要害虫四线奇尺蛾天津亚种幼虫的空间分布研究

四线奇尺蛾天津亚种是近几年新发现的危害柠条的重要害虫,研究四线奇尺蛾种群的空间格局和抽样技术,可为该害虫的危害调查与防治提供理论依据。本文应用6个聚集指标和Taylor幂法则及Iwao的m~*-m回归分析法,对四线奇尺蛾天津亚种幼虫的空间分布型和抽样技术进行了研究,并做了影响因素分析。结果表明:四线奇尺蛾天津亚种幼虫在6个样地均呈聚集分布,分布的基本成分是个体群,通过分布型参数,采用Iwao法计算出了在不同精度下幼虫抽样数公式和序贯抽样模型,该模型可为四线奇尺蛾天津亚种的预测预报及防治提供理论依据。