Delay dynamics of the slug Deroceras reticulatum, an agricultural pest

Not All Slugs Are the Same: Variation in Growth and Development of the Slug Deroceras reticulatum

Models to forecast slug populations make assumptions about growth and mortality in response to environmental factors. To refine these models, the growth trajectories and survival of Deroceras reticulatum, a worldwide pest, hatching in spring and autumn were compared at three rearing temperatures (ambient, 12 °C and 15 °C). Deroceras reticulatum reared under identical conditions showed great variation in growth and strong bimodality in growth rates. At all rearing temperatures, growth was influenced by hatching season; in all cases, fast growers dominated in autumn and slow growers dominated in spring. Survival was influenced by hatching season: autumn-born slugs survived better at ambient temperatures, but spring-born slugs had better survival at 15 °C. Deroceras reticulatum may be partitioned into "slow growers" and "fast growers". Fast growers responded to warmer conditions, growing to large sizes. Slow growers, in contrast, gained weight at comparable rates to ambient reared slugs, regardless of the elevated constant temperatures. The peaks of slug activity in autumn and spring are possibly not distinct generations as some slugs may mature early/late and slip into the alternative cohort. Rather, the observed autumn and spring peaks in slug numbers may be a response of a mixed-age population to the favourable environmental conditions at that time.

Assessment of the Biological Control Potential of Common Carabid Beetle Species for Autumn- and Winter-Active Pests (Gastropoda, Lepidoptera, Diptera: Tipulidae) in Annual Ryegrass in Western Oregon

Simple Summary

Many studies have shown that ground beetles feed on different agricultural pests, but little is known about their species communities from US cropping systems. We assessed the biological control potential of the most common carabid beetle species in Oregon annual ryegrass grown for seed by investigating spatial and temporal overlap of the most common species with those of the most damaging autumn- and winter-active pests (slugs, caterpillars and cranefly larvae) and determined the number of field-collected specimens that had fed on the respective pests using molecular gut content analysis. Only the non-native Nebria brevicollis was abundant during pest emergence and tested positive for all three pest groups. While the other common carabid beetle species—Agonum muelleri, Calosoma cancellatum and Poecilus laetulus—were also found to have consumed pests, they were active only during spring and summer, when crop damage by pests is less critical. We also show that disk tilling did not affect any of the four common carabid beetle species and that only N. brevicollis was significantly associated with a vegetated field margin. This study contributes to expanding our knowledge on conservation biological control in a system where chemical pesticides are still the mainstay of control against invertebrate pests.

Abstract

While carabid beetles have been shown to feed on a variety of crop pests, little is known about their species assemblages in US annual ryegrass crops, where invertebrate pests, particularly slugs, lepidopteran larvae and craneflies, incur major financial costs. This study assesses the biological control potential of carabid beetles for autumn- and winter-active pests in annual ryegrass grown for seed by: (a) investigating the spatial and temporal overlap of carabids with key pests; and (b) molecular gut content analysis using qPCR. Introduced Nebria brevicollis was the only common carabid that was active during pest emergence in autumn, with 18.6% and 8.3% of N. brevicollis collected between September and October testing positive for lepidopteran and cranefly DNA, respectively, but only 1.7% testing positive for slug DNA. While pest DNA was also detected in the guts of the other common carabid species—Agonum muelleri, Calosoma cancellatum and Poecilus laetulus—these were active only during spring and summer, when crop damage by pests is less critical. None of the four carabid species was affected by disk tilling and only N. brevicollis was significantly associated with a vegetated field margin. However, as its impact on native ecosystems is unknown, we do not recommend managing for this species.

Development of a High-Throughput Laboratory Bioassay for Testing Potential Attractants for Terrestrial Snails and Slugs

Invasive snails and slugs are among the most damaging pests of vegetables, fruits, ornamentals, grains, and forage throughout the world. Current control strategies are focused almost exclusively on molluscicides, which are ineffective under some conditions, and which can have serious nontarget effects. A major aim of this study was to develop a generic high-throughput bioassay method for use in identifying attractants for terrestrial gastropods, with the overall goal of developing attractant-based control methods for pest gastropods. Here, we demonstrate the use of the bioassay method in screening a wide variety of foodstuffs and other possible sources of attractants, using the pest snail Cornu aspersum Müller (Pulmonata, Helicidae) and the pest slug Deroceras reticulatum Müller (Pulmonata, Agriolimacidae) as test animals. Among a large number of food items and previously reported attractants tested, chopped fresh cucumber (Cucumis sativus) was the most attractive substrate for both species. Our results also showed that previous feeding experience influences subsequent food choice to some extent, but regardless of previous feeding experience, chopped cucumber was as attractive or more attractive than any other substrate tested.

The effect of climate manipulations on the herbivory of the pest slug Deroceras reticulatum (Müller, 1774) (Pulmonata: Agriolimacidae)

The pestiferous status of the terrestrial slug Deroceras reticulatum and the strong dependence of its biology and ecology on climatic factors have driven research on the potential responses of the slug to predicted scenarios of climate change. Here, we report two short-term experiments performed outdoors, under seminatural conditions, to assess the behavioural response of D. reticulatum to different climate manipulations in terms of herbivory, by measuring over 7 days the damage inflicted by slug populations to lettuce seedlings. The climate manipulations tested emulate predicted climatic conditions for northwest Spain, specifically winter warming and increased summer rainfall, in contrast respectively with normal winter conditions and summer without rain conditions. In a winter experiment, we compared a normal winter treatment with a winter warming treatment; with respect to the normal winter treatment, the winter warming treatment was characterised by higher temperature, lower relative humidity and the absence of rainfall. In a summer experiment, we compared a summer drought treatment with an increased summer rainfall treatment; with respect to the summer drought treatment, the increased summer rainfall treatment was characterised by the presence of rainfall, while the conditions of temperature and relative humidity were similar in both treatments. Neither winter warming nor increased summer rainfall did lead to a significant increase on the number of seedlings damaged by the slugs. However, with both treatments, we found a moderate increase on the amount of damage suffered by the seedlings. The results are discussed in the context of the potential responses of D. reticulatum to future climatic conditions.

Pests of germinating grain crops in southern Australia: an overview of their biology and management options

Grain crops in southern Australia are subject to attack by countless pests, with greater than 40 invertebrate species threatening seedling establishment. Control tactics for crop establishment pests rely heavily on the application of pesticides, especially in canola, which is the most susceptible crop to invertebrate damage. There is genuine interest in integrated pest management (IPM) among growers, but relatively little adoption of classical IPM in broadacre farming in southern Australia. The driving forces behind the lack of adoption are unknown, although over-reliance on broad-spectrum pesticides – which are inexpensive and often applied prophylactically as a means of negating the need to monitor crops – is undoubtedly a key factor. Recent control failures against important pests due to pesticide resistance, increased restrictions on pesticide applications, environmental concerns about pesticide applications and strong support for grain quality assurance programs by exporters, highlight the need to consider IPM principles as a means of reducing chemical inputs. IPM guidelines for broadacre farming systems are limited in scope and there is a need to develop practical management tools that encompass a whole system approach. This paper provides an overview of the main invertebrate pests affecting crop establishment and identifies gaps hindering the wide-scale adoption of IPM.

Biological control of terrestrial molluscs using Phasmarhabditis hermaphrodita--progress and prospects

Phasmarhabditis hermaphrodita Schneider (Nematoda: Rhabditidae) is a nematode that parasitises a wide range of slug and snail species. It has been formulated into a biological control agent (Nemaslug) and was commercialised in 1994. It is now available in fourteen European countries. A review is given of all research on P. hermaphrodita, including basic biology, mass cultivation, formulation, host range, application strategies, field efficacy and effects on non-target organisms. The many critical gaps in present knowledge are highlighted, and future research is proposed that will lead to greater understanding of this unusual parasite and may enable its more widespread use in the management of mollusc pests.

Slug control in Australian canola: monitoring, molluscicidal baits and economic thresholds

Exotic slugs have become serious pests of canola, at establishment, in Southern Australian high-rainfall cropping zones. Slugs were monitored using relatively inexpensive 300 mm x 300 mm terracotta tiles acting as refuges. An investigation was made of the effects of the time of application of chelated iron baits on the slug species Deroceras reticulatum Müller and Lehmannia nyctelia Bourguignat. Baits reduced the number of surface-active slug species. A single application at sowing provided greater efficacy than one application before sowing, and efficacy was comparable with that of two applications. Canola seedling densities showed a negative response to D. reticulatum numbers; the presence of even one individual per refuge trap reduced seedling numbers below optimum densities. Thistles and other vegetation were associated with increased numbers of slugs. European guidelines for slug monitoring and damage appear to be at least partly applicable to Australian conditions.