New insights into slug and snail control

Author: Michael Nash (South Australian Research and Development Institute, Urrbrae, South Australia), Michael Richards (Northern and Yorke Natural Resources Management, Crystal Brooke, South Australia), Helen DeGraff and Greg Baker (South Australian Research and Development Institute, Urrbrae, South Australia). | Date: 09 Feb 2016

Abstract 

Snail and slug baits should be considered as crop protectants. Cultural methods are required to reduce populations and the biological function of farming systems needs to be considered. Baits often perform badly and have to be re-applied due to field degradation and/or pest populations not actively feeding. Research has focused on improving bait performance.

Baits have a limited field life, with the commonly used bran based metaldehyde products (e.g. Meta®) lasting less than two weeks, thus need to be applied regularly. Rainfall not only physically breaks down bran pellets; it causes a reduction in number of snails killed. Temperature, not UV light, also reduces the efficacy of metaldehyde baits. Choose the product based on cost and the context in which it will be applied. Different products perform differently. Rules of thumb: do not use current iron based baits when >10mm rain is expected; do not use Metaldehyde products over the summer and expect them to last.

Observations using time lapse cameras and recording environmental factors highlight different species are active and feed at different times. Even within populations, individuals behave differently depending on the moisture conditions where they hang out. Weather data from the Bureau of Meteorology (BoM) is not as accurate as ground moisture, or soil moisture in predicting behaviour. Using detailed micro-climate information and individual snail’s behaviour to inform bait application and timing of cultural controls resulted in good field control in 2015.

The role of crop rotations needs to be considered; canola leads to a build-up in numbers and linseed has been found to limit slug populations. How will early sown canola influence populations?

Both snails and slugs can be controlled in no-till, full stubble systems once growers understand the context of where and when controls are applied, and follow a few basic guidelines. Bait needs to be applied when snails and slugs are active and feeding, with the timing varying depending on paddock and seasonal conditions and the species present. Basic rules of thumb for applying bait: snails in autumn once feeding prior to egg laying; slugs at sowing to prevent seed and seedling damage.

Background

Several exotic snail and slug species of European-Mediterranean origin have established in Australia and become important pests of grain crops. Snails cause substantial economic losses through yield loss from feeding damage, field control costs, additional harvest costs, grain value loss, receival rejection and threaten market access. Slugs cause major losses from feeding damage at crop establishment, costs of re-sowing and field control.

The biggest threat from snails is market access, with common white or vineyard snail (Cernuella virgata) now listed in formal import standards (February 2015) for wheat and barley into China. The Chinese market is worth $1.5 billion (2014 ABARE) to Australia. Previous contamination issues (e.g. Korea 2012) highlight the potential cost snails pose to the grains industry, when a major market restricts access due to a quarantine breach.

Crop losses are difficult to quantify. Estimates have indicated an overall enterprise cost to farms with snails is $33/ha, with $27/ha spent on bait and $17/t spent on post-harvest cleaning (GRDC 2015 report). For individual farm enterprises, snail costs were equivalent to 14 per cent of the farm profit in the Yorke Peninsula/Lower North region of South Australia (SA) and 34 per cent and 44 per cent of farm profits in the less profitable Victorian Wimmera and Mallee respectively (2000 report). Slugs are particularly damaging to establishing canola, with yield losses in untreated areas of experiments at 60-80 per cent (GRDC DAS00134 data). Bait costs are $30-$50/ha with 95 per cent of canola in western Victoria sown into burnt and/or cultivated ground.

Where snails and slugs are a high risk (south east SA and western Victoria (Vic)) growers have shifted from growing canola and/or have implemented strategic burning and cultivation. That opportunity cost is estimated upwards of $270 million annually lost to the canola industry alone. A five per cent production loss by slug and snail activity would represent >$82 million loss to the Australian canola industry (2012 values).

Research has indicated bait timing is an essential factor to limit snail contamination or slug damage. Growers on Yorke Peninsula have responded by now baiting in March rather than after sowing in May/June. Similarly, in western Vic, baiting equipment has been added to machinery so baits are applied during the sowing operation. In all operations baiting is just one part of the year-round control program with cultural methods vital: cabling and rolling for snails; cultivation and rolling for slugs.

The overall aim of research presented in this paper is to improve decisions on bait applications due to the limited field life of current products and the variable feeding of the targets; that is snails and slugs do not always feed on baits. The first aim is to understand factors that lead to the degradation of bait products and compare the field life of various products. The second aim is to understand activity and feeding triggers, and what seasonal factors lead to greater pest numbers. Due to differences in species the research questions have been split with the focus on snails for presentations at the 2016 Adelaide GRDC Grains Research Update and slugs for 2016 Bendigo GRDC Grains Research Update. The subheadings are the current best practice recommendations, with research questions being addressed below that.

Snails

To improve baiting programs apply baits when snails are actively feeding before egg-lay commences

  • Under what conditions will snails begin moving and/or feeding?
  • Does a snail’s body moisture level change over summer? 
  • Can we use body moisture to predict feeding and/or time to egg lay?

Slugs

To improve baiting programs apply baits to protect seeds and seedlings from actively feeding slugs

  • Under what conditions will adult slugs emerge from soil? 
  • Can we use soil moisture to predict slug activity?
  • Under what conditions and life stage are slugs most damaging to crops?

Methodology

Bait degradation

A number of experiments were conducted to give a range of conditions and test the effect of exposure time, and subsequently tease apart individual factors; temperature, ultra-violet (UV) light, rainfall and mould. Bait products were weathered by spreading approximately 50g of each on the surface of soil (Warooka red loam) in large planter trays (400 x 300 x 120 mm). Trays were placed on benches in an exposed position at the Waite Campus, Urrbrae, SA. Baits were exposed to weathering for seven periods during 2014, at seven day intervals (0, 7, 14, 21, 28, 35) resulting in a variety of conditions. Rain fastness of 16 different products was assessed twice by exposing to rainfall (>35 mm) over 14 days in 2015.

Italian snails (Theba pisana) were used to test the efficacy of molluscicidal baits once they had been exposed to the environment on soil. Five snails, eight replicates per treatment (n=40) were added to each test arena with eight baits as soon as practicable following the completion of weathering periods (usually within one week). Baits were removed three days after initiation of the experiment due to the formation of mould forming, which was scored as present/absent and the number and condition of pellets remaining recorded. Snail mortality was assessed five days after bait was removed.

Snail and slug activity

Ten paddocks across southern Australia have been intensively monitored using cameras to capture slug and snail activity (A Snug Blog). Environmental data was also collected: soil moisture and temperature (10cm); ground leaf wetness, temperature and relative humidity. Rainfall and barometric data was obtained from BoM. Snails were collected monthly (more frequently during autumn) for assessment of size (n=90), moisture content (n=45) and reproductive stage (n=45) achieved using digital calipers for shell diameter, before and after oven-drying weights (40°C for > four weeks), and albumen gland dissections, respectively. Slugs were collected each month if active from 20 surface refuges per site, which were also used to assess abundance. Slugs were weighed within 24 hrs and dissected to determine reproductive maturity.

Results and discussion

How will the latest research findings affect management strategies and packages?

Research findings regarding the field degradation of baits are presented to inform management about their likely efficacy under various weather conditions. Manufacturers often make claims about rain fastness; however these are based on physical integrity that was found not to influence actual efficacy (Table 1). Conclusions from this work:

  • Rainfall erodes physical integrity of bran-based baits.
    • Mould on products did not influence bait consumption nor efficacy
    • Reduction of a.i. by rainfall (metaldehyde and iron chelate) is important. Individuals are more likely to consume a sub lethal dose.
  • Don't use current iron-based baits when >10mm rain is expected
    • Temperature, not UV light, degrades metaldehyde baits
    • Don't use metaldehyde products over the summer and expect them to last >2 weeks.
  • Commonly used bran products need to be re-applied < 2 weeks, more expensive products will last 3-4 weeks
    • Work out the cost benefit yourselves! 

Figure 1: Product degradation in response to exposure to various weather conditions in 2014.

Figure 1: Product degradation in response to exposure to various weather conditions in 2014.

Overall estimates from seven experiments are presented as mean mortality and 95% confidence intervals, calculated from estimating number of dead snails using a generalized linear model (GLM) (Poisson, log link). Significant effect of exposure time is indicated by ***(P < 0.001).  

Table 1: Response of various products to rainfall (>35mm) over 2 week period in 2015.

Product Active ingredient Conc. Different groups no rain Mortality Rainfall Sig. Dif. Rank after Rainfall
 2 weeks 35 mm
Multiguard Iron 6% a 97% 0% Y 12
Mesurol Methiocarb 2% a 94% 56% N 1
Eradicate Iron 6% a 90% 0% Y 12
Ex 5 Methiocarb 2% ab 80% 43% Y 2
Ex 1 Metaldehyde 3% bc 59% 21% Y 6
Ex 3 Iron NS cd 53% 0% Y 12
Metakill (green) Metaldehyde 5% cd 51% 24% Y 4
Metarex micrp Metaldehyde 5% cd 51% 23% Y 5
Ex 2 Metaldehyde + 1.8% cde 49% 6% Y 9
Ex 4 Metaldehyde + 5% cdef 44% 23% N 5
Metarex Metaldehyde 5% cdef 43% 39% N 3
Slugout Metaldehyde 1.8% defgh 29% 16% N 7
Metakill (blue) Metaldehyde 5% efghi 24% 6% N 9
Meta Metaldehyde 1.5% ghi 17% 7% N 8
Sluggoff Metaldehyde 3% hi 11% 4% N 10
Slugger 2.5mm Metaldehyde 1.5% hi 7% 1% N 11
Placebo Nil 0% i 0% NA NA NA

Combined mortality data (%) from the two experiments (14 reps per group) are presented with significant differences between ‘exposed to rain’ and ‘exposed to dry’ two-week treatments for a single product indicated by different letters. Rainfall resulted in a significant reduction (Χ216 = 658, P <0.001) in efficacy that interacted with product, hence significant differences between individual products is indicated with “Y” (HSD < 0.05), but due to variability we were not able to detect significance when less than a 25 per cent effect size.

Slug research

GRDC fast track project (SAM0001) found the accuracy of bait placement around emerging seedlings gave no significant improvement in protecting the crop from slugs. Canola can be established with disc seeders into stubble in the high rainfall zone (HRZ) as long as some basic rules around timing of bait application are followed.  Soil moisture at 50-60cm was associated with increased slug activity at the soil surface as recorded by using surface refuges during this fast track project. Black keeled slug activity is suspected to be triggered by moisture deeper in the soil, hence the current hypothesis is 75mm-100mm of rainfall is required over a three week period in the autumn for populations to become active. Ongoing research is testing this hypothesis and investigating species differences.

Snail activity

Observations, including video footage, have led to more questions than answers. Wetness at 10cm above the ground seems to be best associated with round snail activity, although BoM relative humidity can be used; in summer snail activity is triggered by 90 per cent relative humidity or higher. By late March this response is at 80 per cent relative humidity coinciding with the commencement of mating. Common white snails move more during the night, whereas small pointed snails often move during daylight in the early morning. During wet conditions pointed (conical) snails will move at similar times to round snails. It seems pointed snails (both species) need more moisture to become active, such as longer periods of high humidity or light showers. Observed species differences could be due to temperature, but also behavioural differences. For example, pointed snails have a staggered activity; some are active early in the season before the majority become active mid to late season. One theory is that staggered activity is similar to germination of weeds where buried seeds germinate at a different time to those on the surface. That is, pointed snails in the soil become active at a different time to those hiding up on stubble. This research highlights different species behave differently - bait application needs to match feeding activity. 

Snail research 

The other observation is that snails often move straight past a bait pellet, making us question claims about pellet attractiveness. Is their response to bait related to their physiological state? That is, does body moisture content of the snail or reproductive stage, as measured by albumen gland size, influence feeding on baits, hence ingestion of a lethal dose. Body moisture content of snails fluctuates during the year in response to available water in the environment. Albumen glands swell during reproductive activity. Enlarged glands indicate active egg-laying. Dissections of two populations of Italian white snails and three populations of common white snails within SA in 2015 indicated that glands began to swell in March 2015, peaking late April to mid-May. Common white snails at Palmer SA displayed a potential relationship between these characters (Figure 2), however a specific body moisture ‘trigger value’ for albumen gland enlargement and subsequent egg-laying is yet to be determined. Incorporating climate data will help predict when body moisture increases, and combined with camera observations the aim is to use site-specific weather data to help pin point optimal baiting periods. In 2015, information from a camera and weather station was used to inform bait application (10kg/ha Metaɸ) in mid-January when snail body moisture was >65% and prior to breeding as indicated by reduced size of albumen glands (Figure 2). This baiting followed rolling in early January, which resulted in >90% mortality. Despite no other bait applied for the season there was limited build-up of common white snails in that 2015 wheat crop.

ɸRegistered label rate is 5-7.5kg/ha.


Figure 2: Changes in common white snail moisture and reproductive stage from summer to winter.

Figure 2: Changes in common white snail moisture and reproductive stage from summer to winter.

Addressing some specific questions that arose from 2015

Where have the slugs gone – does cropping after certain crops (e.g. beans) play a part? 

Grey field slug numbers were less in 2015 compared to 2014, however black keeled slug numbers remained constant between seasons (Figure 3). Analysis of camera data indicated that slug activity was reduced during colder conditions, thus damage observed was less in 2015 for most areas. For grey field slugs the question remains: what caused a reduction in their numbers? There were two theories discussed via twitter, email, etc. (i) seasonal conditions meant it was too dry in the 2014 spring for slug numbers to increase, or (ii) the pendulum had swung in favour of natural control, such as an increase in parasites. What is certain is that spring baiting and early autumn baiting prior to sowing rarely prevents damage to emerging crops (Nash et al., 2007); pre baiting does foster complacency. 


Figure 3: Different slug species relative abundance at one site in western Victoria. Crop type for that season displayed at the top of the figure.

Figure 3: Different slug species relative abundance at one site in western Victoria. Crop type for that season displayed at the top of the figure.

Laboratory experiments were used to determine if nutritional differences between crop types can influence slug abundance and thereby contribute to year-to-year fluctuations in slug populations in the field. Both the number of slugs surviving and the growth rate of individuals differed between six crop types, but most importantly the number of offspring produced was greatest when reared on canola. Peas and canola were the most favourable crop for grey field slugs to increase populations as measured by the intrinsic growth rate (Rm ) (Table 2). With values obtained comparable with previous studies; Rm = 0. 03 (Carrick, 1938, South, 1982). The lack of reproduction on PBA Rana faba beans is not supported by field observations. One likely explanation is, despite beans being a poor food source for slugs (hence beans are easily established where slugs are present), the micro habitat created by bean crops favours slug populations. Interestingly, linseed which had the second lowest intrinsic growth rate has an additional advantage as it is thought to dry out the soil. Preliminary data on volumetric soil moisture from paired paddocks comparing the different 2014 crops, recorded in the winter 2015 were; barley 34.0 per cent, beans 34.1 per cent, linseed 32.7 per cent, canola 34.1 per cent, wheat 41.5 per cent. Significantly lower slug populations (mean ± SE) were observed the following year after linseed paddocks, compared with adjacent barley paddocks: linseed 8 ± 3.4; barley 20 ± 7.3; T test P = 0.014, n=8. Two factors are at play when assessing population responses to crops in the field; microhabitat and nutrition. Investigations are needed to understand why populations increase on wheat, and is this a factor in canola crops being damaged as they follow wheat? Farming groups have a role in extending this laboratory research to the field looking at rotations as a way to limit slug populations.

Table 2: Intrinsic growth rate (Rm) of grey field slugs raised on various crops under laboratory conditions (100% rH, 10-14°C).

Crop first eggs laid eggs std. dev. eggs neonates hatch rate Rm
Canola 17/11/14 1986 192 1204 61% 0.026
Peas 24/12/14 1003 195 530 53% 0.038
Barley 5/12/14 1510 182 505 33% 0.021
Wheat 17/11/14 2178 285 492 23% 0.019
Linseed 5/12/14 438 56 66 15% 0.009
Beans 5/12/14 23 NA 0 0% 0

What are the risk factors that lead to increasing snail and slug activity?

Improving soils and moisture holding capacity, which includes increasing macropores (porosity), organic matter and available calcium makes a more favourable habitat for snails and slugs. Moisture is the biggest determining factor for breeding, so in seasons that are more favourable for growing crops, populations will build up. Separate tables for slugs and snails have been included for a quick reference to risk factors. 


Table 3: Risk factors for slug outbreaks.

Factor High risk Reduce risk Low risk
Annual rainfall Irrigated and/or >500mm 500mm-450mm <450mm
Spring conditions Above average spring-autumn rainfall Dry spring hot finish Drought
Establishment conditions Slow - i.e. cold wet conditions Quick - i.e. warm dry conditions
Stubble management No-till, stubble retained Burnt only Tilled and burnt
Tillage Press wheels, raised beds, cloddy seed bed
Full disturbance sowing compacted seedbed
Grazing livestock No sheep in enterprise Sheep in stubbles
Soil Soil with improved moisture holding capacity; i.e. increased clay content and organic matter
Poor moisture holding capcity; i.e. sand no OM
Weeds Summer volunteers
No volunteers
Crop establishment TT varieties hybrid varieties canola seed >2mm
Previous paddock history Slug damage
Beans/canola
Sclerotinia
Clean cereal crops No slugs
Poor cereal crop
No sclerotinia

Table 4: Risk factors for snail outbreaks.

Factor High risk Reduced risk Low risk
Annual rainfall Above average autumn and summer rainfall
Drought
Stubble management No till stubble retained Tillage or burnt only Tillage and burnt stubbles
Grazing livestock No sheep in enterprise Sheep on stubbles
Soil Alkaline calcareous soils Un clayed non wetting sandy soils Acid soils with low organic matter
Weeds Summer volunteers/
Brassica weeds
Previous paddock history of snails

No volunteers

No history of snails
Previous paddock history Snails appear to build up most rapidly in canola, field peas and faba beans but can feed and multiply in all crops and pastures Clean cereal crops Poor cereal crop

Other considerations

Baits should be considered as crop protectants. Cultural methods are required to reduce populations and the biological function of farming systems needs to be considered. Baiting for snails occurs at a different time to slugs. If you have both snails and slugs, one bait in April may not be good enough. Assess your risk and context in which you are trying to control these pests. Different products will suit different situations. 

Snails

For snails consider crop rotation in high risk situations; bait snails in the summer/autumn when actively feeding which depends on moisture. Use cultural methods to control when hot. Consider what other pests are eating the snail bait. Metaldehyde baits are not reliable to control snails in crop as they do not work as well when cold. 

Slugs

For slugs consider crop rotation in high risk situations; if growing canola sow early, ensure quick establishment by using either hybrid seed or grade open pollinated varieties (<2mm seed), surface apply bait directly after sowing and rolling, re-monitor and re-apply cheap bait in high risk situations or use a better quality product. Consider use of foliar N to seedlings (GS 1.4) to ensure quick establishment in lower risk situations as a follow up after baiting at sowing.  

Useful resources


Ground Cover TV episodes on slug monitoring and bait timing

References 

Carrick, R. 1938. The life history and developement of Agriolimax agrestis L. the grey field slug. Transactions of the Royal Society of Edinburgh, 59, 563-597.

Nash, M. A., Thomson, L. J. & Hoffmann, A. A. 2007. Slug control in Australian canola: monitoring, molluscicidal baits and economic thresholds. Pest Management Science, 63, 851-859.

South, A. 1982. A comparison of the life cycles of Deroceras reticulatum (Muller) and Arion intermedians (Normand) (Pulmonata: Stylommatophora) at different temperatures under laboratory conditions. Journal of Molluscan Studies, 48, 233-244.

Acknowledgement

We would like to thank Jon Midwood and Paul Breust, SFS; Ken Young and Jen Lillecrapp, GRDC; Allan Mayfield, SAGIT; and Felicity Turner, MFMG, for comments and intellectual input.

Funding for this work was provided through the GRDC Project DAS00134 and SAM00001 and their support gratefully acknowledged.

Contact details 

Michael Nash

SARDI Entomology Unit
GPO Box 397, Adelaide SA 5001
08 8303 9537
michael.nash@sa.gov.au
@merindie1