New insights into slug and snail control

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?

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.

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.

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.

Addressing some specific questions that arose from 2015

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.

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 (R_m ) (Table 2). With values obtained comparable with previous studies; R_m = 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 (R_m) of grey field slugs raised on various crops under laboratory conditions (100% rH, 10-14°C).