Local insights into slug control

Summary of research

The context in which slug control is applied needs to be understood for successful control. 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. Due to differences between regions, this paper summarises information presented at GRDC adviser updates on current snail and slug research, but the main aim is to provide specific information to growers in western Victoria (VIC). The results from local research are presented in detail. 

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 and understanding factors that limit field life. Recommendations on bait degradation are:

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

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.  For grey field slugs the question remains: what caused a reduction in their numbers? There were two theories discussed via twitter, email, etc.: seasonal conditions meant it was too dry in the 2014 spring for slug numbers to increase, or the pendulum had swung in favour of natural control, such as an increase in parasites. What is certain is spring baiting and early autumn baiting prior to sowing rarely prevent damage to emerging crops (Nash et al., 2007); pre baiting does foster complacency. 

Slug activity in 2015 has been observed in six paddocks across southern Australia using cameras (see here). Environmental data were also collected: soil moisture and temperature (10cm), ground leaf wetness, temperature and relative humidity. Rainfall and barometric data were obtained from BoM and soil moisture from Southern Farming Systems through the soil moisture probe network (see here). Analysis of camera data indicated that slug activity was reduced during colder conditions, thus damage observed was less in 2015 for most areas. Initial results indicate soil moisture at 50-60cm depth is positively associated with grey field slug activity, suggesting more than 75mm of rainfall in the autumn that wets the soil to depth may be a rule of thumb for slug activity. Ongoing research will test this and provide regionally specific information to improve baiting programs. 

The ongoing research questions looking at slug ecology and biology are:

• 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?

Using precision agriculture to increase slug bait efficacy (GRDC fast track project: SAM0001)

Take home message 

The most important thing is getting the bait on at the right time and that is at sowing.

Background

The black keeled slug and grey field slug are common pests of the high rainfall zone (HRZ) in VIC causing damage to emerging crops, especially canola. Most growers have implemented an IPM strategy that includes burning crop residue and disc ploughing soils to reduce slug habitat. This is combined with baiting to reduce slug numbers to a safe level to allow the establishment of canola crops. These control techniques have led to an increase in potential erosion from ploughing and loss of stubble to protect the soil, as well as nutrition losses from burning residues. Some committed no-till growers have resisted the temptation to plough and burn their paddocks, but are spending far more money on baiting strategies to protect their emerging crops with variable results. Hence, we ran trials in 2014 and 2015 to test whether increasing the number of bait points around the emerging crop, by placing the baits immediately above the crop row, is more effective compared to  spreading the same rate over the entire field, as is the common grower practice at the moment. 

Aim

Test for increased slug bait efficacy by concentrating the bait over the crop row, hence improved crop establishment in no till cropping systems.

Methods 

Slug activity and density were estimated using tiles, with numbers assessed pre sowing and treatments. Post sowing assessments were made every four days, weather dependent, and presented as Weeks After Application (WAA). Seedling density was estimated until 6-leaf stage. Initial counts were used to design trial layout, and treatments applied in strips that best intersected slug populations. Plots were systematically laid out at distances greater than 10m to ensure statistical ‘independence’. Width of treatments was determined by the machinery fitting controlled traffic tramlines but was wide enough to ensure plots were independent as determined by spatial analysis. Four trials (three canola, one sunflower) were run in total over two seasons (2014 and 2015) with results presented from one trial, although overall conclusions were the same. Treatments were applied at sowing with baits (Meta and Metarex) applied either in a band (three rates) or surface spread (S, single rate) or insecticides, those being Lannate® (472g/L methomyl) alone or Lannate® plus Regent® (200g/L fipronil) with both applied as a concentrated drench in a band equivalent to registered field rates. 

Results: Trial 4 canola sown 11 June 2015

Initial counts on 11 June 2015 indicated 2.2 (s.d.= 5.5) black keeled slugs per tile. Slug populations were considered lower than previous seasons at this site by the grower. The population was significantly aggregated (Ia = 1.59, P = 0.0087), with significant clump (mean vi = 1.69, P = 0.0013) in the middle of the trial; hence the treatments were laid out in a way to best intersect those areas. Black keeled slug numbers declined during this trial in the untreated plots (Fig 1), which was associated with declining soil moisture at 500mm depth. From the regression analysis, the equation was slugs/refuge = 20 (s.d. 8.6) multiplied by mm water @ 500mm soil depth – 808 (s.d. 353), R2 = 0.40, F = 5.3, P = 0.050, N =11). 

Combined data indicate baits either applied in a band or evenly spread were not significantly different (model estimate 0.13 ± 0.87, Z = 0.15, P = 0.99).  The insecticides applied as a drench along the seed row were significantly less effective than either method of applying baits: banded vs spray model estimate -2.85 ± 0.52, Z = -5.45, P < 0.001; spread vs spray model estimate -2.72 ± 0.78, Z = -3.47, P = 0.001. The two bait products were not found to be significantly different in efficacy: Metarex vs Meta model estimate -0.90 ± 0.85, Z = -1.05, P = 0.54. Lannate by itself or mixed with another product was significantly less effective than either bait product: Metarex vs Lannate model estimate -3.36 ± 0.76, Z = -4.45, P < 0.001; Meta vs Lannate model estimate -2.46 ± 0.53, Z = -4.66, P < 0.001.

Canola was slow to establish, however once data was transformed (log(x+1)) overall significant differences between treatments (repANOVA F10,44 = 3.256, P = 0.003) were observed (Fig 1). However, slugs were not the only cause of seedling loss as indicated by poor fit of the logistic regression (R2 = 0.07; F 1,53 = 4.06; P = 0.04) and a strong linear pattern in the residual plots. No further analysis was conducted, due to the confounding influence of other pests, including earwigs, on canola seedlings.

Figure 1: Abundance of black keeled slugs per refuge pre-treatment and two and four WAA and canola seedlings m-2 response at four WAA to treatments applied in June-July 2015 at Roseneath, VIC. Slug pellets were applied on top of seed row (banded) at various rates (kg/ha) or spread (S). 

Conclusions

The targeted placement of bait around emerging seedlings gave no significant improvement in protecting the crop from slugs. Canola can be established with disc seeders into stubble in the HRZ as long as some basic rules around timing of bait application are followed.  Soil moisture at 50-60cm was associated with increased grey field slug activity at the soil surface as recorded by using surface refuges during this Fast Track project. 

Further considerations for successful slug control

• Change crop rotation in high risk situations; look at linseed or faba beans as an alternative break
• If growing canola sow early, ensure quick establishment by using either hybrid seed or grade open pollinated varieties (<2mm seed). Look at the 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.
• Under no-till and wet situations, do not use bran based products that contain 1.5 per cent metaldehyde
• Under cold conditions, look at using products that contain iron chelate, when no rain is expected.

Golden rules for successful slug control in western Victoria

• Quick crop establishment
• Roll then bait at sowing
• Follow up monitoring of emerging seedlings at least every couple days; watch for a wave of slugs mid late May especially if wet
• Re-bait in problem areas as needed depending on weather.

Useful resources

Pestnotes on snail and slug biology

Ground Cover TV episodes on slug monitoring and bait timing

Slug control: Slug identification and management

Acknowledgements 

We would like to thank Jon Midwood and Paul Breust at Southern Farming Systems for comments and intellectual input.

The research undertaken as part of this project is made possible by the significant contributions of growers through both trial cooperation and the support of the GRDC.

Contact details 

GPO Box 397, Adelaide SA 5001
08 8303 9537
michael.nash@sa.gov.au
Michael Nash's Twitter account