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Using Precision Agriculture To Increase Slug Bait Efficacy

GRDC Fast Track Project Code: 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 in Victoria causing damage to emerging crops especially canola. Most growers have implemented an integrated pest management strategy that includes burning crop residue and disc ploughing soils to reduce slug habitat, and 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 farmers 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 farmer 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 six leaf stage. Initial counts were used to design trial layout, 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 with insecticides. Those being Lannate Lɸ (472g/L methomyl) alone or Lannate L plus Regent (200g/L fipronil) with both applied as a concentrated drench in a band equivalent to registered field rates.

ɸ There is currently no Lannate L or Regent products registered for use on snails/slugs in any crops.

Results

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 (Figure 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 at 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 versus spray model estimate -2.85 ± 0.52, Z = -5.45, P < 0.001; spread versus 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 versus Meta model estimate -0.90 ± 0.85, Z = -1.05, P = 0.54. Lannate L by itself or mixed with another product was significantly less effective than either bait product: Metarex versus Lannate L model estimate -3.36 ± 0.76, Z = -4.45, P < 0.001; Meta versus Lannate L 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 (Figure 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 2 and 4 WAA and canola seedlings m-2 response at 4 WAA to treatments applied in Jun-July 2015 at Roseneath Vic AUST. Slug pellets were applied on top of seed row (banded) at various rates (kg/ha) or spread (S).

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

ɸMetarex label rate is 5-8 kg/ha and Meta label rate is 5-7.5kg/ha. Growers should always follow label recommendations.

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 project.

Research has been reported by GRDC at the following links

https://grdc.com.au/resources-and-publications/groundcover/ground-cover-issue-121-mar-apr-2016/slugs-best-beaten-by-baiting-at-sowing

https://grdc.com.au/resources-and-publications/grdc-update-papers/tab-content/grdc-update-papers/2016/08/new-insights-into-slug-and-snail-control

https://grdc.com.au/news-and-media/news-and-media-releases/south/2015/05/merging-precision-ag-and-slug-control

https://grdc.com.au/resources-and-publications/grdc-update-papers/tab-content/grdc-update-papers/2016/07/new-insights-into-slug-control