Using copper sulphate to control snails

Managing rice snails
with copper sulphate
Mark Stevens1 & Greg Doran 2
NSW Department of Primary Industries, Yanco Agricultural Institute
School of Agricultural and Wine Sciences, Charles Sturt University,
Wagga Wagga
1
2
QUICK TAKE
›› Snails in rice crops are becoming a more significant problem
for growers because of increased levels of repeat cropping
aimed at maximising water use efficiency. Repeat cropping
allows dormant snails to survive in the soil.
›› Research on copper sulphate aimed at gaining product
registration and ensuring its ongoing availability for snail control
has shown that its variable performance relates strongly to soil
type. Higher application rates are needed to the water above
soils rich in dissolved organic carbon.
›› Even above soils low in dissolved organic carbon, biologically
active copper concentrations fall dramatically within an hour
of application.
›› Although soil testing could allow copper application rates to be
‘fine-tuned’ for individual fields, finding alternative chemicals
Snails in rice crops can be effectively managed through crop
rotations, however this conflicts with the need for farmers to
minimise total farm water use by using water remaining in
the soil profile from the previous crop. Chemical control is the
only other option for snail control, and copper sulphate is the
only chemical currently available. Although generally effective,
its behaviour in the environment means its use has several
drawbacks.
Crop rotation practices play a significant role in the extent of snail
problems in rice crops. Research has shown that when crops are
drained prior to harvest a proportion of snails enter dormancy in
the top layers of the soil, and survival through to flooding of the next
crop in spring can be around 45%. Dormant snails cannot survive the
additional 12-month period if a summer fallow is used, and consequently
breaking the cropping cycle has been an effective method of reducing
snail problems during crop establishment.
Rice crop rotations come at a price, however. Repeat crops use
approximately 10% less water than crops on ‘new’ ground because
water remaining in the soil profile reduces the amount of water
required for initial bay filling. The need to conserve water has led to
increased repeat cropping and consequent increases in snail problems.
Copper sulphate has been the only chemical used against snails in rice
since they first became a major problem in the mid-1970s. Its use was
legitimised under a number of APVMA permits, however around 2006
the APVMA indicated that no further permits would be issued and
that they required copper sulphate to be registered. This necessitated
research on the efficacy and environmental fate of copper in rice fields,
and copper sulphate gained formal registration prior to the 2012–13
rice season. The results of this work have helped us understand why
copper performance can be so variable between different areas.
Field trials
Trials conducted at Yanco using snails in stainless steel enclosures
proved that copper is effective against adult snails, with conditions at our
trial site requiring around 6.4 kg/ha of copper sulphate pentahydrate for
95% snail mortality. This is at the lower end of the 6–12 kg/ha rate
range that has been in use for many years.
Of particular interest was the rate at which dissolved copper
concentrations declined in the water in the rice field, as only free ionic
copper has significant activity against snails.
unaffected by soil type should be a higher priority.
The native snail Isidorella newcombi is responsible for over 95% of damaging rice
crop infestations.
IREC Farmers’ Newsletter – Large Area No. 189: Spring 2013
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snails in rice
sediment copper concentration (mg/kg)
30
Whilst total copper (dissolved and bound to particulate matter in
the water column) fell by an average of 13.7% in the first hour after
application, the corresponding fall in dissolved copper was much larger
at 46.2%. Of the dissolved copper that remained only a proportion
would be ionic copper with activity against snails; the remainder would
have been bound up with dissolved organic carbon compounds.
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26
24
22
One of the major drawbacks of copper is that it remains in an active
form for only a very limited time, necessitating a high application rate to
expose snails to a lethal dose in a very short time frame.
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18
So where does the copper go?
16
14
12
0
2
4
6
8
Figure 1 shows the copper concentrations in sediment samples from
the bottom of our trial bays 30 days after copper application.
Trial 1
Trial 2
r 2 = 0.91
10
12
copper sulphate applied (kg/ha)
Figure 1. Effect of copper sulphate application on sediment copper
concentrations 30 days after treatment.
dissolved copper (mg/L ±­SE)
1.0
Murrami
Yanco
Leeton
0.8
Factors affecting site to site variability
To examine how local conditions might be affecting the performance
of copper against snails, we collected three soils from Yanco, Leeton
and Murrami, and used them in soil/water systems to bioassay rice
snails against copper.
0.6
0.4
We found that approximately twice as much copper was needed to
kill 90% of the test snails above the Murrami soil than above the other
two soils.
0.2
0.0
0.1
1.0
10.0
100.0
hours since addition (log scale)
Figure 2. Dissolved copper in the water column of soil/water systems
involving three different test soils. Starting copper concentration 0.8 mg/L,
irrigation water.
250
snail LC90
soil DOC
overlying water DOC
200
Initially, we thought that the dissolved copper might be adsorbing to
the soil surface. To test this we set up large numbers of soil/water
systems, treated them with copper, and measured the dissolved
copper concentrations over time.
Our results are shown in Figure 2, and were essentially the opposite
of what we expected—more copper was remaining in solution above
the Murrami soil than above the other soils, so losses due to adsorption
to the soil surface were not the answer. The copper remained in the
water column, but its toxic effect was somehow being neutralised.
None of the measured soil or water parameters such as pH, clay
content or water hardness correlated with our toxicology results, so
we looked at dissolved organic carbon (DOC).
Analyses of the soil, and also of the water in the soil/water systems
at the time the snail bioassays were conducted showed that variable
levels of dissolved organic carbon in the soil are almost certainly the
main cause of variable copper performance across different sites.
units
150
100
50
0
There is a strong linear relationship between the amount of applied
copper and the subsequent sediment copper concentration—at the
high end of the scale, an application of 12 kg/ha of copper sulphate
has almost doubled the sediment concentration from a baseline
concentration of 15 mg/kg to about 28 mg/kg. On average, only 22%
of applied copper was recovered from the sediments 30 days after
treatment, however all applied copper not removed at trace levels in
the crop will ultimately find its way into the soil. In the longer term this
has the potential to cause sustainability issues.
Murrami
Leeton
Yanco
Figure 3 shows the toxicology results combined with the dissolved
organic carbon levels of the soil and water. The Murrami soil contained
around three times as much extractable dissolved organic carbon as
either the Leeton or Yanco soils, and three days after inundation of the
soil/water systems, the dissolved organic carbon levels in the water
were twice as high above the Murrami soil relative to the other two
test soils.
soil type
Figure 3. Relationship between the amount of copper necessary to kill 90%
of test snails (LC90), soil dissolved organic carbon (DOC), and DOC in the
overlying water of soil/water systems.
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IREC Farmers’ Newsletter – Large Area No. 189: Spring 2013
snails in rice
Insights into copper behaviour
Aside from facilitating the registration of copper sulphate and ensuring
its ongoing availability to rice growers, this project has provided insights
into how copper behaves in the rice field environment.
Copper applied to control water snails remains active for only a very
short period of time, and ultimately ends up in the soil. Adsorption
to the soil surface appears not to be the main factor affecting copper
efficacy. Rather, leaching of dissolved organic carbon into the water
column from the underlying soil is apparently a key factor, and leads to
the copper losing its biological activity whilst still in the water column.
Crops grown on soils high in dissolved organic carbon will require
copper application rates at the higher end of the 6–12 kg/ha allowable
rate range to compensate for this effect. Although soil dissolved organic
carbon is clearly important, dissolved organic carbon and hardness
levels in source water are also likely to have a role in mitigating copper
toxicity.
Although it is difficult to generalise, darker soils rich in organic matter
are likely to be more problematic when snail control is attempted with
copper sulphate. With further research it may be feasible to develop
a model that, along with soil testing, would allow growers to optimise
the copper sulphate application rate within the registered range for
individual paddocks, however this would not overcome the underlying
problems associated with copper sulphate, including its limited
persistence in a biologically active form even when used above soils
low in dissolved organic carbon, its lack of effectiveness against snail
eggs, and its capacity to accumulate in soils.
Identifying and commercialising alternative snail control chemicals
should continue to be a high priority.
RIRDC Project PRJ-005685
Further information
Mark Stevens
T: 02 6951 2644
M: 0477 047 216
E: mark.stevens@dpi.nsw.gov.au
Simple soil/water systems were used to determine how snails respond to applied copper above different soils, and to gain an
understanding of how soil type affects copper toxicity.
IREC Farmers’ Newsletter – Large Area No. 189: Spring 2013
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