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This article was published in 1954
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INSTITUTE OF INSPECTORS OF STOCK OF N.S.W. YEAR BOOK.
Recent Developments in Molluscacides
M. ROBINSON, B.V.Sc., Veterinary Research Officer, Glenfield.
(Now Inspector of Stock, Cooma)
Ever since the inception of copper sulphate as a molluscacide it has been realised that this chemical had certain serious drawbacks: not least among these being the rapid rate at which the concentration fell below the amount required to kill snails. Furthermore, in alkaline water it rapidly formed basic copper carbonate, which is insoluble.
Research into new molluscacidal compounds started in the U.S.A. in 1947 and by late 1948 some eighty compounds had been found which showed molluscacidal activity. Most of these compounds were phenols or closely related chemicals, including naphthoquinones and benzolquinones. The molluscacidal activity of phenols has been known since at least 1932, when Humphreys used a commercial phenol compound to prevent the spread of schistosomiasis in Anglo-Egyptian Sudan. Batte and Swanson (1952) reported similar favourable results against lymnaeid snails, the intermediate hosts of liver fluke.
Trials were carried out (M. O. Nolan, Howard W. Bond and Eliz. R. Mann) using Australorbis glabratus, intermediate host of Schistosoma mansoni, this snail proving to be one of the most resistant to chemicals, of which 1800 were screened (701 of them being phenols or related compounds).
As the snails are aquatic and cannot live for long periods out of water, all chemicals used had to be in the form of aqueous solutions. Compounds which did not dissolve readily had Tween 80 (polyonyalkalene derivative of sorbitan monooleate) added to them and it was found that such solvents at the dilutions used, had no toxic effect on the snails.
All chemicals were tested at a standard dilution of 10 parts per million and at the end of the contact period snails were washed and replaced in aerated fresh water. Food was made available, and on the following day mortality rates were determined by microscopic examination. If snails were moribund or not fully recovered at this examination, they were held for further daily observations until death or revival occurred.
Ten control snails were held for each group tested with a chemical.
Criteria for death were absence of heart beat and lack of response to prodding with a curved blunt needle that could be inserted into the curved shells of the snails, as far back as inner whorls if the bodies of the snails were so deeply retracted.
Recovery was based on the snail's ability to move about or cling to the bottom and sides of the glass container.
As a result of these screening tests, it was found that the phenols contained a large number of molluscacides, and although the lower halogenated phenols were ineffective, the higher ones were among the most efficient tested. Bromine seemed a little more effective than chlorine.
Finally a number of metallic salts of pentachlorphenol were made and all were 100 per cent. effective at 10 ppm. The results with these at 5, 3 and 1 ppm. closely paralleled those of pentachlorphenol itself.
Later, pentachlorphenol, Cu pentachlorphenate, tribromphenol, natriodo phenol and triiodophenol and "Santobrite" (sod. P.C.P. 79 per cent.; other sodium salts of chlorphenols 11 per cent.) were tested for toxicity. Symptoms resembled a very sudden acute D.D.T. poisoning.
Field trials with pentabromphenol, Na PCP and other chemicals showed that the two former were the only ones which possessed any moluscacidal activity in the field.
Emulsions of Na PCP were not as effective as acetone solutions.
Berry, Nolan and Gonzalez, working in Puerto Rico on the control of schistosomiasis, used Copper P.C.P. as a fine suspension in alcohol or mixed with talc and a wetting agent. It had the advantage over other compounds in that it destroyed large mats of algae and thus facilitated its own penetration. It was very toxic to fish.
Sodium P.C.P. also was used and produced good kills, but was not as effective as the bromphenols; pentabromphenol 246 tribromophenol; or copper P.C.P. (which seems unusual as the Cu P.C.P. is insoluble, whereas Na P.C.P. is highly soluble). It is very toxic to fish even in concentrations as low as 1 in 100.
Toxicity trials on various animal species indicated that sodium P.C.P. in concentrations up to 10 ppm, was satisfactory.
N.S.W. TRIALS
These were carried out at Oberon, N.S.W., in conjunction with parasitologists of the McMaster Laboratory, CSIRO, Grazeos and Timbrols.
The first trials used Cu P.C.P. as a 2 per cent and 10 per cent dispersible powder: with talc as the spreading agent. "Wafex" (Na sulpholignate) was incorporated at the rate of ¼ per cent, and 1% per cent respectively. This acts as a sticker and has some wetting properties. The size of the powder was approximately 200 mesh.
Five areas were treated with Cu P.C.P., using a knapsack duster.
RESULTS TO DATE ARE AS FOLLOWS:
AREA 1.
7.5 lb/acre - original snail population low - 24 hours later some dead snails found. Live snails found in lower_reaches but were sluggish and did not respond well to tactile stimulus. Thirteen days later only small number of live snails seen - definite reduction in population.
AREA 2.
3.6 lb/acre - moderately heavy in wet areas - 24 hours later no live snails found. 13 days later no live snails and still many dead ones.
AREA 3.
2.5 lb/acre-few snails, but very heavy vegetation - 24 hours later, some dead snails but many live ones. Thirteen days later some dead snails but many live ones.
AREA 4.
2 lb/acre-boggy stream but little vegetation - 24 hours later, many dead snails, some live ones, but very sluggish. Thirteen days later many live snails - no real reduction in population.
AREA 5.
Bluestone 25 lb/acre - no reduction at all.
AREA 6.
Bluestone 40 lb/acre - no reduction at all.
AREA 7.
This area was treated only round periphery to see if Cu P.C.P. would leach into area over a period and bring about gradual toxic effect. No apparent result.
Results (Summary)
Cu P.C.P. acted more slowly than CuSO4. Higher rates of Cu P.C.P. were more effective.
Cu P.C.P. seems to have a residual toxic effect. Results 14 weeks after treatment:
Snails wiped out in area 2 - 3.6 lb/acre. Snails very reduced in area 1 - 7.5 lb/acre.
Low application rates broke down. Further trials using Na P.C.P. and Cu P.C.P.:
Three areas treated with Na P.C.P. - 2 per cent. and 10 per cent. dusts. Four areas treated with Cu P.C.P. - spray.
Area 1: Na PCP 3 lb/acre. Good kill in 1 hour and in 24 hours majority of snails in area dead. Fairly good kill in stream below treated area.
Area 3: Na PCP 1.5 lb/acre. Snail concentration varying from heavy to nil - 24 hours - some dead and some alive.
Area 4: Cu PCP. Densely overgrown spring. 10 lb/acre. Light population - only few dead snails seen-no live ones.
Area 5: Cu PCP 10 lb/acre. Heavy snail population (Area 4 in previous tests) 24 hours only dead snails found. Dead snails found downstream from treated area.
Area 6: Cu PCP. Densely overgrown - not much free water. Heavy snail population. 4 lb/acre. All snails seen at 24 hours were dead.
Area 7: Cu PCP. Boggy spring draining over grassy patch and into a channel. Very heavy snail concentration. 3 lb/acre. 24 hours; many dead snails found.
Comments on above:
Na PCP lethal to numbers of snails within 1 hour. Cu PCP slower than Na PCP in action. Higher rates of application of Cu PCP more rapidly effective than lower rates.
On 16/3/54 the property was visited again and the results of treatments put down ten days previously were determined. These treatments consisted of treating all boggy and swampy areas in a large paddock, using a mixture of Cu PCP and sand 1 to 25 and distributing this material by means of a super-blower. Some areas which were covered with dense vegetation were sprayed with weedicides. Application rates of the Cu P.C.P. were in the vicinity of 12 to 15 lbs. per acre. At the time of treatment most of the areas were quite wet, but at the time of re-examination several areas had dried up and it is possible that some snail mortalities were due to this desiccation. Some ten areas were examined and only two live snails were discovered; one in each of two areas. In one of these areas at least it seemed evident that small patches of ground had been missed with the treatment.
Several of the superficially dried areas were dug over with a spade, and although the ground six inches down was quite damp and in some instances water would spring into the bottom of the hole, no snails could be detected in the ground.
It seems reasonable to assume that snails can penetrate only into the underlying mud to any depth while the soil is covered with water and in a semisolid state. Any dead snails found on the surface of dried-up areas almost certainly were killed by the molluscacides and not by desiccation.
Two other areas, one which had never been treated and one which had not been treated for seven months (and which on previous occasions had contained snails), were both carrying a medium to heavy population. It now remains to be seen how long these effectively treated areas will remain snail-free, and whether the cost of treatment at the application rates used will be economical.