Current Edition: 26 July 2003
Farm Management
A Methaldehyde - bait is key to success
The molluscicide Methaldehyde has long
been regarded as an active ingredient that plays second fiddle to methiocarb. Most would say that this was correct based on its visual effectiveness under a range of field conditions. But DeSangosse, the manufacturers of Metarex which is based on methaldehyde, state that in a high quality baiting material this active it is as good as the best of the competition, with far fewer environmental side effects.
Andy Doyle reports
The image given to methaldehyde for decades was that it functioned as a desiccant and if slugs did not consume enough of it they could survive, especially in a damp climate. But DeSangosse R & D manager, Antoine Livran, says that for any molluscicide, a slug must consume a lethal dose or else it can recover. And to do this the slug must be active and the pellet must be palatable and water resistant. Basically for any slug pellet to be successful it must be attractive to a slug.
Antoine fully accepts that many formulations of methaldehyde fail on one or more of the three essential components of a molluscicide:
Process
There are three main production processes used for slug baits. These are:
1. Dry Process: uses bran or ground wheat mixed with actives, additives and glues - 30 minutes per batch
2. Steam Process: similar to the dry process but steam is used to help extrusion so fewer additives are needed which helps palatability - 30 minutes per batch
3. Wet Process (Metarex): uses durum wheat groats mixed wet with additives and preservatives, power pressed and then dried - 12 hours per batch.
The different processes indicate the complexity and cost of production and this is reflected in the cost of the final pellet. Pellets that become mushy easily lose their attractiveness and so have limited activity.
Additives and glues can be used to increase the working life of a pellet but these decrease its palatability. Antoine said that glues tend to stop the slug from eating the lethal dose but they will help the long-term integrity of the pellet. "Put simply," he said, "Production process influences efficiency with dry worst and wet best".
The spread characteristics of pellets are also important. Uniform size and density are needed to give uniform application across wide tramline widths. Antoine said he wants the Metarex pellets to give:
The best bait palatability
Great water resistance
No dust
A well polished and regular pellet
The best value for a quality pellet
Efficacy
It is important to know how to judge the efficacy of a molluscicide, Antoine commented.
Trials show that methaldehyde treatments only killed 20% of slugs on day one post-application but the remaining 80%, sickened on day one, were dead one week later.
Other products will show higher initial kill levels on day one but may not increase much thereafter.
Rate
The full rate of Metarex is 7.0 kg/ha. This supplies 35 pellets/metre squared. Antoine commented that a lower application rate is adequate if slug numbers are low but higher slug numbers need more pellets to get control of slugs. But, he said, it is important to have more baiting points (higher rate) to kill juvenile slugs, as they do not travel as far on the surface.
One pellet contains enough active to kill 17 slugs.
Antoine recommends retreating after 30mm of rain. This is not an issue of durability of the pellets but rather that rain, especially very heavy showers, could splash soil around the pellet thus making it less attractive to the slug.
Mode of action
Methaldehyde damages the organelles that produce the mucous released during movement and this process is irreversible. This means that there cannot be recovery.
"There is no problem with the active", Antoine said. "All actives will work if the bait is right." Neurotoxic products show death faster but methaldehyde will stop the slugs from feeding immediately after consuming the pellet.
While methaldehyde has been on the market for many years, its effect on the slug was really only discovered in 1995.
This work showed that it hits the slug's stomach very quickly and stops feeding instantly.
After this it makes its way around the body of the slug and hits the "feet" where it causes the slug to desiccate. Antoine said that weather (wet) may have some effect on survival but that this is the same for all products.
Desiccation is not the primary cause of effect. Methaldehyde destroys the slug's mucous membrane thus disabling its ability to move. This also hits its digestion and skin protection properties.
And visually it can cause the slug to produce excess mucous thus speeding up the dying process.
Methaldehyde has very little effect on non-target organisms like earthworms, beetles or leather-jackets. And like all pellets, colour is used as a deterrent for birds. Antoine said that birds do not take artificial colours and so pellets can be sold in a range of colours including blue, green and orange.
From a human perspective the pellets are very bitter to taste and so are very unlikely to be eaten.
The company
DeSangosse was established in the south of France in 1926, initially as a fertilizer company.
Almost 50 years later it moved into agchem and seeds and the current company was established in 1973.
In recent years it has become involved in the Home & Garden business and the rodent bait business. It is currently 44% owned by the DeSangosse family with the employees owning 56%.
Its business is mainly agriculture related. In 2000 the company had a turnover of e180 million.
At that point 68% of turnover came from the company's agchem distribution business, which distributes for many of the big manufacturers.
The company's own pest control products accounted for a further 31% of turnover and this involved products to control slugs, rodents and ants.
Chemical distribution
DeSangosse distributes agrochemicals for all manufacturers across all of France. The business has been built up for many years and is currently undergoing upgrading of its facilities to meet the most recent EU standards for chemical storage.
The bulk of the storage is conducted from its depot near Agen where it has 21,000 square metres of warehouse capacity.
A number of other centres more than double this storage capacity. Product is moved to and from the warehouses by forklifts, all of which are fitted with barcode scanners to show the specific shelf and the specific product being moved. The scanning devices send radio signals instantly back to computers so that the control room knows exactly what's where at any point in time.
This is very important with up to three million packages handled per year. This gives rise to 112,000 statements to be issued a year and 64,000 tonnes of product to be carried.
The new warehouses (600 metres squared each) are fitted with foam machines in the ceilings to help prevent fire. In the event of a fire, the doors close and foam can fill the full warehouse in less than six minutes. This can be done in three of the warehouses simultaneously.
Main points
Methaldehyde can be equally effective as other actives when it is formulated in a good bait;
Methaldehyde stops feeding immediately after intake;
It destroys the slug's mucous membrane thus rendering it immobile;
Death is caused because the mucous-producing organs are disrupted.
