Garden Pests & Diseases

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Slugs and snails

Among exotic ornamental plants these are usually the only serious threats, the strong rapid growth of the plants being sufficient to cope with most other problems most of the time. Slugs and snails, however, can destroy whole plants, particularly when they are young.

The commonest method used by gardeners to combat this menace is to sprinkle slug pellets around those plants particularly prone to attack, usually far more than are necessary or recommended by the manufacturers. Professional advisors these days recommend always covering them with a stone or slate to prevent birds, hedgehogs and pets from taking them (and being poisoned), or even to confine their use to underground to destroy the especially harmful small keel slug which rarely comes to the surface. Unfortunately, the latter method still leaves our exotic plants prone to above ground attack.

A second method advocated by many advisors and practised by many gardeners wanting to avoid poisonous chemicals is to trap the slugs. This can be done with "beer" traps, which are tin cans or even old plastic yoghurt pots with holes cut just below the lid, half filled with milk, dilute beer or lemonade, and sunk into the ground, so the slugs, attracted by the smell, fall in and drown. The top must be covered to keep out the rain, but with one or more openings to admit the slugs.It is also done by simply leaving pieces of orange peel on the ground and collecting the slugs from it after dark when they are feeding. The beer trap method suffers from the serious problem that it also catches and drowns ground beetles, which are wonderful predators, eating large quantities of pests including slugs, so beer traps can do more harm than good as slug controls in the long term unless certain precautions are taken. The best way to use them is to ensure that the opening is at least an inch above the surface of the soil and has a vertical smooth side which ground beetles cannot climb. This will be no obstacle to slugs, so they will still be caught but the beetles will not.

All the above methods, even the pellets, suffer from a fundamental flaw in ignoring the biology of slugs and snails. These creatures in fact control their own population in a way which totally defeats the methods described. They lay enormous quantities of eggs, most of which remain tiny, relatively harmless juveniles. As slugs move about, they leave behind trails of slime. Only when the density of slime trails is diminished will more juvenile slugs and snails develop - which is exactly what happens as soon as a significant number of the creatures has been killed, so the numbers are replenished almost immediately.

There are now two methods of control I am aware of that counteract this problem. One is to apply aluminium sulphate, sold under various brand names for slug control (and under quite different names and higher prices for changing the colour of hydrangeas!). This substance can either be sprinkled on the soil around endangered plants, or can be dissolved in water and watered on the soil in the area. It affects only slugs and snails (and, presumably, hydrangeas), and kills them (but not, presumably, hydrangeas) by interfering with the slime producing organs. The reason it is effective is because it kills not only adult slugs (like pellets) but also the juveniles and even unhatched eggs. After a few applications it will more or less clear a small area. Of course the cure is only temporary, because there are always more not far away which will soon move in, so repeated applications are necessary, especially after rain, which washes it away. Unlike pellets and traps, it does not attract the pests but is just a contact killer, so more thorough coverage of the ground is necessary. The powder can be put in a continuous ring around a particular plant to ensure protection. Also unlike pellets and traps, you do not have the satisfaction of seeing the dead slugs, because they die underground, out of sight, even if they take the poison on the surface.

The second effective method (I am told - but see my comments below) became available in 1995 at a very high price, and is now sold for a little more than half the cost it was then (but still more expensive than pellets). This is a biological control called Slugsure (change of name since last year), which is a nematode. It is sold as a powder to mix with water and spread on the soil with a watering can. Like the aluminium sulphate it acts both on and beneath the surface and can almost entirely rid the area treated of slugs. The effect lasts about 6 weeks, after which a new lot of slugs will have moved in from surrounding areas, so a repeat application is needed. It has little effect on snails and none at all on anything else. The supplier is Defenders, from Kent, UK. I have removed the full address and phone number from this page for reasons which will be obvious from the following paragraphs.

I am told (by them) that an associated company will take overseas orders, but I don't know yet who they are. They say they have their own web site, and said they would send me the URL with a supply of the product - the goods arrived, but no URL.

A pack to protect 40 square metres for about six weeks now costs 12.95 pounds sterling. A pack to cover 100 square metres costs 25.95 pounds. This is very expensive compared with all the alternatives, and that is not the only problem.

The shelf life is only a few days, so you have to be ready to use it almost immediately on receipt. Not a serious problem in itself, but of course this means you have to use the whole pack at once - not much use if you only want it to protect a few especially vulnerable and precious plants.

Only after opening the pack do you find the detailed instructions, which inform you among other things that it must be applied to bare soil, not to foliage (so how do you protect a large patch of hostas?), and furthermore that it is not suitable for use on heavy clay (which is all I have). I regard this as a con. I will not be buying anything again from that company. They sell a number of other chemical-free pest controls, some biological, some mechanical, but so far as I am concerned, they can keep them.



Neighbours' cats can be a real problem for the gardener, as I know from bitter experience. From what I have read I get the impression that the most common problem they cause is in scratching up seed beds and areas containing young plants, apparently in covering up deposits they have left. I have not experienced quite the same thing, mainly because the untrained cats (they can be trained always to use a provided sand tray) in our area don't bother with covering up the mess - they do it on bare soil, on the lawn and on top of plants such as heather, and then simply walk away.

A second problem is that they will hunt, often successfully, almost anything small that moves, such as birds, frogs, etc. If mice or rats are a problem this can be an advantage, but if you try to encourage birds, or hope frogs will help keep down your slugs, then the continual visits of hunting cats are highly undesirable. Even if not hunting, their presence will frighten birds away.

I have come across many supposed remedies, most of which I tried and found to be useless. Two I did not try because the cure seemed as bad as the disease were keeping a dog and keeping a cat of my own.

Recommended methods relying on smell included bits of orange peel, moth balls and green crystals manufactured and sold for the purpose - none had any noticeable effect on our local felines.

I was told that leaving a few large plastic bottles on their sides half filled with water would deter the cats because of the strange reflections in them - but I found the bottles placed in common cat pathways quickly became crushed by the weight of the cats walking over them as they entered the garden.

Cocoa shell mulch was recommended on the grounds that it is uncomfortable for them to walk on (it is supposed to repel slugs and snails similarly) - I found they selected the areas covered with it to use for toilet purposes in preference to other areas, and it also failed to deter slugs, although I would still recommend it as a simple mulch. It is excellent at keeping down weeds.

Both a catapult and a large water pistol caused them to run off in a panic, but it did nothing to deter them from returning - they simply learnt to flee as soon as they heard my hand touch the inside door knob.

This leaves the one method I have more recently tried which, so far, does work. This is an electronic device which uses infra-red to detect movement and then emits a sound outside the range of human hearing but intolerable to a cat. It does them no harm, but they quickly learn to avoid the area where it operates. After years of frustration, my garden at last appears to be virtually a cat-free zone. Long may it continue! It is claimed that the device has no effect on birds, dogs or other animals - this is certainly true so far as birds are concerned, and I have watched a local hedgehog trigger it but totally ignore it. It can be powered by either a 9 volt battery or via an adaptor by mains electricity. Mine are battery powered, but I find they run down rather fast when they are frequently triggered by birds feeding regularly in front of them or by shrubs in front of them waving in the wind.


Useful pest control links

The Sources for biological pest controls site gives the addresses of UK biological pest control suppliers, with a table showing which is cheapest for each product, and information about the controls available.

Wild animals can be a nuisance for your garden, as well; if you have some hanging around the yard, call in a humane animal control service to get them back to their natural habitat. [N.B. This is a commercial service which I have not used, but it seems a useful idea.]


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