British Cactus and Succulent Society

Highlands & Islands Branch

Natural Control

© Dr Alan Silverside - Paisley University

The difference between Biological Control and Natural Control as management strategies is that Biological Control involves human intervention whereas Natural Control does not; however, that does not mean 'doing nothing'. What it does mean is ensuring that the immediate habitat where we want these predators and parasitoids to operate contains what they need so that they can feed, breed, find shelter, and over-winter. In other words do things which will encourage them to take up residence. Firstly, this scheme of things can only apply when the scale of operation is small - as in a small to medium greenhouse and garden. It is quite impractical in a commercial operation where full-scale biological and chemical control is more appropriate. Most important, it means no chemical fertilisers or other interference which will cause them harm, and cause residue problems. They benefit if sources of nectar and pollen are available, and bushes or plants for shelter.

The author has adopted Natural Control since the time he had a very serious infestation of red spider mite on perpetual carnations. A certain spray was used (taking plants outdoors of course). Shortly afterwards a BBC radio gardening programme gave him cause for concern - so that was the end of using modern chemicals.

What can be done to implement the four needs of our 'garden friends'. Feeding is relatively easy as strategically placed bruised fruit and jam will attract them. Actually, many (even most) of them need some form of food (pollen) to enable fertile eggs to be produced. Umbelliferious flowers such as Cow Parsley (Anthriscus sylvestris) will please hover flies, because they also feed on nectar. The average gardener will probably not think much of Cow Parsley, but should be contented with the Poached egg plant (Limnanthes douglasii), a low-growing border plant with pleasant white-edged yellow-flowers, and described as nectar-rich. It seems that wasps and hover flies favour yellow flowers. Breeding takes care of itself provided enough of them are 'resident' so to speak. Shelter is more of a problem, especially over-wintering. Bushes, shrubs and tall, bushy plants are usually sufficient, although wooden structures like sheds, even inside the greenhouse(my way), pergolas, garden fences which are not sprayed, will all help. Over-wintering is more of a problem as a suitable type of shelter is required. Examples are shown for Ladybirds, and similar accomodation for Lacewings.

Does Natural Control work? Well, it does in the wild. Look at any wild wood where things are in equilibrium, that is, no interference from humans. The author's experience in a small garden is also exemplary. No sprays of any kind are used. As a result, he has had over 8Kg (18lb) of raspberries (Glen Proven) from five plants. Some are lost to raspberry beetle and birds, but not much. In fact, he finds the buzz of insects quite pleasant and redolent of summer. Some of the buzzing will be predators and parasitoids so it is not all doom. A few years ago, over 25Kg (56lb) of strawberries (Royal Sovereign) from a small patch which represents the yield (3.3 tonnes per acre) realised in Victorian Times when they fertilised strawberry fields at 25 tonnes of horse-manure per acre!! Also, heavy crops of fruit from five minarette trees on an bed just over two metres long. So Natural Control definitely works for him, and should for anyone else.

What can be done when infestations occur under Natural Control? It is assumed that the problem is identified early on, before it becomes serious. When mealy bugs are discovered it is possible to remove them with a water-colour-type paint-brush dipped in methylated spirit. The bug is picked off with the brush and put into a jar of warm soapy water. The brush is then re-charged with spirit and another bug picked off. It is desirable to pick off the woolly tufts as well as they contain eggs. A carefully aimed jet of water is effective in removing mealy bugs, too. For aphids it is quite effective to put the infected plant outdoors where wasps can get at it. The author has seen such an infection cleared up in less than half an hour by wasps (Vespa vulgaris). But the wasps need to be 'resident' so to speak - you cannot rely on chance visits. Red spider is more problematical as that relies on a limited range of specific parasitoids.

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