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.
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.