Agriculture Reference
In-Depth Information
established
in Britain and Ireland for many years
(e.g. apple) often have benefi cial predators and
parasites (e.g.
Aphelinus mali
, a wasp parasite
on apple woolly aphid), introduced accidentally
or deliberately from their country of origin, that
reduce pest numbers. It is quite likely, however,
that for the more
recently imported
plant species,
there may not be appropriate predators or parasites
to control a recently introduced pest occurring on
the plant species in Britain and Ireland.
X
Some horticultural practices can
disturb natural
balances
. In a natural habitat such as a woodland,
a
climax population
of plants and animals
develops (see Chapter 3). Here, a complex
balance exists between indigenous pests and their
predators/parasites
. The
food webs
include
several types of predator/parasite found on each
plant species that control (but do not eliminate)
the pests. This development of food webs is
not achieved to such an extent in most gardens,
since a natural succession of wild plant species
mentioned above is not desirable to gardeners who
are aiming for optimum production of edible crops
or for a pleasing aesthetic layout of decorative
plants free from weeds.
X
Regular
movement or removal
of cultivated
plants without particular thought to the natural
balance between predator/parasites and pests
will make pest attacks more likely in the garden/
nursery situation. For example, if a gardener stops
growing the 'poached-egg plant' (
Limnanthes
douglasii
), the number of useful hoverfl ies (feeding
on its pollen) may be reduced.
X
The
removal
of the rotting hollow stems of
herbaceous perennials and branches of decaying
wood, which are common sheltering sites for
parasitic wasps, predatory beetles and centipedes,
may reduce the potential for control of pests.
X
The lack of good
soil structure
(see p. 147)
resulting from poor cultivation or inadequate
incorporation of organic matter in a garden may
hinder the movement of useful predatory animals
such as centipedes in their search for underground
soil pests.
X
A poor physical preparation of soil and lack of
attention to
pH and nutrient
levels in soil may
result in poor soil microbial action (see p. 151).
X
The
repeated planting
of crops or ornamentals
into the same area of soil often leads to serious
attacks of persistent soil-borne pests or diseases.
Notable examples are club root disease on
brassicas (see p. 256) and potato cyst nematode
pest on potatoes (see rotation, p. 192). A
comparable unbalanced situation is found when
young trees and shrubs (such as roses) are planted
into a soil previously occupied by an old specimen
of the same plant species, with the resulting
problem called '
replant disease
' often caused by a
high level of nematodes (see p. 246) and
Pythium
fungus.
X
The unconsidered use of
pesticides
may result in a
rapid decrease in predators and parasites and may
considerably delay their appearance and build-up in
the following growing seasons.
Examples of biological control
species
There are two sources of 'natural enemies' to pests
(and occasionally diseases): the
indigenous
(i.e.
they are locally present, in wild plant communities in
Britain and Ireland) species and the
exotic
ones (from
other countries). Garden pests may be controlled by
predators
that eat the pest, or by
parasites
that lay
eggs within the pest (see also
food chains
, p. 38).
These beneficial organisms are to be encouraged,
and in some cases deliberately introduced. A range
of important organisms useful in horticulture is now
described.
Indigenous predators and parasites
Wild birds can contribute greatly to the control of
horticultural pests. A pair of
blue tits
(
Cyanistes
caeruleus
) can consume 10,000 caterpillars and a
million aphids in a 12 month period. They will also eat
scale insects, which are otherwise quite difficult to
control. The installation of tit boxes (Figure 16.6a) is a
worthwhile activity.
Hedgehogs
(
Erinaceus europeus
) belong to the
insectivore group of mammals. Although their
preferred diet is beetles, caterpillars and earthworms
(up to 200 g per day), they will also eat slugs.
Sometimes saucers of water or half-strength milk
are placed in the garden for them to drink during
dry summer periods. Care must be taken that they
are not exposed to dead slugs which had previously
consumed slug bait containing methiocarb or
metaldehyde, as these slugs will be toxic to the
hedgehog. Placing slug pellets in containers that
prevent hedgehog entry is strongly recommended
(see p. 229). Hedgehogs are encouraged to enter
gardens by means of small holes cut into the base
of a fence panel. Wooden hedgehog-shelters are
commercially available for placing in quiet corners of
large gardens. Heaps of logs and piles of leaf litter
in a quiet location are suitable for their daytime and
overwintering retreat. Care should be taken in winter
that hibernating hedgehogs are not burnt in bonfires.
