Agriculture Reference
In-Depth Information
(b)
(a)
Figure 16.7
(a) Lacewing adult; (b) lacewing larva control aphids (courtesy of Bioline Ltd)
feed on a range of aphid species. Wooden ladybird
shelters and towers are now available to encourage
the overwintering of these useful predators. A
worrying development in the last few years has
been the rapid spread and increase of the
harlequin
ladybird
from South East Asia. This species is larger
(6-8 mm long) and rounder than the seven-spot
species (4-5 mm). It has a wider food range than
other ladybird species, consuming other ladybirds'
eggs and larvae, and eggs and caterpillars of moths.
Furthermore, it is able to give humans a slight but
irritating bite.
Hoverflies
(Figure 16.9), superficially resembling
wasps, are commonly seen darting or hovering above
flowers in summer. Many of the 250 British species
(e.g.
Syrphus ribesii
) lay eggs in the midst of aphid
colonies, and their legless light green-coloured grubs
(resembling small green maggots) consume large
numbers of aphids. The flowers of some garden
plants are especially useful in providing pollen for
the hoverfly adults and therefore encouraging aphid
control in the garden. Summer flowering examples
are poached-egg plant (
Limnanthes douglasii
), baby-
blue-eyes (
Nemophila menziesii
) and Californian
poppy (
Romneya coulteri
). Later summer and autumn
examples are
Phacelia tanacetifolium
and ice plant
(
Sedum spectabile
).
Other beneficial insects include the common wasp,
parasitic wasps, anthocorid bugs and ground beetles.
Predatory mites and parasitic nematode worms are
also important in controlling pest numbers.
Insect pests may also be parasitized by specialized
parasitic fungi
. The numerous species of web-
forming and hunting
spiders
(Figure 16.9d) are very
important in the reduction of many types of insects.
Occasionally, weeds are controlled biologically. The
cinnabar moth
caterpillar (
Tyria jacobaeae
) may
remove the foliage of groundsel and ragwort. A
rust
(
Puccinia lagenophora
) is commonly seen infecting
groundsel, but unfortunately also attacks cinerarias.
Increased attention is being given by horticulturists
to the careful
selection of pesticides
(if they are
needed) to avoid unnecessary destruction of the
predators and parasites described above (see also
p. 203).
Exotic (introduced) predators and
parasites
In greenhouses and polythene tunnels,
high
temperatures
(often all year round) and subtropical
species of plants bring with them
exotic
pests and
diseases from other countries and they increase very
rapidly, and may become resistant to pesticides.
Biological control of exotic pests requires exotic
predators and parasites
; and so, the health of the
major greenhouse crops in Britain and Ireland is due
in large measure to these introduced predators and
parasites. Two organisms, a South American mite
that eats all stages of the glasshouse red spider mite,
and a tiny South-East Asian wasp that parasitizes the
glasshouse whitefly, have been used for many years
and are briefly described below.
Phytoseiulus persimilis
(Figure 16.10a) is a 1 mm
globular, deep orange, predatory tropical mite used
in greenhouse production to control glasshouse red
spider mite (
two-spotted mite
, see also p. 245).
The predator's short egg-adult development period
(seven days), laying potential (50 eggs per life cycle)
and appetite (five pest adults eaten per day) explain its
extremely efficient action.
Encarsia formosa
(Figure 16.10b)
is a tiny (2 mm)
wasp, which lays an egg into the glasshouse whitefly
third and fourth
scale stage
(see also p. 236), causing
it to turn black and eventually to release another wasp.
It requires temperatures above 22°C to be effective.
