Agriculture Reference
In-Depth Information
Encarsia
wasp parasite mentioned above. It is,
however, parasitized by the exotic wasp
Eretmocerus
mundus
, which is commercially available.
species. It has been shown to transmit some plant
viruses.
Moths and butterflies
Leaf hoppers
This insect group characteristically contains adults
with four large wings and curled feeding tubes.
The larva (
caterpillar
), with six small legs and
eight false legs, is modified for a leaf-eating habit
(see Figure 18.10). Some species are specialized
for feeding inside fruit (
codling moth
on apple),
underground (
cutworms
, see Figure 18.18), inside
leaves (
oak leaf miner
, see Figure 18.19) or inside
tree branches (
leopard moth
). The gardener may find
large webbed caterpillar colonies of the
lackey moth
(
Malacosoma neustria
) on fruit trees and hawthorns,
or the
juniper webber
(
Dichomeris marginella
)
causing webs and defoliating junipers.
These squat light brown or green insects are about
8 mm long and affect a wide variety of crops (e.g.
potato, rose,
Primula
and calceolaria). The adults, as
their name suggests, can avoid danger by hopping
off a leaf. They are also able to fly from plant to plant.
They live on the under-surface of leaves, causing a
fine mottling of the upper surface. In strawberries,
Euscelis lineolatus
is a vector of green-petal disease.
On rhododendron,
Graphocephala fennahi
carries
the serious
bud blast
fungal disease that kills off the
flower buds (Figure 18.15c). August and September
sprays of products containing
fatty acids
prevent
insect egg-laying inside buds of rhododendron.
Large cabbage white butterfly (
Pieris
brassicae
)
Common green capsid (
Lygocoris
pabulinus
)
Damage
. Leaves of cabbage, cauliflower, Brussels
sprouts and other hosts such as wallflowers and
the shepherd's purse weed are progressively eaten
away. The defoliating damage of the larva may result
in skeletonized leaves, with only the main veins
showing.
Life cycle
. This well-known pest on cruciferous plants
emerges from the overwintering pupa (chrysalis)
in April and May and, after mating, the females lay
batches of 20-100 yellow eggs on the underside of
leaves. Within a fortnight, groups of larvae emerge
and soon moult to produce the later instars, which
reach 25 mm in length and are yellow or green in
colour, with clear black markings (Figure 18.16b).
They have well-developed mandibles. The pupa
stage occurs usually in June in a crevice or woody
stem, and is held to its host by silk threads. A second
generation of the adult emerges in July, giving rise
to a more damaging infestation than the first. The
second pupa stage overwinters. Care should be taken
not to confuse the cabbage white larva with the
large smooth green or brown larva of the cabbage
moth, or the smaller light green larva of the diamond-
backed moth, both of which may enter into the hearts
of cabbages and cauliflowers, presenting greater
problems for control.
Spread
. The species is spread by the adults.
Control
. There are several forms of control against
the cabbage white butterfly. A naturally occurring
small wasp (
Cotesia glomerata
) lays its eggs inside
the caterpillar, and the yellow cocoons emerge (see
Figure 18.16d). A virus disease may infect the pest,
This is
a light green insect resembling a large (5 mm)
active aphid. It occurs in small numbers on fruit trees,
shrubs and flowers. Although it is rarely seen, it can
cause considerable damage to bud, leaves and fruit
by injecting toxic juices that cause distorted plant
growth (see Figure 18.15d and e). Aphid sprays are
sometimes used against this pest.
Thrips (order Thysanoptera)
Owing to their increased activity during warm humid
summer weather, thrips are sometimes called
'thunder flies', and are known for their ability to get
into people's hair in sultry summer weather and cause
itching. They may be important pests in greenhouses.
Onion thrips (
Thrips tabaci
)
Their toxic salivary juices cause a plant reaction: silver
streaks in onion leaves, straw-brown spots several
millimetres in diameter on cucumber leaves and white
streaks on carnation petals. The
pupa
stage
occurring
in
the soil may cause difficulties in control.
Gardeners
are able to use a product for thrip control containing
natural plant extracts
that blocks the insect's
breathing holes. The predatory bug
Orius laevigatus
is
an effective biological control.
Western flower thrip (
Frankliniella
occidentalis
)
This, sometimes found on greenhouse and
outdoor flower and vegetable crops, is an introduced
