Agriculture Reference
In-Depth Information
Control . Alternate host weeds such as groundsel
and sow thistle should be controlled. Yellow sticky
traps (see p. 191) remove many of the adult flies in
greenhouses. Certain chrysanthemum cultivars show
some resistance. Gardeners using products containing
abamectin have some control of the pest, especially
in the early growing season.
Other leaf miners
The occurrence of South American leaf miner
( Liriomyza huidobrensis ) and American serpentine
leaf miner ( Liriomyza trifolii ), which are able to
damage a wide variety of greenhouse plants, has, in
recent years, created problems for horticulture.
Carrot fly ( Psila rosae ) is a widespread and serious pest
on umbelliferous crops (carrots, celery and parsnips),
where tunnelling by the grubs makes the roots useless
(Figure 18.20b). The cabbage root fly ( Delia radicum
brassicae ) causes similar damage in brassica crops.
Leatherjacket ( Tipula paludosa ) is an underground pest,
the larva (see Figure 18.10) of the 'daddy long legs'
(crane fly) seen in late summer. It is a natural inhabitant
of grassland, causes most problems on turf, but also
damages potatoes cabbages, lettuce and strawberries,
especially after in wet springs and early summers.
Fungus gnat or sciarid fly ( Bradysia spp.) have larvae
that are small (3 mm), translucent with a black head.
They feed on fine roots of greenhouse pot plants such
as cyclamen, orchid and freesia, especially when the
plants are overwatered. This species is also a pest in
mushroom crops. Biological control by tiny nematodes
( Steinernema feltiae ) and a small beetle ( Atheta
coriara ) are available to the gardener. The adults are
also caught by yellow sticky traps (see page 191).
A recently introduced species Hemerocallis gall
midge ( Contarinia quinqenotata ), which distorts flower
buds of day lilies, is becoming increasingly common in
southern Britain.
Figure 18.19 Holm oak leaf miner (symptoms are
caused by small moth caterpillars)
Chrysanthemum leaf miner
( Chromatomyza syngenesiae )
Damage . This leaf miner symptom is caused by
a small fly species (contrast the leaf miners in
FigureĀ 18.19), the larvae of which can do serious
damage to horticultural crops by tunnelling through the
leaf. This species is found on members of Asteraceae
plant family, which includes chrysanthemum, cineraria
and lettuce.
Life cycle . The flies emerge at any time of the year
in greenhouses, but normally only between July and
October outdoors. These adults, which measure
about 2 mm in length and are grey-black with
yellow underparts, fly around with short hopping
movements. The female lays about 75 minute eggs
singly inside the leaves, causing small white spot
symptoms to appear on the upper leaf surface. The
larval stage is greenish white in colour, and tunnels
into the pallisade mesophyll (see p. 115) of the leaf,
leaving behind the characteristic mines seen in Figure
18.20a. On reaching its final stage, the 3.5 mm long
larva develops into a brown pupa within the mine.
The adult soon emerges from the pupa. The total life
cycle takes about three weeks during the summer
months.
Spread . This pest is spread by the adult stage.
18
Beetles
This group of insects have adults with hard, horny
front wings (elytrae) which, when folded, cover the
delicate hind wings used for flight. The meeting point
of these hard wing cases produces the characteristic
straight line down the beetle's back over its abdomen.
The thick skin (cuticle) of beetles enables many of
them to live successfully underground. Most beetle
species are beneficial , helping in the breakdown of
humus (e.g. dung beetles), or feeding on pest species
(see ground beetle, p. 199). A few beetles such as
vine weevil, wireworm, chafer and, more recently, lily
beetle can cause serious plant damage.
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