Introduction - Pests of Ornamental Trees, Shrubs and Flowers - page 9

'eelworms'. Plant-parasitic nematodes are microscopic,
commonly no more than 0.1-0.5 mm long; they are
unique in possessing a distinctive spear-shaped structure
in the oesophagus, with which they pierce the walls of
plant cells. The detailed form of this spear is often useful

for distinguishing between genera. To be active, a
nematode is dependent upon the presence of moisture;
individuals usually travel through the soil or over plant
tissue in films of water, progressing with serpentine
movements of the body.

Slugs and snails

Slugs and snails (phylum Mollusca: class Gastropoda)
are soft-bodied, non-segmented invertebrates with the
body composed of three regions: head, foot and visceral
mass. The last-mentioned is covered by a layer of
epithelial cells, called the mantle; this secretes a shell of
calcium carbonate and encloses a mantle cavity. The
mouth usually contains a rasping tongue or radula,
armed with thousands of minute chitinous teeth. Most
molluscs, such as clams, cuttlefish, octopuses, oysters,
sea-slugs and squids, are marine animals; several species
live in freshwater habitats but only certain slugs and
snails in the order Pulmonata are able to survive on land.
Terrestrial slugs and snails are hermaphroditic

creatures with a slimy, asymmetrical body. Their mantle
cavity is vascularized and functions as a lung, with an
aperture on one side of the body. The visceral mass is
often contained within a hard, helical shell. The head is
well developed and bears a long and a short pair of
retractile tentacles, each longer tentacle having a simple
eye at its tip. The foot is muscular, broad and flattened,
and functions as the propulsive organ, the animal gliding
along on a bed of slime. Slugs and snails are most active
in warm, humid conditions. They feed on plant material
of various kinds, and some species are regarded as
important pests.

Earthworms

Earthworms (phylum Annelida: class Oligochaeta) are
well-known hermaphroditic creatures with long, thin
and distinctly segmented bodies. They burrow in the soil
and feed mainly on decaying vegetative matter, thereby

contributing to soil fertility, aeration and drainage.
Although primarily beneficial, a few species can be a
nuisance in lawns and sports turf.

Birds and mammals

Birds (class Aves) and mammals (class Mammalia) are
of only minor importance as pests of ornamental plants.
Birds are of particular significance as pests of flower

buds or open blossoms. Mammals are mainly damaging
to young plants, new shoots, bulbs, corms and seeds;
they may also strip bark from trees.

Pest damage

The kind of damage inflicted upon plants by pests varies
according to feeding habit and methods (e.g. whether the
pest's mouthparts are adapted for biting, piercing,
rasping or sucking). Some pests attack the roots or other
underground parts, but most affect the leaves, stems,

shoots, buds or flowers. Damage also varies from minor,
often imperceptible blemishes, colour changes or loss of
vigour, to complete death of plants. Leaves, for example,
may become blistered, discoloured, disfigured,
distorted, dwarfed, galled, malformed, mined,