Garden pests - Principles of Horticulture - page 230

18.6). In moist, warm weather it often causes above-
ground damage to leaves of bedding plants, new turf,
lettuce and Brussels sprouts. Snails, while not being
as serious pests as slugs, nevertheless are capable
of causing considerable damage in private gardens to
young plants, to particularly 'snail-attractive species'
such as hostas, and to crops in greenhouses such as
tomatoes and petunias.
Life cycle . Slugs and snails are hermaphrodite
(bearing in their bodies both male and female organs).
They mate in spring and summer, and lay clusters
of up to 50 round, white eggs in rotting vegetation,
the warmth from which protects this sensitive stage
during cold periods. Slugs range in size from the
keeled slug ( Milax ), 3 cm long, to the black garden slug
( Arion ), which reaches 10 cm in length. Slugs move
slowly by means of an undulating foot, the slime trails
from which may indicate their presence. The three
species of mottled slug ( Testacella ), occurring mainly
in southern Britain, are quite common, but rarely seen,
living predominantly underground. The mottled slugs
are carnivorous, feeding on earthworms and compost
worms, sometimes on other slugs (Figure 18.6).
Control . Predators such as centipedes, ground beetles
and glow-worm larvae are natural controlling species
against slugs and snails. Digging of garden plots
exposes slugs to birds such as thrushes and reduces
slug egg numbers. There are some examples of
slug-resistant cultivars. While most lilies are prone
to slug attack, Lilium hennyi and Lilium tigninum are
are rarely affected. Hosta species , which compete
with lettuce as the slug's favourite food, may be
grown with more confidence if blue-leaved cultivars
are chosen. Coarse sand placed round the base of
pots growing plants such as hostas helps prevent
leaf damage. Copper strips placed around pots
(Figure 16.2a) or on the edge of glasshouse benches
deter snails. A parasitic nematode ( Phasmarhabditis
hermaphrodita ), drenched into soil in the warmer
months, is increasingly being used to limit slug
numbers. A spray containing extracts from a Yucca
species is now available for small areas of control. The
most effective methods are the two ingredients, ferric
phosphate , a pellet that is increasing in popularity
with amateur gardeners, and metaldehyde used as
small, coloured pellets (which include food attractants
such as bran and sugar). Metaldehyde is also available
as a liquid formulation. Pellets containing methiocarb
(that acts as a stomach poison) are progressively less
used now by gardeners, for reasons given below.
(Use of metaldehyde and methiocarb in gardens has
recently been claimed to be a major contribution to
the decline in the numbers of birds such as song

thrushes that eat the poisoned slugs. Hedgehog, frog
and insect predator numbers may also be similarly
affected. A simple device, such as that seen in Figure
18.6d, helps gardeners to prevent the entry and
poisoning of mammals and birds. As can be seen, it
uses a modified plastic milk carton (containing the
slug pellets) which retains the dead slugs within the
container, and prevents entry of mammals and birds.
Trade product are available that give similar results.)

Insects

Belonging to the large group of Arthropoda , which
include also the woodlice, mites, millipedes and
symphilids (Table 18.1), insects are horticulturally the
most important arthropod group, both as pests and as
beneficial soil animals.

Insect structure and biology

The body of the adult insect is made up of segments
and is divided into three main parts: the head, thorax
and abdomen (Figure 18.7). The head bears three
pairs of moving mouthparts. There are two main
methods of feeding (see Figure 18.8). Caterpillars,
sawfly larvae and beetles have biting mouthparts.
In the aphids and their relatives, the mandibles and
maxillae are fused to form a delicate tubular stylet
that sucks up liquids from the plant phloem tissues.
Thrips adults and larvae tear and rasp plant tissues.
Insects remain aware of their environment by means of
compound eyes which are sensitive to movement (of
predators) and to colour (of flowers). The thorax bears
three pairs of legs, and in most insects, two pairs of
wings. The abdomen bears breathing holes (spiracles)
along its length, which lead internally to a breathing
system of tracheae. The blood is colourless, circulates
digested food and has no breathing function. The
digestive system, in addition to its food-absorbing role,
removes waste cell products from the body by means
of fine, hair-like growths located near the end of the gut.
Since the animal has an external skeleton made
of tough chitin, it must shed and replace its 'skin'
( cuticle , see Figure 18.7) periodically by a process
called ecdysis , in order to increase in size.
The two main groups of insect develop from
egg to adult in different ways. In the first group
( Exopterygta ), which includes the aphids, thrips
and earwigs, the egg hatches to form a first stage
(instar) called a nymph , which resembles the adult
in all but size, wing development and possession of
sexual organs. Successive nymph instars more closely
resemble the adult. Two to seven instars (growth
stages) occur before the adult emerges (Figure 18.9).