Agriculture Reference
In-Depth Information
horticultural unit at any time of year. Many plant
cultivars (such as lettuces, potatoes and
Hosta
)
are highly bred to have succulent tissues that
unfortunately favour slugs. Slugs exploit these
situations and are rated by gardeners as the
worst
garden pest.
Examples of molluscicide active ingredients:
X
Ferric phosphate
is an inorganic salt, formulated
in pellets. It is relatively non-toxic to mammals
and birds. This ingredient is acceptable to organic
growers.
X
Metaldehyde
causes the slug to produce
excessive amounts of mucus, and consequently
to become dry and die. Metaldehyde is applied
as pellets (or sometimes as a spray formulation).
There is some danger to children, pets and wild
animals.
X
When using selected biological control species
such as
Encarsia
wasp against glasshouse whitefl y
(see p. 198) in
greenhouses
, the selection of
appropriate insecticides/acaricides such as
fatty
acids
for control of other pests (see integrated
control, p. 201) reduces
Encarsia
deaths.
X
Insecticide products are dangerous to animals in
ponds
(fi sh, snails and insects), and care is needed
not to spray near ponds.
Restoring and maintaining natural balances:
X
Read the pesticide product label carefully to
check whether the chosen active ingredient kills
biological control and pollinating species. Internet
sources will give more detailed information.
X
Be particularly careful when spraying in
glasshouses using introduced biological control
species, or near ponds.
Insecticides and acaricides (used
against insect and mite pests)
Entry point for insecticides into insects
The insects and mites have
three
main points of
weakness for attack by pesticides: their
waxy
exoskeletons
(see p. 232) may be penetrated by
wax-dissolving contact chemicals; their abdominal
spiracles
allow fumigant chemicals to enter tracheae
or are blocked by 'sticky' pesticide formulations; and
their
digestive systems
, in coping with the large food
quantities required for growth, may take in stomach
poisons.
Benefits and limitations of insecticidal control: see
general points for chemical control on p. 199.
Safe practice and environmental impact of insecticidal
and acaricidal control: while most acutely toxic and
persistent insecticides are no longer approved for
garden use in Britain, there are still important points to
be remembered:
X
Care needs to be taken when mixing and applying
insecticides. A
concentrated
active ingredient
within the trade product represents a particular risk
if adequate protective clothing (see p. 201) is not
worn. Products for gardeners are chosen for their
low level of toxicity to humans but gardeners still
need to be careful.
X
Most of the insecticide ingredients available
to the private gardener have some toxicity to
benefi cial
insects and mites living alongside
pest species. Alternative strategies (see physical,
cultural and biological control in this chapter) such
as deterrents, fl eeces, traps and encouraging
benefi cial predators and parasites should be
considered before taking the option to use an
insecticidal spray.
Natural balances:
X
The use of insecticides/acaricides can seriously
affect natural balances. While the increase in
insect and mite numbers is balanced in nature
by
benefi cial
predators and parasites such as
ladybirds and parasitic wasps (see biological
control, p. 196), careless spraying of an insecticide
such as
deltamethrin
may kill off these useful
species.
16
Examples of insecticide active ingredients
Three groups of insecticides are described.
X
Natural plant extracts
. Such products
contain natural products such as
alginates/
polysaccharides
and act by blocking the
breathing holes (spiracles) of pests such as
aphids, thrips and mites. They have been given
clearance for use by organic growers. A smoke
formulation based on extracts from
garlic
enables
the gardener to fumigate the glasshouse against
pests such as glasshouse whitefl y, aphids and
two-spotted mite, without needing to remove the
glasshouse plants.
X
Deltamethrin
belongs to the pyrethroid group. It
has both cuticle and stomach action. It is effective
against caterpillars, and outdoors. It is residual for
a period of up to three weeks. It may reduce the
effectiveness of biological control by killing off
useful predators and parasites.
X
Potassium salts of
fatty acids
work by contact
action, dissolving the cuticle of pests such as
aphids, whitefl y, spider mites, mealy bugs and
scale insects.
