Hoverfly
(R. Lucas -
What's That Pest?)
Smart strategies
Getting started
Attracting beneficial bugs
Good reasons not to spray
A garden in harmony
Self defence
Plants to Repel Pests |
 Planting for pest control
It's a jungle out there. Plants compete with plants and are preyed
upon by insects, but there are some that help each other in the struggle
to survive. We look at ways to encourage a natural relationship between
friend and foe in the garden.
In a move away from treating bad bugs with chemicals, there is a revival
of interest in how plants can protect each other from attack. Companion
planting, or growing plants together for their mutual benefit, has been
around since Roman times. The wisdom of centuries of observation is that
certain plants (the "companions") will assist others (the "hosts"
which attract pests) by fending off the pests, or attracting beneficial
insects. For gardeners wanting to deal with pests in a way that doesn't
harm the environment, it's an appealing idea.
 Putting
it to the test
There are countless reports that companion planting works, most of them
based on personal experience or anecdotal evidence. There have been relatively
few scientific studies to substantiate the claims, and the results have
often been contradictory. Gardening Which?, our sister organisation in the
UK, trialled some popular combinations: cabbage and celery (to protect the
cabbages from caterpillars), onion and carrot (mutual protection from their
respective flies) and garlic and rose (for aphid-free roses). In their trial,
all the plants were attacked equally by pests, whether they were grown alone
or with their companions. They also tried using marigolds in greenhouses
to control whitefly, but with little success.
In contrast, trials by other organisations have found benefits. Growing
onions and carrots together reduced carrot rust fly attack, provided there
were at least four rows of onions for each row of carrots. There were fewer
aphids on potatoes when shallots and garlic were grown within 75cm, but
the numbers of thrips increased. Underplanting Brussels sprouts and cabbages
with clover and French beans resulted in fewer cabbage aphids and caterpillars,
but reduced the yield. Dense plantings of marigolds have been very effective
against some soil-dwelling nematodes (eelworms) that infect the roots of
flowers, and crops such as potatoes. |
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It seems that companion planting can work under certain conditions,
but that the interaction is very complex. Other factors such as the weather,
the level of pest attack the previous year and the variety of plants, may
all affect the result. The closeness of the planting necessary for it to
be effective can mean that the plants are competing for light, water and
nutrients and so may produce poor quality and/or lower yields.
Smart
strategies
Plants have a fascinating armoury with which to repel their enemies.
Camouflage You can use plants to deter a pest that locates its
host by sighting. For example, it's the contrast of brassicas against bare
soil that the white butterfly recognises, so planting them among a crop
of unrelated plants (as illustrated) such as dwarf beans, clover or tomatoes
will reduce their visibility.
Masking Strongly scented plants such as mint, sage, rosemary and
garlic are reputed to discourage those pests that find their hosts by smell.
Their own powerful scent can mask that of a host plant, or it can directly
repel a pest. An example of mutual protection is with carrots and onions:
it's thought the smell of onions confuses the carrot rust fly, while the
smell of carrots confuses the onion fly!
Decoys or traps A more attractive plant may lure a pest away from
a plant you don't want damaged. Aphids are said to ignore most plants if
there are nasturtiums nearby.
Attraction Some plants attract beneficial insects such as ladybirds,
hoverflies and parasitic wasps, which prey on harmful insects. The flowers
provide them with the nectar and pollen necessary for them to breed. These
good bugs then find pests in the nearby crops; predators lay their eggs
close by the pest, while parasites lay them inside the pest. The larvae
then develop by feeding on the pests.
Chemicals Some plants secrete chemicals that can kill pests outright,
or alter the environment so they can't survive. Recent trials by Wrightson's
research division indicate that brassica roots release chemicals that suppress
some soil-borne diseases.
Getting
started
You may like to try some of the combinations suggested in the table;
most of these have some scientific support. Grow the plants in alternate
rows, mix plants together in a row, or set them out without distinct rows.
Decoy or trap plants can be planted as a protective barrier around the desirable
plants. Experiment with the spacing and arrangements. The idea is to confuse
the pests by disrupting the visual and chemical cues they use to find their
hosts. Don't expect immediate or complete control you'll need to accept
a certain amount of damage. If you're happy with the results, try other
combinations you suspect may work, but leave some areas as they are, so
you can compare the effectiveness. |
Ladybird
(R. Lucas -
What's That Pest?) |
Attracting
beneficial bugs
Growing plants that attract beneficial insects is usually considered
to be the most effective type of companion planting. Plants in the Apiaceae
(Umbelliferae) family carrots, celery, parsnips, anise, caraway, chervil,
coriander, dill, fennel, lovage, parsley, Queen Anne's Lace are particularly
attractive. They have large open flowers, so the nectar and pollen is easily
accessible. Flowers of the Asteraceae family such as daisies, tansy, golden
rod (Solidago) and yarrow, also encourage welcome visitors. Other plants
to try: tansy leaf (Phacelia), buckwheat, honesty, cosmos and alyssum. If
you can stand it, leave a part of the garden to go wild. Areas of rough
vegetation provide beneficial insects with food and a place to overwinter.
To be successful, you need to provide overwintering insects with plenty
of pollen- and nectar-producing flowers in spring. This will encourage them
to start breeding at the same time as pests become active. Otherwise, their
numbers won't build up until summer, by which time the damage has been done.
Put in some early-flowering plants such as alyssum and Phacelia (sow in
late winter), or leave some winter brassicas to flower. In summer, parsnips,
carrots, parsley, angelica, Queen Anne's lace, celery, dill and fennel are
in flower. As autumn approaches, the beneficial insects are less active,
but it's useful to provide flowers such Solidago and artemesias (mugwort)
to take them through to hibernation.
Be patient. It takes time for the good bugs to build up possibly
a whole season if you don't already have overwintering populations.
Good reasons not to spray
Spraying may seem like a quick fix, but as well
as being only a short-term solution, and possibly causing us harm, pesticides
can:
- Kill harmless species.
- Kill useful predators and parasites.
- Encourage resistance in pests.
- Contaminate the environment.
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Improve your soil.
Add organic matter such as compost and mulch to encourage worms and beneficial
micro-organisms. |
A garden
in harmony
- Choose suitable plants. Plants that are struggling because they're
not suited to the climate or aspect Keep plants healthy. Water during dry
periods, mulch and fertilise regularly.
- Practise crop rotation. This will reduce build-up of pests and disease
in the soil .
- Practise garden hygiene. Remove and dispose of diseased plant parts.
Keep weeds under control in most areas. Even though they can act as a refuge
for natural enemies, they can also harbour pests and disease.
- Choose disease- and pest-resistant plants.
- Check for pests and diseases regularly. Early diagnosis allows you
to control them before they become a problem. Try picking, hosing or brushing
off pests, then squash them or drop them into a bucket of soapy water.
Prune and clean away diseased leaves and plant parts. Put them in the rubbish
don't compost.
- Grow a range of plants. Diversity encourages beneficial insects and
makes it harder for pests and diseases to spread.
Self-defence
Each plant has its own particular smell, taste, colour and shape which help
pests locate them. But they're not entirely defenceless once attacked. Several
studies have shown that "good" parasites are attracted to substances
released by the plant on which their victims may be found. Some plants also
sent out "distress flares"' substances which not only attract
beneficial species that eat the pests, but also promote a defence response
in nearby plants of similar species. |
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See Plants to Repel Pests.
© Consumers' Institute, Issue 53, August 1999 |
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