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hoverfly

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.

onions

 

cabbages

 

 

 

 

 

 

 

 

 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.

hands

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.

See Plants to Repel Pests.

 

© Consumers' Institute, Issue 53, August 1999


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