The Online Gardener's Handbook
Chapter 2: Integrated Pest Management
Cultural Control Methods
Table of Contents
- Companion Planting
- Crop Diversity
- Crop Rotation
- Plant Selection
- Plant Hardiness
- Planting Time, Depth and Spacing
- Trap Crops
- Water, Irrigation and Drainage
- Weed Control
- Mechanical Control Methods
- Physical Removal
- Row Covers and Other Barriers
Cultural Control Methods
Cultural control methods involve manipulating planting, cultivation and other practices in ways that make the environment less favourable for the development and spread of pests. They are typically implemented early, sometimes even before planting your garden, because the best way to manage pests is to prevent them from occurring in the first place. This is always more effective than attempting to control pests after they are infesting your plants. Some of the most important involve keeping garden plants as healthy as possible, because healthy plants can withstand pest attack better than plants growing under one of several different environmental stresses.
The following are cultural pest management options that must be considered if pesticide use is to be minimized or eliminated.
Climate can greatly influence the health of your garden, and though you have no control over climate, you can be aware of its effects, and take counter-measures when appropriate.
Climate has an effect on insect populations. Fewer generations are produced in cold areas than in warm. Late spring frosts and cold, wet springs are harmful to newly hatched larvae. Heavy rainfall washes small insects such as mites and aphids from plants.
Climate can also influence the severity of a disease. A cool wet spring, for example, increases the severity of Anthracnose on white oak, sycamore, and ash. Frequent summer showers increases the incidence of botrytis on flowers such as tulips, geranium, roses and petunias.
Companion planting is the inter planting of crops with other plants thought to repel or prevent pest outbreaks. It is advocated by some as an effective control technique. Unfortunately, a three year study conducted by staff of the Pest Diagnostic Clinic disproves this belief.
Cabbage, potato, and roses were planted in test plots, and companion plants thought to have insect repellent characteristics were planted around and within them. These included basil, bean, catnip, celery, chives, dill, flax, garlic, geranium, hyssop, horseradish, French and African marigold, nasturtium, parsley, peppermint, sage, tansy, thyme, tomato, and wormwood. All plants were maintained using organic methods.
At the end of the study, it was found that not only were pests generally unaffected by the companion plants, but the companion plants competed with the crops for moisture, nutrients and sunlight. The overall result was reduced crop yield and quality. The only combination which consistently showed some promise was the interplanting of beans and potatoes to reduce injury from Colorado potato beetles and potato leafhoppers.
If you still want to try companion planting as a pest control measure in your garden, several techniques may improve the likelihood of success:
- avoid close planting of highly competitive, densely foliated plants near the crop. For example, avoid planting African marigold, catnip, celery, tansy, tomato, and wormwood as companions unless sufficient space is allowed;
- avoid wormwood, as it produces a chemical substance that reduces the growth of nearby plants;
- avoid companion plants that serve as alternate hosts for pests or diseases of the crop. For example, flea beetles will attack nasturtium as well as cabbage;
- choose plants of aesthetic or culinary importance which could also provide enjoyment or food;
- always leave some of the crop without companion plants to objectively assess whether or not any benefit is resulting from this technique.
One exception to this may be companion planting with pollen or nectar-producing plants to attract beneficial insects. By serving as a source of food for many predatory insects and mites and parasitic wasps and flies, they may help to attract and increase populations of these beneficial organisms near your garden plant. For more information, refer to the section on Promoting Beneficial Insects.
If a wide variety of plants are grown in a small location, it is more difficult for flying insects to find the right plant on which to feed. This can also slow the spread of diseases and provide a habitat for beneficial insects. For this reason, inter-cropping, or planting alternating varieties of crops, can help reduce pest problems more than if crops are planted in large blocks.
Crop rotation involves the planting of non-related crops in a particular location in successive years to minimize the chances of plant-specific disease organisms and pests building up in the soil. This is practical only in large gardens and fields. In small home gardens where rototillers, foot traffic, water and wind readily move soil from one part of the garden to another, it is of limited value. Crop rotation can be effective when certain crops are not planted for two to four years if a particular disease is present, or if certain plants can be successfully planted in remote parts of the garden over successive years. Crop rotation is also only effective on certain types of pest, namely those that have a limited host range, are not extremely mobile and do not survive in the soil for more than a year or so. Despite the limitations to the use of crop rotation, it is still a good practice to avoid planting the same crop in the same location of your garden every year.
Proper fertilization is key to ensuring that plants are healthy enough to withstand pest attack. However plant fertility can influence pests in other ways as well. Excess fertilizer promotes excess vegetative growth, which can attract aphids and may also increase humidity, thereby favouring disease development. Finally, plant symptoms associated with over- or under-fertilizing can be easily mistaken for pest damage, which may lead to misdiagnosis.
Crops left too long in the garden are more susceptible to pests or storage rot diseases. It is important therefore both to harvest on time and to properly prepare the crop for storage. Avoid rough handling of fruits and vegetables. This can cause wounds which, in turn, can become infected by secondary disease agents. In some cases it may also be possible to avoid applying pest controls by harvesting the crop early, before pests reach damaging levels. This will only be effective if the pest occurs late in the season.
Carefully select the types of crops to be grown in your garden, discarding those that are likely to require pest control measures. For example, hybrid tea roses suffer from blackspot, a disease that will defoliate the plants by mid-to-late summer, while some of the old-fashioned and species-type roses are not susceptible to this disease.
Plant varieties can be extremely variable in their response to insect and disease pests, with some being much more able to tolerate damage than others. The resistance to damage can be due to the possession of physical characteristics or chemicals that repel or kill pests, through an ability to remain vigorous in the face of heavy pest damage or for other reasons. Plant breeders have incorporated some of this genetic resistance to various diseases and pests into plants. It is now possible, therefore, to select non-susceptible or resistant plant varieties for your garden. Information on resistant varieties is available from seed catalogues or you can ask nursery or garden centre staff when buying plants. Remember, though, that some resistant varieties can still suffer damage if pest populations are extremely high. Additionally, resistant varieties will also vary in temperature, fertility and other requirements, and these should also be considered to determine if they will adapt to your garden's growing conditions.
Not all plants are adapted to all sites, climatic zones, soils, soil pHs, light availability or similar environmental conditions. Plants that grow in unsuitable locations are predisposed to diseases, insects and secondary problems. Woody and herbaceous perennial plants are given hardiness zone ratings that correspond to geographic regions of Ontario and Canada. Buy only those plants that carry a hardiness rating equal to or higher than your location.
Planting Time, Depth and Spacing
It is sometimes possible to alter planting time, depth and spacing to avoid or repel pests. If you know the biology of the pest attacking your plant, you may be able to time planting to avoid damage. For example, altering planting times to avoid the egg-laying or spore-discharge stage of some pests may prevent damage. In other cases, planting early (before pests arrive) may allow young plants to estabilish and develop to a point where they are better able to withstand pest attack. Optimum planting times are provided for several garden vegetables (see Table 9). You may also want to time planting to avoid environmental conditions that are conducive to pest development (e.g. cool, wet early spring weather favors many fungal diseases). Planting time can also be critical to minimize the potential for seed and seedling rot. Do not plant large seeded vegetables when the weather is cool and moist, as seed rot can easily overcome the seedlings before they emerge. Wait until the soil is warm and moist.
Planting at the proper depth ensures that seedlings emerge quickly and reduces the risk of damping-off fungi. Even, wide spacing guards against crowding, which can lead to spindly succulent growth, poor ventilation and poor drying conditions. These increase the likelihood that problems such as botrytis, white mould, root rots, damping-off and foliar diseases will develop.
Sanitation involves the removal of material which allows pests to survive or be transported between plants or crops. Because many insects and diseases overwinter in plant debris and in weeds, the most important sanitation measure involves the regular removal of such material. Pull up all infected plants and weeds promptly and dispose of them outside the garden. For larger plants, such as trees, cut off diseased or damaged branches. Pest-infested plant material should either be sent to the garbage or buried at least 60 cm deep. Do not put diseased plants in the compost pile, as the sustained high temperatures required to kill problem-causing insects, nematodes, fungi and weed seeds cannot be guaranteed. Some insects overwinter under boards, bricks, stones and similar objects. Remove all such hiding places and store them away from the soil. Additionally, do not leave vegetables or fruits on the soil from one year to the next, and dispose of prunings from woody plants promptly.
Sanitation also includes careful attention to the health of new plants or seeds introduced to your garden. Use only high quality, pest-free and disease-free planting material and seeds. If possible, quarantine new plants for a period of time before placing them in your garden to allow time for possible pest symptoms to develop.
Some insects and diseases can be transferred between plants on gardening tools or on the gardener. Always examine borrowed tools such as rototillers to ensure potentially infested soil is not being transferred from another area to your garden. Try to enter and work in pest infested areas of your garden last, and disinfect pruning shears and other tools after working on infected plants. Thoroughly wash and disinfect all pots and planters before reusing them, ensuring all soil is rinsed off.
Tilling is important not only to prepare the soil for planting, but to protect the health of your plants. When you work the earth, diseased plant material is incorporated deep into the soil. It is then more rapidly broken down by soil micro-organisms. As a general rule, harmful pathogens survive poorly on rotted plant material or when deep in the soil, and so their levels are effectively reduced.
In addition, tilling exposes soil borne insects, slugs and other invertebrates to adverse weather conditions and to predators such as birds. Many will also be killed by the tilling process itself.
Trap crops or plants can sometimes be used to lure an insect away from the plant you wan to protect. Plants that the insect prefers to eat are planted near the garden crop, luring the insect over to the trap crop. The trap crop may then need to be destroyed or a control applied. For example, mustard trap crops have been used to help reduce flea beetle populations on cole crops. However, to be effective the trap crop has to be much more prefereable to the insect than the garden crop, and you need to ensure that the insect has no way of moving back to the garden plants after the trap crop is destroyed.
Water, Irrigation and Drainage
Many diseases are spread by spores which require moisture on leaves and stems to germinate and cause infection. Fungi and bacteria are also often worse when plant surfaces and soil are excessively wet for long periods of time. When, how and how much you water your plants can therefore be important in limiting the spread of disease. Ensure plants are neither over nor underwatered. Overwatering promotes root rots and other diseases, while underwatering contributes to the development of unhealthy plants which are less tolerant to pest attack. If diseases that are spread by splashing are present in your garden, avoid overhead watering. Water in the morning so that plants dry before nightfall, or only on dry days. Do not work in the garden when plants are wet, as you may unwittingly spread disease when you brush against them. Finally, when planning your garden, avoid areas with poor drainage. Good soil drainage is essential as soil saturated with water for several days may cause roots to rot. If you cannot avoid a poorly drained area, grade beds to allow water to flow out, or prepare raised beds or hills.
Some weeds serve as reservoirs for insects and diseases. They also compete for nutrients and moisture. Always keep weeds under control both within the garden and in surrounding areas. Weed control should begin before garden plants emerge, to avoid pests moving onto them from the destroyed weeds. Weed debris should also be removed before plants begin to emerge.
Mechanical Control Methods
Mechanical control methods are those that physically prevent the pest from attacking or injuring the crop. Hand weeding and use of fences to exclude deer and other wildlife are examples of mechanical, or physical controls.
Physically removing pests by hand or other means can be effective if just a few pests are present and it is done frequently. Soft eggs or soft-bodied insects such as aphids can be crushed when they are found, while caterpillars, slugs, snails, cutworms, grubs and other larger pests can be picked off and dropped into a bucket of soapy water. Caterpillar tents or webs can be ripped off trees and dealt with similarly. Larger groupings of pests can be dislodged from sturdier plants by aiming strong jets of water at them, however care must be taken that the plant does not get so wet that disease development is encouraged. Similar removal of diseased plant material may also be used to slow the progress of certain diseases, for example pruning of disease portions of branches, however this is generally not as effective with diseases as with insects.
A mulch is any covering placed on the soil surface to help plants, including wood chips, fabrics and plastics of various colors. Mulches are often used to help warm the soil or prevent it from drying out, however they can also help in controlling weeds and some insect pests. Black plastic mulch is often used for weed control, however exercise caution as it also heats the soil and so can damage some crops. Reflective mulches made of aluminum foil or other silver coatings may help keep aphids and whiteflies from seedlings and smaller plants. Different colours of mulches can be used for different purposes.
Row Covers and Other Barriers
Various types of barriers can be constructed around and over plants to help protect them from insect and vertebrate attack. Row covers and plant cages, which are hung over young plants, are among the most common. Row covers have traditionally been used to promote plant earliness by increasing temperatures, however they can also be used to keep migrating pests, such as cucumber beetles, flea beetles and aphids, off young, vulnerable plants. Typically the covers are removed once plants are old enough to withstand damage - this allows pollinating insects access to flowers, and prevents plants from being overheated later in the summer. Homemade cages, often made of fine mesh fabric covering a wooden frame, are often used. Alternately a variety of synthetic barriers are available for purchase.
Trapping can be useful in ridding gardens of some vertebrate pests,
but traps aimed at insects are also available. Some insect pests
of ornamental and fruit trees that climb up and down trunks can
be intercepted by applying a layer of sticky glue (such as Tanglefoot)
around the trunk of the tree. Application must be timed to intercept
adult stages as they climbing up the tree to lay eggs. Sticky traps
consisting of sticky material applied to yellow, white or blue cards
are sold by many garden supply centres and can be used to intercept
flying insects. However, while they can be useful as a monitoring
tool, to detect insects as they enter your garden, they generally
do not work as stand alone traps, and can also trap beneficial insects.
Pheromone traps, which use a sex hormone to attract male insects
to a trap, are also sold commercially for many insect pests. While
some may work to reduce insect mating, they are generally most effective
as a monitoring tool. Exercise caution in some cases. Commercially
available Japanese beetle pheromone traps are very attractive to
these insects and can actually draw more insects into your garden
than would have occurred if the traps weren't present.
For more information:
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