The Future is Abundant
A Guide to Sustainable Agriculture

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Insectary Plants

Kate Burroughs

Most ecologists agree that the practice of monoculture has led to an increase in pest and disease problems. When the same crop stretches for miles, unbroken by other plant species, it is much easier for herbivores and wind-borne plant disease spores to spread. Native plants are removed in the process of clearing the land for agriculture, and native insects have to switch to crop plants for their survival. Introduced pests which are adapted to agricultural crops have compounded the problems.

Increasing the diversity of plants within and surrounding the agricultural ecosystem is one strategy for controlling pests using biological restraints. Careful consideration must be given, however, to the particular plant species to be added, because simply increasing the number of different plants may have the undesired result of increasing plant pests. Therefore, it is important to observe and understand the biology and ecology of potential crop pests and of their natural enemies. Adding the appropriate species, even in small numbers, is the key to establishing stability.

Crop and non-crop plant species affect the colonization of pests by their visual appearance and smell, or by acting as a physical barrier or diversionary host. Some introduced insect pests, such as the cabbageworm (Pieris rapae) and the cabbage aphid (Brevicoryne brassicae) naturally exploited the temporary habitats of early succession plants on disturbed ground. These pests are especially attracted by the visual appearance of open stands of green plants silhouetted against brown soil, similar to the conditions created by plowed-field agriculture. Other species, such as the corn borer (Ostrinia furnicalis) and the fruit fly (Oscinella frit), which formerly exploited dense strands of wild grasses, are less attracted to widely-spaced crops.

The dispersal of insect pests, nematodes and diseases can be impeded in several ways when host and non-host plants are grown together. Because insects often locate their host plants by smell, most companion planting schemes are designed to mask the smell of crop plants by interplanting them with other strong-smelling plants. For example, cabbages are sometimes protected against flea beetles by intercropping with tomatoes. Companion planting also decreases plant-to-plant colonization by the relatively immobile pests living in the soil. Non-susceptible plants help prevent the dissemination of wind-borne plant disease spores, such as powdery mildew. Intercropping can also provide a physical barrier to protect crops. For example, the cabbage root fly (Erioischia brassicae) can be impeded from laying its eggs in fields which have a cover crop of clover.

Another way in which increased plant diversity helps decrease the presence of insect pests is by supporting beneficial insects which prey on pests. At some point in their life cycle, most beneficials need nectar, pollen, alternate hosts or other food sources. Proper nutrition of beneficials is essential because it affects their ability to survive and lay eggs. Again, plant species added for this purpose must be selected carefully because they may also cause an increase in the population of pests.

Plantings can be diversified in several ways. Depending on the situation, one or more of the following systems can be used.

Hedgerows: A mixture of plant species, from annual herbaceous plants to perennial shrubs and trees, can be planted on the edges of crop fields, along fencerows, or in woodland clearings.
Polyculture or Companion Planting: Two or more crops can be interplanted, or one crop can be sown into another before the first crop is harvested. These strategies are especially useful for annual crops, and also help keep troublesome weeds under control.
Encouraging beneficial weeds and/or insectary plants: Specific species can be planted or native volunteers allowed to grow. Some insectary plants are also valuable as cover crops and can be used to enrich the soil as well.

Hedgerows

Hedgerows can increase the number of beneficial insects by providing shelter and alternative sources of food. They provide nectar and pollen for beneficials such as ladybird beetles (ladybugs) and lacewings when crop aphid populations are low. Most of the parasitic wasps also require nectar, pollen, and/or honeydew during their adult stage. Wild plants and hedgerows growing on uncultivated land also provide alternate sources of food for beneficial insects when prey populations are low.

Hedgerow plants leaf out in the spring, well before most crop plants. They support insect populations which provide food for the beneficial insects and allow the beneficials to produce an early generation. This early generation of natural enemies will then move into the crop fields as pest populations begin to build. In the same manner, a diverse community of insects in the hedgerow increases the diversity of insects in the adjacents fields. The closer the hedgerow is to the field, the greater the beneficial influence.

Upstanding hedges interfere with air currents, catching weakly flying insects and depositing them on both lee and windward sides of the hedge. Most strongly flying insects, such as syrphid flies and wasps, tend to congregate on the lee side. Hedgerows also facilitate insect migrations after hibernation and from spring to summer habitats, so their beneficial influence may extend far beyond their immediate area.

Polyculture or Companion Planting

The practice of growing many crops together in the same field has been used for millenia throughout the world, and it is still being practiced in many places. One combination which has been used extensively by native people in North America is corn, squash, and beans. Polyculture increases crop production by copying the structure and function of natural ecosystems, and maintains diversity in crop rotations. Pest infestations in polyculture agroecosystems are significantly lower than those of monoculture systems. This effect is due to a greater number and more effective dispersal of natural enemies, or the disruption of colonization and dispersal of pests, or both.

The population of the diamondback moth (Plutella xylostella), a pest of tomatoes, has been regulated by a chemical repellency, or masking, of the tomatoes by intercropping them with cabbage. White and red clover intercropped with cabbage has been shown to reduce the native cabbage aphid and exotic cabbageworm populations by interfering with the colonization of the pests and increasing the number of predator ground beetles.

Beneficial Weeds and Insectary Plants

Nectar and pollen are important for the survival of beneficial insects when hosts are scarce. Access to nectar and pollen sources increases the longevity and egg laying ability of the beneficial insects and thus their effectiveness in controlling pests.For many predators, hosts are scarce during midsummer. When designing the system it is important to determine when pest populations are likely to be at their peak so the main bloom of insectary plants will attract beneficial insects at the same time. A good strategy when using native weeds or planting insectary plants (in the case of annuals or biennials), or those with overlapping bloom periods (in the case of perennials) to provide flowers throughout critical times.

Secondary plants growing with the crops in the field often attract beneficial insects or repel insect pests. Certain predators of aphids, for example, are able to colonize brussel sprout plants grown on bare ground. Ground beetles (Lebia spp.) are associated with certain weed species such as Solanum and Amaranthus. Removing these weeds also removes the beneficial ground beetles.

The presence of an alternate host for beneficials on insectary plants should not coincide with the periods critical for control of insect pests. If alternate hosts are not controlled, the beneficials could be satisfied by feeding on them instead of the pests.