Conservation of natural enemies is arguably the most important concept in the practice of biological control and fortunately is also one of the easiest to understand. Simply put, conservation of natural enemies means avoiding practices which are harmful to natural enemies and implementing practices which benefit them. Sounds like good common sense! The tricky part comes in understanding exactly what practices are harmful and how beneficial practices can be integrated into your production system. That requires that we understand the biology of the natural enemies and are willing to modify our practices to accommodate them.
Natural Enemies as Livestock. Everyone understands that a dairy cow needs food, water, shelter and protection from adverse conditions. To perform her best she also needs protection from biting flies, diseases and in some cases, predators which may injure or kill her. The dairy producer knows her requirements change through the year and makes provisions to provide for these needs. In the winter, shelter is critical, while in the summer adequate water and shade are necessary. In some months, grazing may provide her total food requirements, but as pasture growth slows, supplemental food may need to be provided.
Natural enemies have exactly the same types of needs as the dairy cow. To perform their best, they need food, shelter and protection from adverse conditions as well. Frequently, we do not fully understand or make allowances to provide for these needs. The result is many instances where biological control could be very effective, but has failed or resulted in less than adequate control because we did not provide for the natural enemies' basic requirements. So what do natural enemies need and how can we help them?
Avoid Harmful Practices. The most obvious practice to avoid is the use of insecticides at times when natural enemies will be harmed. Insecticides can have direct effects on natural enemies by poisoning them or indirect effects by eliminating their hosts and causing starvation. In some cases, insecticides can be successfully integrated into the system without causing harm to natural enemies. This may be through the use of a selective insecticide such as Bt, timing the application to avoid times of the day or season when important natural enemies would be exposed, or placement of the insecticide in a location where natural enemies will not contact it. In other cases, adequately protecting natural enemies may require the elimination of insecticide use.
Certain cultural practices can also be detrimental to natural enemies. Plowing, cultivation, mowing or harvesting operations which disrupt natural enemies at critical points in their life cycle should be avoided. Excessive amounts of dust from roads or cultural operations can also disrupt the activities of predators and parasitoids resulting in reduced control. Burning of crop residues or inappropriately timed irrigation can also kill many natural enemies. Finally the ambiguous category of "clean farming" which includes removal of weeds and noncrop habitats has been identified as detrimental to many natural enemies.
Incorporate Beneficial Practices. Here is where a detailed understanding of the biology of the important natural enemies in your system becomes extremely critical. If you do not know what natural enemies you want to manage, it is doubtful that you will be successful. So the first step is to gather information on the types of natural enemies you want to conserve. Then consider these points:
Where does the natural enemy overwinter? In
England, a group of researchers discovered that important
predators of aphids in wheat overwintered in areas of
tussock grasses in hedgerows on the edges of fields. The
predators migrated into the fields in the spring, but got
there too late to control aphids it the center of the
fields. By planting a one meter strip of tussock grasses
in the center of the field, overwintering predator
numbers soared and aphid damage was controlled.
What alternate food sources do my natural enemies need?
Are these present at the right times and close to my
field? After emerging from overwintering, spotted lady beetles feed on plant pollen (dandelion, spring beauty,
etc.) for several weeks before moving into alfalfa and
wheat fields to feed on aphids. Many parasitoids also
require the protein-rich pollen in order to develop new
eggs. Sources of sugar (carbohydrate) are needed by many
parasitoids, which they frequently obtain from the nectar
of flowering plants or from aphid honeydew. Having a
diversity of plants in and around fields has been shown
in many cases to improve biological control.
Do my natural
enemies need alternative prey/hosts? Many predators and
parasitoids require alternative hosts at some points in
their life cycle. Lydella thompsoni is a tachnid fly which parasitizes European corn borer. It emerges before ECB larvae are present in
the spring and completes its first generation on common
stalk borer instead. Clean farming practices which have
eliminated hosts for the stalkborer are thought to have
contributed to the decline of this parasitoid.
Alternative prey may also be important in building up
predators in a field prior to the appearance of the pest
you wish them to control. Lady beetles and minute pirate bugs can consume many European corn borer eggs, but
alternative prey must be present in the field prior to
ECB egg laying to have maintained high predator numbers.
What shelter is needed by my natural enemies
during the growing season? The activity of ground
dwelling predators may be limited by high soil
temperatures during the day. Incorporation of cover crops
or intercrops may help to reduce soil temperatures and
extend the activity period of these organisms. Similarly,
many parasitoids require moderate temperatures and higher
relative humidity and may need to leave fields in the
heat of the day to seek shelter in shady areas. For
example, the activity of predatory paper wasps increases
when given wooden nest boxes which provide a protected
place for them to build their nest in and near fields.
Conclusion. Consideration of the biological and ecological needs of natural enemies is critical for the success of any biological control effort. It is one of the easiest ways for producers to initiate biological control on their farms and should be a major consideration in any importation or augmentation program. While there are innumerable practices which may either benefit or harm the natural enemies you are seeking to manage in your production system, understanding the biology and life cycle of the specific natural enemies you want to conserve is the first step to achieving the best results.
For more information:
Mahr, D. L. and N. M. Ridgeway. 1993. Biological control of insects and mites: An introduction to beneficial natural enemies and their use in pest management. North Centr. Reg. Ext. Publ. No. 481.
- Doug Landis, Michigan State University
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