IPM Tactic: Biological Control
Now that we have some background on the biology and identity of pest organisms, we can spend more time intelligently (?!) discussing which tactics might work to suppress which organisms under what conditions. Please review the IPM Steps and Tactics.
Many of the tactics listed for IPM are common-sense and fairly well understood. For example, it is not news to most of you that one can hoe or hand-pull weeds, both physical tactics. Choosing pest-resistant plant varieties and keeping them well-fertilized and waters are cultural tactics that are fairly well understood. However, two categories of tactics require special background preparation before use; these are the use of biological control and chemical tactics, i.e. the use of pesticides. An overview of the dynamics of biological control is discussed below, followed by an overview of chemical pesticides. Class activities follow.
Biological Controls: General Points
"Balance of Nature"
In nature, very few species' populations "explode" into vast numbers. Most are suppressed by an active group of predators, parasites or diseases. In the case of plants, herbivore (plant eater) species keep them in check. From a human perspective, we consider these organisms "beneficial organisms" or "natural enemies" if they are harming something that is a pest to us. However, pests are not completely eliminated or eradicated using biological controls. Think about it. If the natural enemy destroyed all the prey, there would be no food left for their next generation and they would die out. Thus, the balance between predator-prey; parasite-host; disease-host; or herbivore-plant allows some of the host species to survive so that the natural enemy can reproduce and continue, from our point of view, suppressing the pests.
The goal of using biological controls therefore is to suppress pest populations below damaging or intolerable levels.
There are three ways people use these organisms in their gardens, greenhouses or farms to help suppress pests; they import, augment or conserve and/or encourage them.
Three Ways to Use Biological Controls
This method is also called "classical" biological control. The idea is that many organisms we know as "pests" on this continent actually originated elsewhere and were introduced by humans either on purpose or accidentally. Unfortunately, these "introduced species" usually come without their natural enemies back home, so no predators, parasites, diseases or herbivores keep them in check. If well-suited to their new environment, introduced species can explode and even become "invasive", taking over habitats formerly occupied by "native species". In importation or "classical" biological control, governments sponsor expeditions to the locations of origin of the pest to search out and bring back their natural enemies. Researchers look for organisms that are highly specific to attacking only the target pest to avoid causing unforeseen disruptions in the ecosystem in question.
To learn more about importation and use of natural enemies, teachers and their classes can partner with local agencies to help release natural enemies and study the effects of biological control on the pest species over time. A good recent example in the Northeast US is the release of 3 beetle species in the last decade to control the invasive wetland plant, purple loosestrife (see Activity on page ). In this way, students will learn about current on-going issues and outcomes in pest management.
When you buy and release commercially available biocontrol organisms, you are augmenting, or adding to, the naturally occurring population of beneficials. You can use this technique when you know the exact identity of the pest species and which biocontrol agents will attack that pest. This technique can be used most easily by classes that have access to a greenhouse. Greenhouse plants can be used to set up a controlled experiment to test the efficacy of different biocontrol agents. Alternately, a biocontrol program can be designed by students to manage the pests that will most likely come to live in your greenhouse anyway. For example, the greenhouse whitefly can be successfully suppressed by using the tiny wasp parasitoid, Encarsia formosa which is commercially available from a number of sources. Other pests, such as aphids, can be suppressed by other parasitoids and a number of predators. (See Activity on page ).
3. Conservation or Encouragement
Natural enemies are already in great abundance outdoors and are suppressing most of the potential pest populations. Thus, a garden or farm manager would do well to try to conserve them or encourage them.
What sorts of actions can be taken to conserve and encourage natural enemies? Consider the following list and think of how class activities could be built around them.
a) Learn to recognize natural enemies ("Friend or Foe? You Ought to Know!)
b) Learn to "scout" for natural enemy populations as well as scouting pests
Pest managers count the number of pests at regular intervals to take data on whether the pest population is present and if so, is it increasing or decreasing. Sometimes, if you are looking for them and notice that the natural enemy population is increasing also, you can mathematically predict that soon they will suppress the pest all by themselves - meaning you do not have to spray a pesticide if you just wait a bit longer.
c.) If using chemicals, choose those that are "soft" on natural enemies.
Chemical pesticides are all different; some are harsher than others, so try to pick those that will not kill the natural enemies. Remember, if you destroy them, you inherit their jobs of controlling pest species!
d.) Provide habitat useful to the natural enemies.
Biological control organisms are living, breathing organisms with needs of their own. To encourage their presence, provide for shelter, food or other needs. For example, the tiny wasps that parasitize caterpillars need to feed themselves on nectar from shallow flowers to keep up their energy. There are specific plants you can plant in your garden to help the good guys out!
Practical Uses of Biocontrol:
In private, commercial settings, the most common use of biocontrol is the augmentation method, where natural enemies are purchased and released. Greenhouses are the most common place where this method is used. However, because the natural enemies and pests are ALIVE and form dynamic populations, success of biological control in suppressing pests requires attention to certain important factors.
- Get the right enemy for the pest species.
- Release them at the right time in the pest population cycle.(Start when pest populations are LOW so enemies have a chance!)
- Environmental conditions must be favorable (weather / temperature / moisture).
- Food sources must be present for the natural enemy (adults or larvae in some cases).
Advantages and Limitations of Augmentative Biological Control
Advantages of augmentative biocontrol include:
- Reduced reliance on chemical pesticides.
- On-going suppression of pests over time (vs. repeated spraying)
- They reproduce themselves and keep working.
- Can you think of others?
- More knowledge is required to use successfully.
- More attention to detail is required.
- Results are not immediate - it takes awhile before pest populations decline.
- Cost is usually higher than use of chemical pesticides
- Can you think of others?