Lecture 22 – EFB 502 2016
Biological Control
• Use natural enemies (predators/herbivores, pathogens, parasitoids/
parasites) to suppress or reduce the population of a target species.
• The only practical (ecological/economic) method for maintaining
widespread, well-established invasive species at lower population densities
Types of BC
• Classical BC – Reunites an invasive species with its own natural enemies
(Functionally, a planned ‘invasion’.
• Augmentative BC - Releases a BC agent that is incapable of long term
persistence – essentially a biological pesticide
Inoculative – Enemy reproduces but dies out with out further
introductions
Inundative – does not reproduce
• Conservation BC
Changes or preserves habitat features that favor the persistence and
abundance of natural enemies. Can be used in conjunction with either
augmentative or classical BC
Classical Biological Control
• Re-association of an invasive species with its natural enemies
• High rate of failure
• May impose significant risk
• Return on investment is very high if successful
• Eradication of the target species IS NOT the objective of classical
biological control (although it may occur at very local scales!)
Invasive species
Native species
Ecological / Economic
Damage threshold
Figure: Objective of a CBC program. Reduce invasive species below some population
density threshold and maintain it at some lower density in perpetuity
Classical Biological Control
• Control of a non-native species in its invasive range through introduction (reassociation) of its own natural enemies.
• Success rate is variable, ~10% for control of insect pests, >50% for weeds on
islands
• The most attractive aspect of CBC is the very favorable Cost / Benefit ratio.
Payback on successful programs may exceed 200:1
• Until recently, risk was generally thought to be low with modern biological
control
•
Developing classical BC programs
1. Determine the suitability / susceptibility of the target organism for biological
control
2. Search for potential natural enemies
3. Evaluate the ecology of natural enemies
4. Select candidate species
5. Conduct host range testing
6. Small-scale experimental releases
7. Post-release evaluation and follow-up study
8. Large scale general release
Under current guidelines, this can take 5-10 years and millions of dollars!!
Host Range Testing
• Different organisms are subjected to different levels of stringency
• Agents for control of plants – stringent federal requirements
• Agents for control of arthropods
US – little or no restriction until very recently
EU – developing guidelines
Aus./NZ – strict requirements
Host Range Testing
• Centrifugal Phylogenetic Wheel – Developed for plants
• Sequence of plants chosen from most closely related to more distantly
related species
Includes crop and endangered species
Usually 50 or more species used.
• Subject the candidate BC species to a battery of choice and no-choice
experiments
Evaluate its overall response and assign a risk probability
Accept or reject species
• Pretty good predictor (Pemberton 2000). Of 118 introductions for BC, only
one species attacked plants not closely related to the target species.
Biological Control of Arthropods
• Host-range testing - much more difficult
Few are monophagous (feed on one species)
Often have complex behaviors in three dimensional environments that are
difficult to test in a lab setting
May overestimate or underestimate host range
Many parasitoids/predators have significant capacity to learn and adapt to
unfamiliar environments and/or prey
Rapid shifts in host preference in the field
Success in Biological Control
 Variable – depends on the target species.
Weeds on islands >50% success, higher ~30% world wide in all systems
(Syrett et al. 2000).
Insects in all control programs Worldwide, ~ 2300 introductions for insect
pests: complete control in ~ 100 cases (4%), substantial control 140 cases
(~10%) (Ehler and Andres, 1983).
Gurr et al. (2000) for insects, estimate < 10%.
Cost-Benefit
• Successful biological control programs have very high benefit to cost ratios
(~145:1)
• The benefits of successful programs are so great that they may exceed the
costs of all failed programs
• Indirect benefits include reductions in pesticide/herbicide contamination of
environments