Biological Control of Weeds

1
Biological Control of Weeds

• Peter B. McEvoy
• Ent 420/520
• Oregon State University

2
Biological Pest Control

• Scope of weed biocontrol
• Historical highlights
• Stages in Project Development
• Determining host specificity
• Selecting target and control organisms
• Ecology as an explanation and guide for biocontrol

3
Scope of Weed Biocontrol Excludes

1. Weed species of value
2. Weeds closely related to crop plants, widely
   planted ornamentals, or valued native plant
   species
3. Weeds that require immediate control
4. Weeds that need to be eliminated from an area or
   can be tolerated only at very low densities
5. Weeds that are geographically localized or have
   minor importance
6. Weeds of cropland under intensive cultivation,
   frequent crop rotation, or heavy pesticide
   treatment

4
Historical Highlights

• 1795 - First transfer between countries
• 1903 - Pioneering foreign exploration
  (Lantana)
• 1925 - Outstanding early success (Opuntia)
• 1951 - First control of native weed (Opuntia)
• 1945-46 -A successful transfer project
  (Hypericum)
• 1964 - First aquatic weed (Alternanthera)
• 1961- First annual weed (Tribulus)
• 1960-70s - First control of cropland weed
  (Chondrilla)

5
Stages in Project Development

• Domestic Surveys
• Literature searches to learn about target-weed
• Pre-introduction surveys of fauna
• Scoring system for selecting target weed
• Foreign exploration and selecting promising
  candidates for screening
• Host specificity determination
• Importation, Release, and Establishment
• Evaluation of results

6
Classical Biological Control of Weeds Not a
panacea, Not risk free
Alien Plant
Alien Control Organism
7
Biological Control as a Lottery

• Runaway importation rates
• Monitoring and evaluation gap

McEvoy and Coombs 2000
8
Biocontrol of Leafy Spurge
9
Practicing biocontrol as a lottery can lead to
revenge effects (McEvoy and Coombs 1999)

• Scarce resources are diverted from more
  profitable alternatives for managing pests
• One control organism undermines another, more
  effective control organism, leading to increase
  in pest density
• One pest is replaced by another pest that is even
  harder to control
• Control organisms introduced to promote
  environmental and economic health end up
  undermining it by harming non-target organisms

10
Questions to Answer Before Releasing New
Organisms Into the Environment

• Are new organisms necessary? What is the severity
  of the problem?
• Are they effective? How effective is the proposed
  solution to the problem?
• Are they safe? Will control organism harm other
  plants?

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Are Control Organisms Likely to Be Effective?

• Critical Attributes. Species with desirable
  biological properties can be selected based on
  expert intuition, data bases, and mathematical
  and experimental models.
• Targeted disruption of pest life cycles. Using
  field studies and math models to identify which
  life-cycle transitions and pathways contribute
  most to population growth, which are most easily
  disrupted, and using this information to improve
  targeting of pest vulnerabilities
• Combinatorial Ecology. Coordinated manipulation
  of disturbance, plant competition, and natural
  enemy regimes

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Insects for Control of RagwortSenecio jacobaea
(Asteraceae)
1960 Tyria jacobaeae (Lep Arctiidae) Cinnabar moth
1966 Botanophila seneciella (Diptera Anthomyiidae) Ragwort seed fly
1970 Longitarsus jacobaeae (Coleoptera Chrysomelidae) Ragwort flea beetle
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Traditional Paradigm Focus on Level and
Stability of Pest-Enemy Equilibrium
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Ragwort Biocontrol
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Ragwort Biocontrol by the Numbers
Forbs
Grasses
McEvoy et al 1991
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Biocontrol of Klamath Weed in CA Huffaker and
Kennett 1959
Total Forage
Percentage Cover
Other Weeds
Year
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Lingering UncertaintiesCinnabar Moth Friend or
Foe?

• Necessary. Is cinnabar moth necessary for ragwort
  control?
• Effective. Is it effective in controlling the
  target host?
• Safe. Is it a threat to nontarget species?

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Control Value of Ragwort Seed-Head Fly

• Fly oviposition and ragwort flowering are not
  well synchronized
• Early, aggregated attack by the fly means 70 of
  flowering heads escape attack

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General Questions

• Patterns in system dynamics. What are the
  patterns in the dynamics of weed biocontrol
  systems?
• Regulatory mechanisms. How are these systems
  regulated?
• Response to perturbation. How does the system of
  interactions respond to perturbation?

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Combinatorial EcologyFood Webs and Driving Forces
C

• Disturbance
• Colonization
• Local interactions (successional development)

H
P
R
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Hypotheses

• The Activation Hypothesis is that localized
  disturbances and buried seed combine to create
  incipient weed outbreaks.
• The Inhibition Hypothesis is that natural enemy
  attack and interspecific plant competition
  combine to oppose increase and spread of
  incipient weed outbreaks.
• The Stability Hypothesis is that the balance in
  activation and inhibition leads to a general
  condition of local instability and stable average
  concentration of the pest.

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Goals of Demographic Analysis(McPeek and Kalisz
1993)

• Population growth. Estimate population growth
  rate and project future population dynamics
• Stable stage distribution and reproductive
  values. Understand and project population
  structure and contributions of different types of
  individuals to future reproduction
• Sensitivity and Elasticity. Understand
  contributions of each age or stage class to
  overall population growth rate and fitness

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Design of LTRE
Synthesis
VITAL RATES
POPULATION STATISTICS
T1
E1
A(1)



VITAL RATES
POPULATION STATISTICS
TN
A(N)
EN
Matrix
Treatment
Effect
Analysis
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Matrix Population Model
Life Cycle Graph Age and Stage
Projection Matrix M
D J1 J2 A
D a 0,0 0 0 a 0,3
J1 a 1,0 0 0 a 1,3
J2 0 a 2,1 a 2,2 0
A 0 a 3,1 a 3,2 a 3,3
J2
J1
A
D
Nt1 M Nt At SSD Nt1/Nt ?
Time scale 1 yr
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Life Table Response Experiment (LTRE)Experimental
Design

• 2 Disturbance Times (Fall 86, Spring 87)
• 3 Plant Competition Levels (Removed, Clipped,
  Unaltered)
• 2 Cinnabar Moth levels (Present, Absent)
• 2 Flea Beetle Levels (Present, Absent)
• 4 Blocks
• 96 experimental units

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Treatment Effects on Population Growth
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Prospective and Retrospective Analysis
External Forces Life Cycle Transitions Population Growth Rates

?ij ?

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Elasticity Analysis
Elasticities sum to 1.0 and represent the
proportional contribution of each element to ?
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Elasticity
Percentage of total elasticity associated with
each life cycle transition
Perennial N 8 subpopulations
Biennial N 24 subpopulations
11
50
14
J1
A
14
37
50
24
30
Treatment Effects
X
Plant Competition
Flea Beetle
Cinnabar Moth
X
J2
J2
J2
X
X
X
X
X
J1
J1
A
J1
A
X
A
X
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A Sound Control Strategy for Control of Ragwort
Should

• 1) favor the ragwort flea beetle over the
  cinnabar moth
• 2) promote plant competition
• 3) reduce the intensity and frequency of
  disturbance

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Conclusions Regarding Control Effectiveness

• Driving forces of disturbance, colonization, and
  local interactions (competition and herbivory)
• Parsimonious prescription. Control can be
  achieved using fewer natural enemies by
  coordinated manipulation of disturbance,
  competition, and natural enemy regimes

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Specific Models Have Practical Benefits (Barlow
1999)

• Outcome of introduction predicting the outcome
  and success of a specific introduction
• Agent selection aiding in the selection of the
  most appropriate agent
• Predicting the impact of exotic agents on
  ecosystems and non-target species
• Understanding of the processes involved
• Critical data assisting in the identification
  and interpretation of critical field data

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Prickly Pear in South AfricaLogistic Pattern
35
Opuntia aurantiaca and Dactylopius austrinus in
South AfricaPopulation Cycles
Moran and Zimmerman 1991
36
Insects for Control of Sesbania punicea in South
Africa
Species Resources Treatments Treatments Treatments Treatments
Species Resources A-C D-F G-L M-P
Trichapion lativentre Flower buds x x x x
Rhyssomatus marginatus Developing seeds x x
Neodiplogrammus quadrivittatus Trunk and stems x x
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Number and Identity of Species for Sesbania
Control
Hoffman and Moran 1998