LABORATORY EXERCISE 22
PURPOSE: Population Dynamics, Management Strategies and Population Assessment.
I. The Logistic Equation as a basis for considering nematode management
strategies.
a) Consider a simple logistic description of a plant of genetic potential
size (Q), growth rate per unit size (a), and of current size (P):
then..... dP/dt=aP(Q-P)/Q
b) Consider a nematode population of current size (N), with growth rate (r),
limited by carrying capacity of the root system (K):
then..... dN/dt=rN(K-N)/K
c) The carrying capacity for the nematode at any point in time is
approximated by the size of the plant at that time:
K=P*s, where s is the number of nematodes that can be supported
per unit of plant size.
d) Besides being constrained by its genetic potential, the growth of the
plant is also constrained by the nematode population level in relation to
its current carrying capacity; that is, the closer the nematode
population to the carrying capacity, the less functional the root system:
dP/dt=aP((Q-P)/Q)(K-N)/K)
e) The following general management strategies are suggested by this model,
and are testable:
(i) Reduction of the initial population - e.g. preplant pesticide,
non-host crop rotation, etc.
(ii) Reduction of the rate of population increase - e.g. systemic
nematicide, horizontal resistance, biological antagonists, etc.
(iii) Increase in carrying capacity of the plant, that is tolerance of the
nematode damage - e.g. by improvement of water and nutrient status,
removal of other sources of stress, etc.
II. Life Table Parameters.
The distributed-delay simulation model (POPSIM) is generalized for use
with any nematode population. The model opens reads a soil temperature
weather data file and allows a crop to grow during a standard spring and
summer growing season. The nematode population will only grow and
reproduce in the presence of the crop. The user is able to define the
number of individuals in each life stage on January 1, the developmental
time of each life stage, the standard deviation of this developmental time,
the fecundity rate of females, and stage-specific survivorship for each
stage. The pathogenicity of the nematode, and hence the level of crop
damage can also be established.
For example, the simulation can be performed with parameter values
reflective of r-selected attributes and K-selected attributes in separate
runs. The probability of success of the organism in an annual
cropping system can be determined.
A. Determine sensitivity of the population dynamics and final densities
of the organism to length and variance of life-stage duration, to
stage- specific mortality levels, and to fecundity rates. Use
reasonable estimates of life-stage duration for Meloidogyne
incognita, Xiphinema index, Paratrichodorus minor, and Mesocriconema xenoplax. Some literature sources are provided and can be
researched in the nematology library. Some paramater values will
need to be estimated. A useful approximation is derived
from Shaffer (Env. Ent., 1984), who noted that the standard
deviation for development in 113 species of insects and mites could
be described by:
S.D. = 0.209 X -0.73 where X - is the average development time.
Lifetable estimates: - Degree days
Egg/J1 J2 J3 J4 A E/f/DD
M. xenoplax 120 65 80 90 180 0.4
(25.0) (15.0) (16.0) (20.0) (60.0)
M. incognita 160 120 300 100 500 0.8
(33.0) (30.0) (55.0) (22.0) (110.0)
P. minor 68 50 45 88 100 0.7
(11.2) (7.5) (6.7) (26.9) (70.0)
X. index 200 300 500 500 800 0.1
7. NEMAPLEX Exercise
Main Menu:
Select...Decision Support
Select...Management Strategy Simulator
Use the logistic-based simulation model to explore the effect
of nematode management strategies, and their costs, that:
a) reduce the initial population;
b) reduce the rate of population increase; and
c) change the tolerance (carrying capacity) of the host to
the nematode.
Note the effect on the host, on the nematode population, and
on the net crop value.
Return to Main Menu
Select...Population Simulator
Use a range of initial densities for a single nematode
species on a host to which it is moderately pathogenic.
Plot log final population (Pf) against log initial
population (Pi).
Repeat for a host on which it is highly pathogenic.
Determine the maximum multiplication rate and equilibrium
density.
Determine the effect of host status on these parameters.
Determine the effect of management strategies that:
(i) Reduction in survival of specific life stages;
(ii) Reduction in fecundity;
(iii) Change in life-stage duration;
(iv) Change in life-stage variability.
Consider management tactics for achieving these changes.
Select...Nematode Management
Review: Principles of Nematode Management.
Tactics: (Various tactics listed)
Emerging Research Developments: (Various topics
listed).
Return to Main Menu
Select...Sampling Simulator
Do the sampling exercise and measure the population within
prescribed constraints.
Return to Main Menu
Select...Literature
Select...Management of Nematodes.
Return to Nematology 100 Menu