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