Basic Principles of Nematode Management

Rev 06/13/22

Premises: 

1. Many plant-parasitic nematodes have a wide host range; crop rotation may be difficult.
2. Nematode dispersal is usually passive, but may be active over short distances or vectored over longer distances; immigration and emigration are not short-term considerations.
3. Soil and plant remains are major reservoirs; the population of interest is already resident; it is detectable and assessable. 
4. Knowledge of ecological and population factors affecting nematodes; including age structure, gene pool, growth rate, spatial pattern, relationship with environment, has potential value in designing control strategies.
5. Control has usually been through reduction of the nematode population.
6. There are usually several species of plant-feeding nematodes at a given site.
7. Not all nematode species are harmful, some may be beneficial in nutrient cycling or suppression of opportunistic species.  Conservation of beneficial species may be important.
8. The behavior of a biological system may differ from expectations based on reductionist observation.

See Summary of Management Strategies and Tactics

Important Concepts:

I. The Logistic Equation

Consider the logistic 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:

1. Reduction of the initial population - e.g. preplant pesticide, non-host crop rotation, etc.
2. Reduction of the rate of population increase - e.g. systemic nematicide, horizontal resistance, biological antagonists, etc.
3. 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 and  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. 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 can be determined. 

Reasonable estimates of life-stage duration for Meloidogyne incognita, Xiphinema index, Paratrichodorus minor, and Mesocriconema xenoplax should be used.

 Some parameter 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.

B. An interesting exercise is to plot log final population (Pf) against log initial population (Pi), using a range of initial densities for a single nematode species on a host to which it is moderately pathogenic. 

Repeat for a host on which it is highly pathogenic. 

The maximum multiplication rate and equilibrium density can be determined in each case, as can the effect of host status on these parameters.

C. The potential of management strategies can be determined that:

1. Reduction in survival of specific life stages;
2. Reduction in fecundity;
3. Change in life-stage duration;
4. Change in life-stage variability.

Lifetable estimates: - Degree days + s.d. (%)