Biological Control of Nematodes

Rev. 06/13/22

Contents

 Population Regulation Soil Suppressiveness
Approaches Population Suppression A Case Study
Mechanisms Return to Management Menu  

 

Introduction:

The introduction of a nematode antagonist into a soil environment.  Requires:

  • that the organism has high invasive potential and is able to survive, spread and even multiply in the new environment
  • that the organism can be obtained or cultured in sufficient abundance for field release at realistic densities
  • some aggregation of the target nematode and a unique delivery mechanism: e.g., root dips, irrigation drippers, etc.

Augmentation:

Determination of the presence of a resident antagonist and augmenting its abundance or effectiveness by further introductions or by modifying the environment to increase its reproductive potential.  Requires:

  • knowledge of the biology of the resident antagonist, including constraints on its abundance and effectiveness
  • that the organism can be obtained or cultured in sufficient abundance for field release at realistic densities

Conservation:

Mediation or avoidance of adverse effects of cultural practices, pesticide applications or environmental conditions on abundance and effectiveness of resident antagonists and suppressive elements of the soil community.  Requires:

  • knowledge of the biology of the resident antagonist, including constraints on its abundance and effectiveness

 
Back to Top

 

Strategies

Mechanisms
A. Reduction of the initial population.

B. Reduction of the rate of population increase.


 a. Exploitation

Exploitation of nematodes as a food resource by other organisms has received most attention.  All organisms in the soil foodweb are sources of carbon and energy for other organisms.  The web is ultimately dependant direct grazing, litter, sloughings and exudates from plants, or on other exogenous sources.  The result of the interactions may be regulation or suppression of component populations.
A. Reduction of the initial population.

B. Reduction of the rate of population increase.

  

b. Competition

Has not received much attention but is probably an important mechanism of population regulation and damage mediation.

  • Direct competition for space, feeding sites. Examples: speculated competition for feeding sites between Paratylenchus hamatus and Mesocriconema xenoplax may dampen rate of population increase of the more damaging species;
  • .Apparent competition where the presence of the "competitor" results in increase of predation pressure due to density-dependence or renders the environment less favorable.  Examples: increasing population levels of bacterial-feeding nematodes that share a common predator with a plant-feeding species should result in greater abundance of predators;  parasitism of potato roots by Pratylenchus neglectus may render them less attractive to Meloidogyne chitwoodi (Umesh and Ferris).
A. Reduction of the initial population.

B. Reduction of the rate of population increase.

 

 c. Antibiosis

some preliminary work with rhizosphere bacteria.  Commercial approaches by Abbott laboratories have resulted in an antibiotic product of fungal origin, DiTera, which is currently registered as a nematicide.
Back to Top

 

Population regulation and suppression 

The terms are not synonymous.

  • Regulation implies a buffering impact on population change.
  • Suppression indicates population reduction. 

Both are legitimate goals of biological control.

Regulation and suppression by biological antagonists may be expected to  exhibit the known phenomena of population interaction, including spatial and temporal density-dependence, especially if the mechanism involved is exploitation or 
antibiosis.

  • Density-dependent regulation implies that the level of parasitism or predation that is occurring in the population depends on the density of the population. In the case of fungal infection of nematodes, the probability that infection will spread to healthy individuals depends on the numbers of individuals present. Similarly for predation.
  • Hence, the proportion of the population that is parasitized or killed by predators will increase as the population increases. There may have to be many nematodes present for the proportional mortality to be high. 
  • The residual population may still be above damaging levels.

Spatial density-dependence recognizes that the populations of antagonists are seldom uniformly distributed. 

  • The percent parasitism or predation may be high in high-density aggregates and low in low-density aggregates. 
  • This raises the issue of scale in studying ecological systems, and constitutes the basis of Murdoch's descriptions of the possibility of "local extinction and global stability".

Temporal density-dependence recognizes that population densities change through time. 

  • Consequently, the  proportional antagonism may be higher seasonally when host or prey populations are higher. 
  • The concepts also allow for a time lag in the relationship between the host and antagonist.

Density-dependent population regulation

An example of density-dependence of the ring nematode Mesocriconema xenoplax.  The incidence of parasitism by the fungus Hirsutella rhossiliensis increased as population densities of the nematode increased following soil fumigation.  Nematode population levels increased at a greater rate on Nemaguard than on Lovell rootstock, and at a lower rate when annual applications of phenamiphos (Nemacur) were applied.

Soil Suppressiveness

Consider the notion of population regulation in biologically- buffered soils as a community phenomenon rather than the  result of interaction of two populations. 

Hence the need to develop multiple-decrement lifetables, as in studies of  human ecology and as suggested in entomology (Carey), in  understanding and assessing the suppressiveness of soil.
Back to Top

 

A case study

Consider Jaffee's studies on the relationship between percent parasitism of the ring nematode, Mesocriconema xenoplax, in peach orchards in California. In some orchards 10% of the nematodes are parasitized, in others, 50%. The relationships appear stable in a given orchard.

Potential interpretations are complex: 

  • scale of observation - the probable much larger size of the soil sample than the universe of a nematode population  aggregate;
  • spatial density-dependence among population aggregates;
  • temporal density-dependence within population aggregates;
  • probable asynchrony of population events among aggregates of this stable-age-structured nematode population;
  • consider the idea of the metapopulation, that is, a series of non-interacting populations sharing a defined area, with the individual population units asynchronous and in disequilibrium, but the metapopulation appearing stable - 
    again a question of scale of observation.
  • Consequently, there is a need to study underlying mechanisms in scale-appropriate microcosm to provide understanding of the whole system.
Back to Top

Return to Management Menu