Nematode populations dynamics threshold levels and estimation of crop loss

1. NEMATODE POPULATIONS
DYNAMICS THRESHOLD
LEVELS AND ESTIMATION
OF CROP LOSS

2. Outline
■ Introduction
■ Threshold levels
■ Factors influencing nematode thresholds
■ Population dynamic
■ crop loss assessment
■ Nematode damage models
■ Approaches of estimating yield loss

3. Introduction
■ Although hundreds of different nematode species are associated with plants, not
all are plant parasites.
■ In several cases, phytonematode populations occur in numbers too small to cause
serious plant injury.
■ Limited information is available regarding potential economic losses associated
with synergistic and antagonistic interactions between nematode species and the
involvement of them in disease complexes.
■ The evaluation of population dynamics and threshold levels and the damage is
thus of much significance.

4. Threshold Levels
■ Determination and use of economic thresholds are considered essential in
nematode pest management programs.
■ Economic thresholds refer to the population density of a pest at which the value of
the damage caused is equal to the cost of management.
■ The economic efficiency of control measures is maximized when the difference
between the crop value and the cost of pest control is greatest.
■ Since the cost of reducing the nematode population varies with the magnitude of
the reduction attempted, an economic (optimizing) threshold can be determined
graphically or mathematically if the nature of the relationships between degree of
control and cost and nematode densities and crop value are known (Ferris 1978 ).

5. Cont..
■ Thresholds may vary due to differences in sample collection methods, nematode
assay methods, soil types, climate, etc. Some differences may result from simple
differences of opinion.
■ For each crop, there may be certain plant-parasitic nematode species present in a
sample that are not known to cause damage even at high levels.
■ If a species has not been shown to cause a loss of yield or quality, no threshold is
given and it is assumed that no management is necessary even if that species is
present in high levels.

6. Cont..
■ Accurate and detailed records should be kept for each field to track where each
nematode species occurs and where populations tend to be highest.
■ In fields where a damaging nematode species is detected at subthreshold levels, it is
possible that “hot spots” with above-threshold levels can occur.
■ If such hot spots are identified, localized nematicide applications may be profitable
where application to the whole field would not be.
■ It is common for a sample to contain two or more nematode species each at
subthreshold levels.
■ In these situations, some common sense must be exercised.
■ As an example, if two species are each present at 90 % of their threshold levels, then
management tactics probably should be implemented.

7. Cont..
■ The terms “tolerance limit,”
“damage threshold,” and
“economic threshold” are used by
nematologists when characterizing
relationships between nematode
density and yield.

8. Cont..
■ Economic threshold level depends upon several factors including the type of
parasitism, life cycle, rate of reproduction and survival of the nematode, tolerance
of the crop, length of the growing period of the host plant, and environmental
conditions, because under good growing conditions, type of soil with adequate
moisture and nutrition, plants can tolerate more nematodes than under stress
conditions.
■ Economic thresholds are often difficult to determine because environment and
market values cannot be predicted with certainty.

9. Cont..

10. Factors influencing nematode thresholds
■ Pathogens
■ Soil type
■ Vigor
■ Antagonism
■ Temperature

11. Pathogens
■ A nematode population’s species is commonly recognized as the most important
consideration in determining a nematode threshold. However, it is extremely
difficult to identify the majority of individual nematodes to species in any soil
sample.
■ As a result, identifications are usually made to the genus level even though
management recommendations are made using numbers published for an explicit
species.
■ While this would seem to be a major problem, it is generally considered an
acceptable practice.

12. Soil Type
 Soil type is another factor that may affect nematode thresholds.
 Nematodes are aquatic organisms.
■ Even terrestrial nematodes live in films of water.
■ The available water, the size of pores, and level of soluble inorganic molecules will
all be affected by soil characteristics.
■ Most turf- parasitic nematodes, for instance, are ectoparasitic; that is, they spend
their lives in the soil and only their stylets ever enter into the plant.
■ Even migratory endoparasites (those that move around side of plants) will spend
some time in the soil

13. Vigor
■ Plant health and vigor have a major impact on nematode thresholds. Nematodes
are often considered stress-related pathogens.
■ They usually do not cause significant damage but can cause visible disease
symptoms when plants are under excessive stress.
■ The same population of nematodes that can kill stressed turf may go unnoticed
on a vigorously growing stand.

14. Antagonism
■ Microbial antagonists surely play a role in limiting nematode populations and may
affect threshold values.
■ Many species of fungi live off plant-parasitic nematodes and can reduce total
nematode numbers.
■ While this may not affect a nematode threshold, antagonists like the bacterium
Pasteuria penetrans certainly do.
■ Pasteuria attach to the cuticle of nematodes and slowly invade and reproduce
inside the host.
■ During this process, the nematode continues to survive but its pathogenicity, vigor,
and fecundity will decline.

15. Temperature
■ While climate and temperature are often overlooked, they may dramatically affect a
threshold in the context of nematode vigor.
■ Nematodes are invertebrates, and as such their life cycle is entirely dependent upon
the environmental temperature.
■ Nematode reproductive rates and metabolism are directly proportional and respond to
fluctuations in temperature.
■ Some nematode species have the ability to become dormant in colder climates and
survive in frozen soils while others die.
■ Some become quiescent at high soil temperatures while others thrive.
■ To a certain point, nematodes that experience warmer temperatures will be more
active and cause more damage.
■ Thus, thresholds for the same nematode may vary from region to region.

16. Population Dynamics
■ The term population dynamics is used to convey changes in the numbers, age
class distribution, sex ratio, and behavior of a population through time and space,
determined by inherent characteristics of the individuals and mediated by
environmental conditions, food resources, and interacting biotic agents.
■ Nematodes have various reproductive strategies.
■ Some grow large and have long life cycles with low rates of population increase (K
strategists), others are relatively small, have short life cycles and potentially higher
reproductive rates (r strategists)

17. Cont..
■ An endoparasitic habit with induction of giant cells or other rich and continuously
available food sources, reduces exposure to predation and other stresses and
further increases reproductive potential.
■ A reduction in the number of active juvenile stages further decreases development
time, thereby reducing generation time and increasing the potential for multiple
generations in a season.
■ A wide host range completes the adaptation of pathogens such as some
Meloidogyne spp., which can be regarded as the ultimate plant-parasitic nematode
r strategists.
■ Many Longidorus spp. are examples of K strategists.

18. Cont..
■ It is a characteristic of K strategists that they do best in stable environments where
populations are usually close to the equilibrium density (the population density
that can be sustained).
■ In contrast r strategists increase rapidly where the environment is favourable, often
overshooting the equilibrium density.
■ Severe damage to the host occurs and the population crashes.
■ This can occur with repeated cropping of hosts as Pi increases, environmental
influences on sex determination reduce multiplication, parasites of the nematode
increase in number, and increasing damage to the host and competition for
feeding sites progressively reduces multiplication.

19. Cont..
■ Consequently, nematode multiplication rates are strongly density dependent.
■ Again, the question of how density is defined arises.
■ Usually it is expressed as the numbers of nematodes per gram or ml of soil, but the
units that directly affect the nematode are those that are root related, e.g. number
of root tips and/or length or weight.
■ Hence, a cultivar with twice the root mass of another will, except at low densities
where the multiplication rate is the maximum, support a higher multiplication rate.
■ Similarly, tolerant cultivars that maintain a greater root mass as Pi increases than
intolerant cultivars, will have a greater equilibrium density and maintain a greater
multiplication rate at high pre-planting population densities.

20. Cont..
■ Overall multiplication rates are determined by the intrinsic maximum rate of
multiplication, which is influenced by nematode species, the susceptibility (defined as
all those qualities favouring the nematode) of the host, and the various environmental
factors that influence both the nematode and the host.
■ Nematode multiplication can be modelled in different ways. For migratory nematode
species that multiply continuously, Seinhorst (1966) proposed the following formula
derived from a logistic equation:
■ Pf = aEPi/(a - 1)Pi + E
■ where a is the maximum rate of increase and E is the equilibrium density at which Pf =
Pi. For sedentary nematodes with one generation at a time, e.g. potato-cyst nematodes,
Seinhorst (1967) proposed an alternative model based on the competition model of
Nicholson (1933):

21. Cont..
■ where a is again the maximum rate of multiplication, and 1 - q is the proportion of
the available space which is exploited for food at a density of Pi= 1.
■ Jones and Perry (1978) also proposed a model for sedentary nematodes with a
logistic basis derived from the observation that sex determination is density
dependent. Their model includes parameters that reflect fecundity and the
proportion of the population that does not hatch.

22. Cont..
■ All three models, in their most basic form, show maximum rates of multiplication at
low initial densities.
■ As Pi increases the rate of multiplication is reduced as an upper asymptote is reached .
■ In reality, the shape of this curve is modified as Pi increases due to the increasing
damage inflicted and the loss of roots.
■ With the Jones and Perry model this is exacerbated as space lost as a result of root
damage increases the competition between invading nematodes, resulting in an even
greater shift in the sex ratio towards male production than would otherwise be the
case.
■ Thus the approach to the asymptote is slower and indeed the asymptote is reduced
below the theoretical level.
■ Further increases in Pi can inflict so much root damage that the population increase
becomes negative and the population size is ultimately reduced.

23. Cont..
■ All the equations mentioned require modifying by including a damage function
such as that of Seinhorst, which also allows the differences in tolerance between
cultivars to be taken into account.
■ The damage functions used model proportional differences, and further
modification may be required to account for absolute differences in plant size.
■ Another plant characteristic that affects population increase is the host status of
the plant.
■ Differences can be modelled in terms of the maximum multiplication rate or the
space required for successful multiplication (Seinhorst models), or in terms of
fecundity or effects on the sex ratio (Jones and Perry model).

24. Cont..
■ An important method of expressing and comparing the effects of different cultivars or
cropping regimes is to consider the equilibrium density, i.e. the point at which Pf = Pi.
■ This density is usually observed at a Pi which is larger than that which gives the largest
Pf .
■ In practice this equilibrium density is reached after a period of oscillation about the
equilibrium density.
■ The size of the oscillations will be determined by the tolerance and resistance of the
host.
■ Tolerance and resistance will produce small oscillations, while susceptibility and
intolerance can result in large oscillations.
■ Indeed, these two factors can interact to the extent that a tolerant but partially resistant
cultivar can produce a higher equilibrium density than an intolerant susceptible cultivar

25. Crop Loss Assessment
■ Crop losses are influenced by several factors including the pathogenicity of the
species of nematode involved, the nematode population density at planting, the
susceptibility and tolerance of the host, and a range of environmental factors.
■ Because of this, available models only estimate yield losses as proportions of the
nematode-free yield.
■ Estimating threshold levels further involves various economic calculations.
■ Consequently, predicting yield losses and calculating economic thresholds for most
nematode/crop problems is not yet possible.

26. Crop Losses Due to Nematodes
■ Major objective of crop damage assessment and prediction is to form a basis for
nematode management decisions.
■ Annual estimated crop losses due to nematodes in India have been worked out to be
about Rs. 242.1 billion.
■ Plant pathogenic nematodes are responsible for an annual loss of over $100 billion
worldwide (Sasser and Freckman 1987).
■ Losses also occur as a result of sampling and control using nematicides.
■ Many a times in olden days, nematodes have caused people to migrate due to soil
sickness.
■ It has been estimated by the International Meloidogyne Project that nematodes cause
annual losses of 78 billion US dollars in developed countries and more than 100 billion
in the developing countries.

27. Cont..
■ Nematodes pose a constraint to horticultural development and intensive
cultivation.
■ It has been estimated that annually an average 6 % loss in field crops, 12 % in fruit
and nut crops, and 11 % in vegetable and 10 % in ornamental crops is due to
nematode infections.
■ Besides causing quantitative losses, nematodes are known to reduce vitamins and
minerals in edible plant parts.
■ Nematode damage is less obvious and many a times goes unnoticed. It causes
gradual decline in yield (Seinhorst 1965).
■ Nematodes cause complex diseases in association with other soilborne pathogens.

28. Quantification of Nematode/Disease
■ Nematodes have generally been quantified in terms of their numbers per unit of
soil or plant part.
■ Number of nematodes may be related to the intensity of plant symptoms, which in
turn is a visual indication of the stress imposed on the plant, resulting in
measurable loss.
■ With nematodes, preplant nematode density is the most common descriptor used
in quantitative relationships of loss.
■ Field disease assessment is done normally, by using disease keys, standard area
diagrams, remote sensing, and population counts.

29. Cont..
■ Disease keys and standard area diagrams rely on the determination of severity in
comparison with a predefined key or series of diagrams depicting different degrees of
severity.
■ The severity assesses for a plant part like a leaf includes the infected area as well as
any accompanying chlorosis or necrosis.
■ Remote sensing has been successfully used for assessment of losses due to nematode
pests that cause total plant loss but has been only marginally successful with pests
that affect only plant parts (James and Teng 1979).
■ Population counts are widely used in quantifying diseases caused by nematodes. A
problem in loss assessment is the determination of a representative mean value of the
nematode/disease in a cropping unit using the designated method of assessment.
■ Sampling for diseases populations is a relatively under-researched area in comparison
with insect sampling.

30. Cont..
■ The distribution of a nematode/ disease in any spatial unit may be mathematically
described as a frequency distribution with estimated parameters, for example,
normal or negative binomial.
■ Preliminary indications of the type of distribution are obtained by examining the
mean: variance ration of the sample mean of disease intensity.
■ Knowledge of the type of distribution in a field enables sampling protocol to be
designed to obtain a representative mean in an economic manner (Lin et al.
1979).
■ Nematode populations commonly occur as clusters, suggesting that the pattern of
taking samples from a field is important (Barker and Olthof 1976).
■ Various microprocessor techniques have been designed for use in fields.

31. Cont..
■ Portable, low-cost data acquisition system for measuring canopy reflectances,
which may be used for determining the mean effect of a pest on a crop in terms of
reduced crop vigor; laboratory-based video image analysis unit to measure the
area of infected leaf tissue and proportion of infection; testing the taping of
images of diseased leaves in the field with portable video cameras and analyzing
the images in the laboratory; image analysis is routinely used for measuring the
root area of plants and it is conceivable that there will be developments allowing
its use for measuring nematode number in a sample (Lindow and Webb 1983).

32. Nematode Damage Models
■ The nematode, the host, and the environment are the three interacting variables
influencing the extent of yield loss in infested soils.
■ An understanding of the mechanisms and principles involved in these interacting
relationships is basic to being able to predict yield reductions from estimates of pre-
planting nematode population densities (Trudgill 1992).
■ When modeling the damage caused to plants by root-feeding nematodes, several basic
principles are to be considered like damage is proportional to the nematode population
density; the degree of damage is influenced by environmental factors; and the yield
harvested is determined by the amount of light intercepted by the crop, by how
efficiently the intercepted light is converted into dry matter, and finally by how that dry
matter is partitioned into non-harvested and harvested yield.
■ For some crops significant variations in moisture content will also affect final yield; the
above principles are more complex in practice.

33. Cont..
■ Damage may be proportional to the nematode population density, but there are
several qualifications of this statement, viz., the relationship is usually curvilinear,
increasing numbers of Nematodes having proportionally diminishing effects; there
is some evidence that at low densities the host plant can repair the damage and
that growth may even be slightly stimulated; Seinhorst (1965) termed the
population density (Pi) at which damage first became apparent as the tolerance
limit (T); equally, at very high values of Pi, increasing numbers of nematodes may
not further reduce dry matter productivity.

34. Cont..
■ This minimum yield is termed “m.” There are various reasons for the occurrence of
“m”; there may be some growth before attack starts or after it finishes, and a
significant biomass may be planted (e.g., potato tubers).
■ However, “m” applies to total dry matter and because of effects on partitioning, the
harvest value of m may be greater or less than that for total dry matter; the third
parameter in the Seinhorst equation is z, a constant slightly less than one.

35. Cont..
■ The equation is as follows:
■ An important qualification is that y is expressed as a proportion of the nematode-free yield. Hence, according to
Seinhorst, the greater the yield potential, the greater the loss in tons per hectare for any value of Pi.
■ The Seinhorst equation is usually plotted with Pi on a logarithmic scale, producing a sigmoidal curve.
■ In practice T is usually small and the Pi value at which m is reached is so large that it is only the central part of the
curve that is of practical use.
■ This approximated to a straight line (Oostenbrink 1966). The equation for such a line is y = y (max) − slope constant ×
log Pi.

36. Cont..
■ The slope of the regression varies for several reasons.
■ These include differences in pathogenicity (capacity to cause damage) between
species, e.g., Meloidogyne spp. may be inherently more damaging than Tylenchus
but there is no measure of their relative pathogenicities.
■ Different plant species and varieties within species differ in their tolerance
(capacity to withstand nematode damage).
■ Also, there are large environmental influences on the damage suffered and
particularly how that damage is translated into effects on final yield.

37. Cont..
■ An important consideration, often overlooked, is the basis of measuring Pi. Usually
it is given as numbers per gram of soil.
■ A more appropriate measure is per unit volume of soil as this allows for bulk
density differences.
■ Numbers per gram of root is probably the most appropriate, but is difficult to
measure because it is always changing.
■ This latter aspect becomes important when trying to relate results from
experiments where root densities are very different, e.g., pot and field trials.

38. Cont..
■ A further problem is encountered when considering damage by nematodes that have
two or more generations in the lifetime of a crop.
■ Usually the Pi is measured at planting, but on a good host population of, for example,
Meloidogyne spp., they can increase from below the value of T to a level in mid-season
where they cause significant damage.
■ Even so, it is a race between increasing Pi and increasing plant size that brings with it
increasing tolerance (in Seinhorst terms, increasing m).
■ In such situations suitability as a host (susceptibility) and tolerance can have a marked
effect on the degree of damage.
■ To conclude, both the Seinhorst and Oostenbrink equations are, without the addition of
a substantial amount of additional information, purely descriptive and cannot be used
to predict actual yield losses.

39. Approaches of Estimating Yield Losses
■ Pot studies can be used to determine some of the basic information on yield-loss
relationships, but because of environmental differences and interactions, field
studies are also needed.
■ There are two major approaches, viz., to use nematicides at relatively uniformly
infested sites and to work at sites with a range of population densities but which
are uniform in other respects (Trudgill and Phillips 2006).
■ A combination of both approaches is often a happy compromise.
■ The former gives practical information on the effectiveness and potential value of a
particular treatment but tells little about the nature of the relationship.

40. Cont..
■ It also suffers from the criticism that nematicides have a range of side effects.
■ The latter has the benefit of producing information on the relationship between Pi
and yield, but it requires experimental errors to be minimized.
■ Because Pi estimates have large errors, accuracy is improved by reducing plot size
and by taking and processing multiple samples from each plot.

41. Cont..
■ However, plot size must be large enough to obtain a realistic yield and adequate guard
plants are essential. Another option is to establish many small plots in large but
otherwise uniform fields.
■ These can be at random, in a grid pattern or along known trends in Pi. The plots can be
split and a nematicide applied to one-half. For each plot the Pi and yield are
determined.
■ The results will produce a scatter of points, hopefully with yield decreasing as Pi
increases.
■ Much of the scatter is due to errors in estimating Pi and yield, and it can be minimized
by taking the average of all the results within each error band.
■ Such an approach requires a wide range of initial populations, a uniform field, a large
number of plots (100 or more), and the plots to be part of an otherwise uniform crop.

42. cont..
■ Chemical management of Tylenchulus semipenetrans consistently increased yield of grapefruit on
sour orange rootstock in Texas (Timmer and Davis 1982).
■ In this study, data from chemical control tests conducted from 1973 to 1980 were analyzed to
determine the relationship between nematode counts and grapefruit yield and fruit size.
■ The correlation between yield and nematode counts was negative (r = −0.47) and highly
significant (P < 0.01).
■ The data best fit the exponential decay curve: y = 160.3x (−0.0000429) where y = yield in kg/tree
and x = nematodes/ 100 cm of soil.
■ The correlation between fruit size and nematode counts was not significant because yield and
fruit size were inversely related.
■ Yield loss in an average untreated orchard was estimated to be 12.4 tons/ha.
■ Economic loss to citrus nematode in Texas grapefruit, assuming no treatment and an average on-
tree price of S60/ton, was estimated to be S13.2 million annually.

43. THANK YOU