Rev: 10/30/2019

Howard Ferris

Department of Nematology

University of California

Davis, California, USA

hferris@ucdavis.edu

Patterns in the population dynamics of nematodes are determined by the intrinsic characteristics that regulate rates of births and deaths of individuals and modified by conditions of the environment in which the population functions. Intrinsic factors include the productive capacity of the gonad in relation to resource demands of somatic tissues, the rate and length of the reproductive period and the life history strategy. Modifying factors include availability of food, sperm and other driving resources, and environmental conditions. Various models have been used to describe the dynamics of populations; some are primarily descriptive of observed trends, others more explanatory and mechanistic. All may be relevant in prescribed situations.

Economic thresholds are management tools for minimizing economic losses due to nematodes. They are based on projections of expected crop performance in relation to population levels at a critical point in time or at multiple points in time. The economic threshold is that level to which the population of the target nematode species should be managed under prevailing economic and environmental conditions. In its most comprehensive sense, the economic threshold is based on the integral of expected returns from the current crop and from future crops, given the expected trajectory of the nematode population at this level of management.

At its simplest, if the control cost is $100, it only makes sense (in the short term) to apply the control when the expected crop loss is >$100. So the economic threshold is that population level of nematodes at which the cost of control is equal to the value of the crop loss. If the population is above that threshold, the value of the loss is greater than the cost of control so apply the control; if not, take the loss.

The above reasoning assumes that the applied control will reduce the population to a non-damaging level. If that is not the case, the economic threshold is that population at which the cost of control is equal to the difference between the crop value with and without the control. So, if the cost of control is $100, the expected crop loss at this nematode population level without control is $120 and the expected crop loss with control (because the control is not 100% effective) is $30, the population is below the economic threshold for that treatment. That is you would spend $100 of control cost and only get $90 improvement in returns.

a) Population Increase At any initial population level, if we plant a host crop the population will increase. The amount of increase is determined by the reproductive capacity of the nematode population, the number of nematodes present and the amount of damage they cause to the host. But, for any given population level the amount of increase under a host crop should be predictable.

b) Nonhost Decline In the absence of a host, the population of nematodes should decline. The rate of decline should be predictable for a given field, e.g. 60% per year.

c) Population Change over Time So for any starting population, say 1,000 nematodes, if we grow a host crop the population will increase, say to 10,000 nematodes (10-fold increase). If we follow that with one or more years of host crop, the population will decline. At some time in the future, determined by the rate of decline, it will be back to the starting population level. So in the example above, the 10,000 nematodes will be 60% lower after one year on non-host, that is 4,000. After another year of non-host, the population will be 1,600, and after a third year it will be 640. So, it took 3 years of non host to get back to around the 1,000 nematodes we started with.

If we had started with 100 nematodes, they may have increased 20-fold under the host crop, due to lower plant damage, to 2000 nematodes. At 60% loss per year under non-hosts, the progression of decline would be 800, 320, 128, 52; so, 4 years of non-host to get under the starting population.

Starting with 5,000 nematodes, maybe only 2-fold increase on the host to 10,000. which would decline to under 5,000 in only 1 year of non-host.

d) Economics For any starting population level, we should also be able to predict the amount of damage to the host crop. In the above examples, 5,000 nematodes may cause 90% crop loss, 1,000 say 50% loss and 100 nematodes 10% loss. So, if the potential value of the host crop is $1,000 and that of the nonhost is $500, we can examine the economics of the 3 scenarios above

Example 1. (starting with 1,000 nematodes) $500 from the host and 3x$500 for the non-hosts. That is $2000 in 4 years or $500/year.

Example 2. (starting with 100 nematodes) $900 from the host and 4x$500 for the non-hosts. That is $2900 in 5 years or $580/year.

Example 3. (starting with 5,000 nematodes) $90 from the host and 1x$500 for the non-host. That is $590 in 2 years or $295/year.

If we do those calculations for every possible starting population level, the nonhost rotation length that maximizes the average annual returns is the optimum. And, of course, the optimization calculation can be done more automatically with a spreadsheet algorithm that we will use in class.

References:

Burt, O. R. and H. Ferris. 1996. Sequential decision rules for managing nematodes with crop rotations. Journal of Nematology 28:457-474.

Schmitt, D. P. and H. Ferris. 1998. Pathogenicity and damage levels. Pp 239-265 in S. B. Sharma (Ed.). The Cyst Nematodes. Kluwer Academic Publishers, Dordrecht. 452p.

Explore these concepts:

                            Economic Threshold Analysis