Two efficient, senti-automatic elutriators for assaying soil samples for nematodes are described. The first apparatus is a four-unit elutriator which combines conventional extraction methods with the following major features: atttomatic mixing of 500- to 1,500-cm³ soil samples with water (± air); "turbinate" sample splitters from which fractions of 1/15 or greater are passed onto 26- or 38-μm sieves for collection of larvae and adult nematodes; the capacity for collecting roots, intact egg ntasses, and cysts on 250-425-μm sieves; and a variable speed motorized sieve-shaker. Nematodes, after being collected on 38-μm sieves, are separated from debris by centrifugation or by Baermann trays. Secondary features include: air cylinders, solenoid valves, and time clock for atttomatic dumpittg residual soil and water; relay-controlled coarse spray nozzles activated for 5 sec every 30 sec for washing nematodes through 250-425-μm sieves; adjustable rates of water amt air flow, and tinting. The second type of elutriator operates on similar principles but costs less to construct. It requires somewhat more operator participation; sieve spraying is carried out by the operator, anti elutriators are dumped manually. Both elutriators also show promise for monitoring populations of certain other soil microorganisms.

Two Semi-automatic Elutriators for Extracting Nematodes and Certain Fungi from Soil

A 7-ha alfalfa field (Medicago saliva L. cv Mesa Sirsa) was sampled systematically on a 6 x 6-m grid by removing individual cores (2.54 cm diam) to a depth of 45 cm from each of the 1,936 grid intersections. The soil was mainly coarse-textured with a fine-textured streak running centrally, north to south. Nematodes were extracted by a semiautomatic elutriator and sugar flotation-sieving technique. Five plant-parasitic species were consistently present: Meloidogyne arenaria, Pratylenchus minyus, Merlinius brevidens, Helicotylenchus digonicus, and Paratrichodorus minor. All species had a highly skewed nonnormal frequency distribution that departed significantly from randontness. Goodness-of-fit tests on the distribution of five populations in the entire field showed that three (Meloidogyne, Merlinius, and Helicotylenchus) were described by a negative bittomial. When the samples were categorized by soil texture (coarse vs. fine-textured), all populations in the fine-textured areas, and three populations (Meloidogyne, Pratylenchus, and Merlinius) in the coarse areas, fitted a negative binomial distribution. Nearly all populations titted a negative binomial when the frequency distributions from randomly located one-meter-square areas were examined for each species. Key Words: population distribution, negative binomial model, sampling.

Plant-parasitic Nematode Distributions in an Alfalfa Field

Root-lesion nematode (Pratylenchus thornei) is a serious pathogen of wheat and other crops and is estimated to cost the Australian wheat industry $36M per year in lost production. Use of tolerant and resistant wheat cultivars is the most important component of integrated control of this nematode. A targeted approach has been adopted to develop such cultivars for the northern grain belt of Australia. Methods are described that have been used successfully to screen for tolerance in the field and resistance in the glasshouse. Backcross programs used to incorporate superior tolerance into locally adapted cultivars have doubled tolerance levels, which translates into double the grain yield on severely infested sites. Resistance from the bread wheat line GS5Oa backcrossed into locally adapted cultivars has reduced the nematode multiplication rate more than tenfold in advanced lines. New sources of resistance have been located in Aegilops tauschii, in durum wheat and in synthetic hexaploid wheats derived from them. Other new sources have been identified in landrace wheats and wheat cultivars from the Middle East and North Africa. A concerted effort is required to exploit these new found resistances and knowledge to accelerate the production of suitable cultivars for farmers’ use.

Progress in breeding wheat for tolerance and resistance to root-lesion nematode (Pratylenchus thornei)

CHAPTER 18 – Sampling Nematode Populations

Population-based habitat conservation planning for migrating and wintering waterfowl in North America is carried out by habitat Joint Venture (JV) initiatives and is based on the premise that food can limit demography (i.e. food limitation hypothesis). Consequently, planners use bioenergetic models to estimate food (energy) availability and population-level energy demands at appropriate spatial and temporal scales, and translate these values into regional habitat objectives. While simple in principle, there are both empirical and theoretical challenges associated with calculating energy supply and demand including: 1) estimating food availability, 2) estimating the energy content of specific foods, 3) extrapolating site-specific estimates of food availability to landscapes for focal species, 4) applicability of estimates from a single species to other species, 5) estimating resting metabolic rate, 6) estimating cost of daily behaviours, and 7) estimating costs of thermoregulation or tissue synthesis. Most models being used are daily ration models (DRMs) whose set of simplifying assumptions are well established and whose use is widely accepted and feasible given the empirical data available to populate such models. However, DRMs do not link habitat objectives to metrics of ultimate ecological importance such as individual body condition or survival, and largely only consider food-producing habitats. Agent-based models (ABMs) provide a possible alternative for creating more biologically realistic models under some conditions; however, ABMs require different types of empirical inputs, many of which have yet to be estimated for key North American waterfowl. Decisions about how JVs can best proceed with habitat conservation would benefit from the use of sensitivity analyses that could identify the empirical and theoretical uncertainties that have the greatest influence on efforts to estimate habitat carrying capacity. Development of ABMs at restricted, yet biologically relevant spatial scales, followed by comparisons of their outputs to those generated from more simplistic, deterministic models can provide a means of assessing degrees of dissimilarity in how alternative models describe desired landscape conditions for migrating and wintering waterfowl.

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Estimating habitat carrying capacity for migrating and wintering waterfowl: considerations, pitfalls and improvements

Soil sampling experiments were conducted on 0.01 hectare (ha) plots of Fox loamy sand in areas of low, medium, or high population densities of Pratylenchus penetrans (Cobb). Nematode count data from 1, 20, and 40 core samples were statistically analysed to find normalizing transformations and to compare the efficiency of different sampling schemes. Although negative binomial distributions fitted the observed frequency distributions, normalizing transformations of the form log (x + c), that stabilized variance for all population densities, were obtained empirically. It was estimated that at least seven hours of field sampling and laboratory analysis are required to obtain an estimate of mean population density of a plot within 20 % of the true mean with 95 % confidence. An estimate of nematode population density in a 0.01 ha plot based on a subsample from one 20 or 40 core sample is shown to have very low precision.

Jean R. Proctor

Papers

The Determination of Normalizing Transformations for Nematode Count Data From Soil Samples and of Efficient Sampling Schemes