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N
Number of hosts.

Na
Number of hosts encountered by P searching parasites.

Nn; Nn+1
Population density in generations n and n+1.

N0; Nt
Population density at time O and time t.

NAD
(Nicotinamide adenine dinucleotide) The oxisided form of the coenzyme of an enzyme involved in respiration. NADH2 is the reduced form of the coenzyme. NAD was formerly called DPN (diphosphorpyridine nucleotide).

NADP
(Nicotinamide adenine dinucleotide phosphate) A coenzyme involved in respiration. NADPH2 is its reduced form. NADP was previously known as TPN (triposphorpyridine nucleotide).

Nagana
Disease of ruminants caused by Trypanosoma brucei brucei or T. congloense.

Naiad
The aquatic growing stage of insects with an incomplete metamorphosis.

Nanometer
One trillionth of a metre, or 10-9 (abbreviated hm); a unit used to measure viruses or individual features of microbial cells.

Nares
The openings into the nasal cavity.

Nasopharyngeal
Pertaining to the part of the upper respiratory tract above the mouth.

Naso-pharynx
The part of the pharynx which lies above the level of the soft palate.

Natality
Population 'increase' factor i.e. birth rate.

Native pasture
One composed of native plants or naturalised exotic plants.

Natural control
The collective action of environmental factors to maintain species population size within certain upper and lower limits over a period of time.

Natural enemy
An organism that causes the premature death of another organism.

Natural host
A host in which the pathogenic micro-organism (or parasite) is commonly found and in which the pathogen can complete its development synonymous with 'typical host'.

Natural immunity
Immunity due to innate genetical characters.

Natural openings
Stomata, lenticels, and hydathodes.

Natural selection
The selection among a group of animals by the forces of nature which allows those of the group best fitted to survive in the particular environment to live and reproduce, while those not fitted to survive die. By this means, the species or group survive and adapt to the environment, the less well adapted members being gradually eliminated over many generations.

Nauplius
Typically the earliest larval stage(s) of crustaceans; has only three pairs of appendages: antennules, antennae and mandibles - all primarily of locomotive function.

Neascus
Strigeoid metacercaria with a spoon-shaped forebody.

Necrophyte
A fungus (or other organism) colonising already dead tissues.

Necropsy
A post-mortem examination.

Necrosis
A localised and rapid destruction of a cell or more often a group of cells and a consequent quick death of those which are in contact with or form part of a living tissue; rot and canker are examples of necrotic symptoms. Cell death (used particularly for death of cells in a focal point in a multi-cellular organism) due to anoxia or local toxic or micro-biological action.

Necrotic
Having symptoms characterised by the death or disintegration of cells or tissues.

Necrotroph
An organism that causes the death of host tissues as it grows through them such that it is always colonising dead substrate.

Necrotrophic
Utilising dead plant or animal tissues as a source of nutrients.

Nectarthode
An opening at the base of a flower from which nectar exudes.

Nectary
A nectar-secreting gland.

Negligible residue
A tolerance set on a food or feed crop permitting a minuscule amount of pesticide at harvest as a result of indirect contact with the chemical.

Neighbourhood stability
Ability to withstand perturbations of small magnitude and not be affected; compare with global stability.

Negative contrast staining
A staining procedure used to prepare virus particles for examination in an electron microscope.

Negative stain
A procedure in which the background is stained whereas the specimen is not.

Negri-bodies
Oval or round bodies included within the cell wall, sometimes in the cytoplasm and sometimes in the processes, of certain cells of the brain, usually in the cerebellum, which can be seen with the microscope after staining, and which are accepted as confirmation of diagnosis of rabies.

Nematicide
Pesticide effective against nematodes.

Nematocide
The term 'nematocide' includes all preparations intended for preventing, destroying, repelling or mitigating any invertebrate animals of the phylum Nemathelminthes and class Nematoda, inhabiting soil, water, plants or plants parts.

Nematode
Microscopic unsegmented worms, many of which are parasites on plant roots. Un-segmented round worms with elongated, fusiform, or sac-like bodies covered with cuticle, and inhabiting soil, water, plants and animals.

Neoplasm
An abnormal mass of tissue not required for the repair of organs, the growth of which exceeds, and is uncoordinated with, that of the normal tissues. Persists in the same excessive manner after cessation of the stimulus which provoked the changes in growth pattern. Neoplasms may be benign or malignant. Cancer refers to any type of malignant neoplasm.

Neotenic plerocercoid
Adult caryophyllidean cestode, except for Archigetes, which is a neotenic procercoid.

Neotenic procercoid
Adult Archigetes, a caryophyllidean cestode.

Nephritis
Inflammation of the kidney tubules.

Nephrotoxin
A substance that causes damage to the kidney.

Net assimilation (rate)
Mean rate of dry matter production per unit of leaf area.

Net cash flow
The difference between the money received and the money spent in any one period (week, month, year).

Net production
Production after respiration losses are subtracted.

Net profit
The profit after all deductions except tax.

Net profit margin
Net profit expressed as a percentage of sales.

Net worth (equity)
The value of total assets that the farmer owns minus the value of his total liabilities.

Network(s)
A diagrammatic illustration of complex problems involving inter-related activities. Can be used to depict the principal technical elements of any R & D project, and their technical and scheduling relationships.

Neurotoxin
A substance that causes defects in nerve tissue. A toxin which acts against the central nervous system.

Neurotropic
Turning inwards or having an affinity for nervous tissue. Usually applied to a virus or other micro-organism which prefers to multiply in nerve tissue.

Neutralisation
Reaction between antibody and soluble antigen, such as toxin. A serological test in which a virus in suspension is neutralised by specific antibodies added to the suspension and loses its infectivity. Viruses are also neutralised by antibodies.

Neutral soil
A soil with pH value of 7.0 and therefore neither acid nor alkaline.

Niche
The place or position, in both a physical and functional sense, of a population in an ecosystem as determined by the full complex of interacting and limiting environmental factors.

Nidus
A centre in which infection settles and from which infection spreads. Specific locality of a given disease; result of a unique combination of ecological factors that favours the maintenance and transmission of the disease organism.

Night yarding
Putting animals in a confined space at night.

Nitrification
Formation of nitrates from organic and ammonia forms. The conversion of ammonia to nitrate.

Nitrogen fixation
The conversion of atmospheric nitrogen to nitrogen compounds that can be utilised by plants, accomplished by nodule-forming bacteria on legume roots or by soil micro-organisms.

Nitrogen-free extract
Carbohydrates (mostly) that remain after protein, ash, crude fibre and ether extract have been determined.

No observable effect level (NOEL)
The dosage of a pesticide in chronic toxicity studies that results in no discernible harm to experimental animals.

Nocturnal
Active at night.

Node
The joint on a stem from which roots, leaves and branches have their origin or a point on a chart where the two dimensions of a matrix organisation intersect.

Nodulation
The production of nodules on roots of legumes by specialised legume bacteria.

Nodule
A tumour-like structure produced by the roots of symbiotic nitrogen-fixing plants. Contains the nitrogen-fixing microbial component of the symbiosis.

Noisiness
Errors that may occur during the replication of a virus's genome.

Nomadism
Continual movement of man and animals with no fixed settlement generally in search of food and water.

Non-controllable costs
Cost elements that are outside the control of a manager over a specified time period.

Non-crop area
Areas of land not being used or intended to used for cropping.

Non-cyclic photophosphorylation
Light-driven ATP synthesis in which water is the electron donor and NADP the electron acceptor.

Non-flammable
Not readily ignitable, with a flash point above 55°C as determined by a closed cup method.

Non-infectious disease
A disease which cannot be transmitted from one plant to another that is caused by an environmental factor, not by a pathogen.

Non-ionic
In which there is no residual charge.

Non-ionic surfactant
A surfactant that does not ionise in solution and is therefore compatible with both anionic and cationic surfactants.

Non-occluded
Term to describe viruses in which the virions are not occluded in a dense protein crystal; preferable to 'non-inclusion'.

Non-parametric methods
Methods used for testing hypotheses that do not explicitly involve assertions about parameter values e.g. normal distribution of data.

Non-pathogen
A micro-organism not known to cause disease in a plant.

Non-persistent virus
A virus that persists in its vector for a few (usually less than 4) hours at approximately 20°C, transferred from infected to healthy plants.

Non-persistent transmission
A type of insect transmission in which the virus is acquired by the vector after very short acquisition feeding times, and which is transmitted during very short inoculation feeding periods. The vector remains viruliferous for only a short period unless it feeds again on an infected plant.

Non-preference
The behaviour of insects when they avoid or exhibit negative reactions to a plant of a host species; also used to describe resistance trait that induces such behaviour (antixenosis = preferred term).

Non-protein nitrogen
Urea, biuert etc.

Non-reciprocal density-dependence
Mortality inflicted on a population by a biotic mortality factor whose own numbers are not changed as a consequence.

Non-response
In a sample survey the failure, for any reason, to obtain information from a designated individual or element.

Non-selective herbicide
A chemical that is generally toxic to plants without regard to species; toxicity may be a function of dosage, method of application etc.

Nonsense mutation
A mutation that changes a normal codon into one which does not code for an amino acid.

Non-septate
Absence of apparent cross-walls in filamentous organisms when viewed with a light-microscope.

Non-target organisms
Those organisms which are not the intended specific targets of a particular use of a pesticide.

Non-virulent
A variant of a pathogenic organism incapable of causing severe disease.

No observed effect level
The maximum dose or ambient concentration which an organism can tolerate over a specific period of time without showing any adverse effect and above which adverse effects are detectable.

Normal distribution
A continuous probability distribution commonly used in statistics and completely characterised by a mean µ, which is symmetric about, and a variance _², which describes the spread of the distribution. The distribution is denoted by N(µ_²).

Normal flora
The micro-organisms which ordinarily grow on the various surfaces of a plant or animal.

Normal probability distribution
A symmetric bell-shaped probability distribution of infinite range represented by the equation F(x) = 1 e-(x-µ)²/2_² (- _ < x < _) __2_ where µ is the mean and _² is the variance of the distribution.

normative theories
theories concerned with 'what ought to be', as opposed to 'what is'.

no-see-um
biting midge of the family ceratopogonidae.

nosema disease
a disease of adult honey bees caused by the micro-sporidian nosema apis.

nosemosis
infection with micro-sporidia of the genus nosema.

nosography
a branch of pathology that deals with the description of diseases.

notifiable disease
a disease which by law has to be reported to the appropriate authorities.

noxious
a term applied to seriously harmful weeds or pests.

noxious weed
a weed arbitrarily defined by laws as being especially undesirable, troublesome and difficult to control. definition will vary according to legal interpretations.

nozzle
i) air-blast. nozzle using high-velocity air to break up liquid supplied at low pressure ii) anvil. nozzle in which liquid jet strikes smooth solid surface at high angle of incidence iii) cone (or swirl). nozzle in which liquid emerges from orifice with tangential velocity component imparted by passage through one or more tangential or helical channels in swirl chamber iv) deflector. nozzle in which fan shaped sheet of spray is formed by directing liquid over sharply inwardly curving surface v) fan. nozzle in which two streams of liquid are caused to converge, producing on impact fan-shaped sheet of liquid

nuclear magnetic resonance
a technique for elucidating molecular structures by utilising the principle that nuclei of some isotopes behave like spinning magnets which when exposed to a particular magnetic field and electromagnetic wave length (radiowaves) can absorb energy.

nuclear polyhedrosis virus
(npv) a disease virus of insects, mainly the larvae of certain lepidoptera and hymenoptera, cultured commercially and sold as a microbial insecticide.

nuclease
an enzyme which catalyses the hydrolysis of nucleic acids.

nucleic acid
a dna or rna molecule which may be composed of one or two strands. a compound of high molecular weight that consists of pentose (ribose (rna) or deoxyribose (dna)), phosphoric acid, and nitrogen bases (purines and pyrimidines), present in all living things including the infectious parts of plant viruses.

nucleic acid hybridisation
joining together of a strand of nucleic acid (dna or rna) from one organism with a strand from another by base sequence homology.

nucleocapsid
the structure composed of the capsid with the enclosed viral nucleic acid; some nucleocapsids are naked, others are enclosed in an envelope.

nucleoid
resembling a nucleus. the region in a procaryotic cell where the dna is located. although frequently diffuse, sometimes the dna is sufficiently contracted so that a defined region can be detected.

nucleolus
a dense protoplasmic body within the nucleus having a high rna content; the site of ribosomal rna synthesis.

nucleopolyhedrosis
a viral disease of insects, mainly the larvae of certain lepidoptera and hymenoptera, it is characterised by the formation of polyhedral inclusion bodies (polyhedra) in the nuclei of the infected cells.

nucleoprotein
a compound of nucleic acid and protein. most plant viruses that have been chemically characterised are nucleoproteins.

nucleoside
an individual nucleic acid base (adenine, cytosine, guanine, uracil or thymine) bonded to a single ribose or deoxyribose sugar molecule.

nucleosome
a compact, highly folded unit of a eucaryotic chromosome which contains about 200 base pairs.

nucleotide
an individual unit of a nucleic acid, composed of a nucleic acid base bonded to a ribose or deoxyribose sugar molecule which is itself bonded to one, two or three phosphate groups. the building blocks of dna and rna.

nucleotide sequence
the order of nucleic acid bases in a dna or rna strand.

nucleus
membrane-enclosed protoplasmic body structure within eucaryotic cells containing the genetic material (dna) organised in chromosomes.

null hypothesis
in a statistical test it is the statement of the hypothesis to be tested.

nurse crop
a companion crop.

nutrient
a chemical which an organism obtains from its surrounding environment and uses either as an energy source or a source of the elements needed to biosynthesise cell constituents.

nutrient medium
a liquid broth or semi-solid jelly containing nutrients which stimulate and sustain the culture and proliferation of bacteria, higher plant cells or animal tissue.

nymph
the intermediate growing stage in the life cycle of arthropods with an incomplete metamorphosis, usually having much the same morphological appearance as the adult.

nymphochrysalis
non-feeding, pre-nymph stage in the life cycle of a chigger mite.