Before contemplating taking any pest management measures against an insect species on a crop, the species must be correctly identified; then, presuming its biology is known, it should be clearly established that the species in this particular context is a pest and that it could be profitable to attempt population control.
In this article, the various terms used to describe pests are defined. It should be noted that some terms are more or less synonymous, but they are all well-established in the literature. For example, a major pest is very often a serious pest, and all economic pests are serious.
The definition of a pest can be very subjective, varying according to many criteria; but in the widest sense any animal (or plant) causing harm or damage to man, his animals, his crops, or possessions, even if just causing annoyance, qualifies for the term pest.
From an agricultural point of view, an animal or plant out of context is regarded as a pest (individually) even though it may not belong to a pest species. For instance, cattle on a farm are a pest but caged in a paddock is not and is in fact a valuable national asset there.
Similarly, volunteer cabbage plants growing in a field with onions have to be regarded as ‘weed’ pests. Many insects belong to generally accepted pest species, but individual populations are not necessarily always pests; that is, of course, not necessarily economic pests.
In agriculture, we are concerned when the crop damage caused by insects leads to a loss in yield (quantity) or quality, resulting in a loss of profits by the farmer. When the yield loss reaches certain proportions the pest can be defined as an economic pest.
Clearly, the value of the crop is of paramount importance in this case, and it is difficult to generalize, but as a general guide for most crops, it is agreed that most species reach pest status when there is a 5– 10% loss in yield.
Obviously, a loss of 10% of the plant stand in a cereal or rape field (note that this is not the same as a 10% loss in yield!) is not particularly serious, whereas the loss of a single mature tree of Citrus, apple or peach is important.
This is the amount of damage done to a crop that will financially justify the cost of taking artificial control measures, and will clearly vary from crop to crop according to its basic value, the actual market value at the time, and other factors.
Economic injury level (EIL)
This is the lowest population density that will cause economic damage and will vary between crops, seasons, and areas. But it is of basic agricultural importance that it is known for all the major crops in an area. The major components in a simplified equation (from Pedigo et al., 1986):
EIL = (C/V) (1/L)
C = Pest Management Costs (
N per unit measure)
V = Market Value of Product, Managed Resource, etc. (
N per unit measure)
L = Loss Caused to Product, Managed Resource, etc., per Pest (loss per unit measure per pest)
Although this equation is only a simplified version, it is easy to see how considering costs, values, and losses can assist the pest manager in determining when the pest is actually causing economic losses.
The ease of determining the values of C, V, and L will vary with the pest management situation. To date, EIL values have primarily been determined for agricultural crops.
However, the specific values in the equation will vary with many factors including geographic region, production practices, and market fluctuations.
Despite these variables, it is still helpful for a pest manager to assess what these costs, values, and losses might be for his/her specific situation, and then to consider these values when making pest management decisions.
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It is desirable that control measures be taken to prevent a pest population from actually causing economic injury. The economic threshold is the population density of an increasing pest population, at which control measures should be started to prevent the population from reaching the economic injury level.
The normal situation in a field or plantation crop is that it will be attacked by a number of insects, mites, birds and mammals, nematodes, and pathogens which together form a complicated interacting pest complex.
The control of a pest complex is complicated and requires careful assessment, especially as to which are the key pests, and careful integration of the several different methods of control that may be required.
This, of course, makes the process of evaluation difficult, and generally, much money and time are usually wasted on uneconomic pest control, either through carelessness or lack of knowledge.
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This is the actual (total) number of different species (and numbers of individuals) of pests found on either a crop or an individual plant at any one time, and, as already mentioned, this would usually be a pest complex, but could also be a mono-specific population, although this would be rare.
In any one local pest complex, it is usually possible to single out one or two major pests that are the most important; these are defined as key pests, and are usually perennial and dominate control practices.
A single crop may have one or more key pests, which may or may not vary between crop type, cultivation areas, and seasons.
It is of course necessary to establish economic thresholds for these key pests in order to be certain when to apply control measures, for it has been often observed that the mere presence of a few individuals of a key pest species in a crop may cause undue alarm and lead to unnecessary pesticide treatment.
Key pests owe their status to several factors, including their usually high reproductive potential, and the type of damage they inflict on the host plant (e.g. Stem borer on Maize; Boll Weevil on cotton).
This is a species that is both a major pest and an economic pest of particular importance, being very damaging and causing considerable harm to the crop plants and a large loss in yield. It almost invariably occurs in large numbers.
These are the species of insects and mites that are either serious pests of a crop (or crops) in a restricted locality or are economic pests over a large part of the distributional range of the crop plant(s).
Thus the species here regarded as major pests usually require control over a large part of their distributional (geographical) range, most of the time.
In any one crop, in one location, at one time, there is usually only a rather small number (say 4–8) of major pests in the complex that actually require controlling.
For example, although the pest spectrum for cotton worldwide is 1360 species, on any one cotton crop there will probably only be about five species requiring population control.
Usually for most crops in most localities the major pest species remain fairly constant from year to year, but several entomologists have commented recently that in some areas they have observed that the major pest species complex has been gradually changing over a long period of time.
So over a period of some 10–50 years, it is expected that the complement of major pests for a crop may change. It must be remembered that evolution continues all the time, though it is not often obvious, and that in an artificial environment, such as agriculture, it can be expected that evolution will be accelerated.
These are the species that are recorded feeding or ovipositing on the crop plant(s) but usually do not inflict damage of economic importance; often their effect on the plant is indiscernible. They may be confined to particular crop plants or may prefer other plants as hosts.
Many (but not all) pests listed as minor pests are potentially major pests. Many species that are major pests of one crop will occur in a minor capacity on other crops.
Sometimes a major pest of a particular crop in one part of the world (e.g. Africa) will be a minor pest of the same crop in a different part (e.g. Australia).
This term is used occasionally in the literature and refers to a minor pest species that could become a major pest following some change in the agroecosystem.
Only a relatively small proportion of the species listed as minor pests are really potential pests in this sense, because of their basic biology.
Secondary or sporadic pest
These are species whose numbers are usually controlled by biotic and abiotic factors which occasionally break down, allowing the pest to exceed its economic injury threshold.
A most important point to remember is that an insect is only an actual pest (in practice) at or above a certain population density, and most control measures are aimed only at reducing this population to a lower level.
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