A pest is not a biological phenomenon; it is an anthropomorphic designation. For example, we consider termites as beneficial organisms when they live in forests, converting dead trees into soil organic matter. The same insects are pests when they feed on wood-associated human structures. In both cases, the termite behavior is the same, but in one case we place no value on the wood being consumed, and in the other we assign high economic value.
Sometimes the mere presence of insects causes concern or alarm. When simple insect occurrence or very minor feeding is the basis for designating an insect as a pest, the insect is said to be an aesthetic or cosmetic pest. However, when insects decrease the value of a commodity they are said to be economic pests. There is no absolute distinction between aesthetic and economic injury, especially with respect to vegetable crops. A home gardener may consider a dimple on a tomato fruit caused by the feeding of a stink bug to be an insignificant blemish, an aesthetic injury, but the same type of blemish can cause a produce buyer to downgrade the value of a crop, causing significant economic loss to a tomato farmer. Similarly, a few holes in the leaves of cabbage or lettuce plants caused by flea beetles is of no significance early in the life of a crop, because the affected foliage is not harvested. The same type of injury, should it be more frequent or appear late in the development of the crop, could constitute injury either by reducing the growth rate of the plant or by affecting the appearance of the harvested commodity.
It is very useful to differentiate between direct and indirect pests. Direct pests are those that attack the portion of the vegetable plant that is harvested for food. Indirect pests attack some other portion of the plant that is not harvested. A stink bug feeding on a tomato fruit is a direct pest; a leaf miner burrowing in tomato foliage and a wireworm feeding on tomato roots are indirect pests. Ultimately, the presence of large numbers of indirect pests can be damaging, but on an individual insect basis indirect pests are less injurious than direct pests. Knowing the identity of pests, and their feeding behavior, allows us to distinguish between a severe and modest threat to a vegetable crop, and determines our course of action in dealing with the threat.
In commercial vegetable production, economic considerations are very important. Commercial vegetable producers normally are aware of the "economic injury level," the point at which pest suppression is economically feasible. There is little value of spending more money on production costs such as pest suppression than can recovered in increased yield. Therefore, a grower may not be very concerned about indirect pests, relative to direct pests. Similarly, growers may be unconcerned about direct pests that are few in number unless they burrow into the produce and are not easily detectable; such cryptic damage can dramatically lower the value of a crop, because consumers are intolerant of even slight insect contamination of food products. Commercial growers often make decisions that affect large acreages, and have a huge financial investment at stake. Under such circumstances, it is appropriate to assume a worst-case scenario, and to act decisively to protect crops from insect damage. This approach is viewed as a form of crop insurance, and in many cases, it is a good investment. However, in many cases good crop monitoring can establish precisely how abundant and damaging insects might be, and because they often are not numerous enough to cause injury, significant financial savings can be realized by treating crops for pests only when treatment is actually needed. It is important to bear in mind that unnecessary treatments for pests also are a financial loss. Such unnecessary treatments also encourage the development of insecticide resistance in pest populations. In the home garden environment there is greater latitude for risk, and it is often possible to abstain from pest suppression actions unless pests are observed. In either the commercial or home garden situation, we can make sound decisions about the need for pest control only when we have information about pest identity, biology, and damage potential.
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