Categories of Pests in Agriculture
Insect pests have been categorized as key, major, minor sporadic and potential pests based on their population and damaging capacity. these terms have been used rather loosely by different workers. In order to arrive at a definite and practicable concept, different categories of pest are defined in terms of the general equilibrium position (GEP), the economic injury level (EIL) and the damage boundary (DB).
GEP is the mean value of pest density around which the pest population tends to fluctuate as changes occur in the biotic and abiotic components of the environment without accompained by a permanent modification in the composition of the environment. A permanent modification of any component of the environment may alter the GEP. The lowest level of injury where the damage can be measured is called the DB while the lowest number of insects that will cause economic damage is referred to as EIL. The EIL is also defined as a level of pest abundance or damage at which the cost of control equals the crop value gained from instituting the control procedure.
These are the most severe and damaging pests. The GEP lies well above DB and EIL. Human intervention in the form of control measures may bring the population temporarily below the EIL. However, it rises back rapidly and repeated interventions may be required to minimize damage. These pests present a persistent and perennial threat to our crops and are not being satisfactorily controlled with the available technology.
There is need to lower their GEP below EIL by permanent modification of one or more components of Environment. Cotton bollworms, diamondback moth, chickpea pod borer, sugarcane borers and some vectors are the frequently occurring key pests.
Here the GEP is close to the EIL and in some cases both may essentially be at the same level. Thus, the population crosses EIL quite frequently and repeated control measures are necessary, but economic damage is avoided by timely interventions.
Many of the important sucking pests like cotton jassid and whitefly, brown planthopper and leafhopper on rice, sugarcane whitefly and scale insect fall in this category. Rice stem borer, gall midge and leaf folder are also frequently major pests.
The GEP in case of minor pests lies below both EIL and DB. Under favorable environmental conditions, the population may cross EIL and DB for usually a short interval. These pests are easily manageable to available control measures and a single application of insecticides is usually enough to prevent economic damage.
Cotton stainers (Red cotton bug), grey weevil, thrips and mites; rice hispa and root weevil; sugarcane mealy bugs, thrips and mites; and Spodoptera litura on oilseed and vegetable crops frequently occur as minor pests.
The population of these pests is usually negligible, but in certain years under favorable environmental conditoins, they appear in a virtually epidemic form crossing many times over DB and EIL. Under these conditions, the pest has to be controlled by undertaking suitable management strategies. These pests are highly sensitive to abiotic conditions and once the favorable season is over, only a residual population survives.
Many of the sporadic pests like white grubs, hairy caterpillars, cutworms and grasshoppers are polyphagous (feeding on or utilizing many kinds of food). But some oligophagous (i.e. eating only a few specific kinds of food) pests, e.g. sugarcane pyrilla may also be sporadic in nature.
These insects are presently not causing any economic damage and, therefore, as such should not be labeled as pests. Their GEP lies below the DB and does not cross EIL even under favourable conditions. Any change (cropping pattern, cultural practices) in the ecosystem may, however, push their GEP higher and there is a danger of economic damage from these pests, if control operations against the other categories of pests are undertaken in an indiscriminate manner.