Concept of Pest & Pest Outbreak Dr. Jamba Gyeltshen 19/8/2011

Content 1.1. Definition of pest 1.2. Types of pest 1.3. Pest outbreak

1.1. Definition of pest: Pest is a general term used to describe any organism (usually insect or animal) that is harmful to our health and properties including crop and livestock. The term, in its broader sense also includes micro-organisms, parasitic plants and weeds.

1.2.1 Classification based on morphological difference Insect pests Mite pests Micro-organisms (pathogens) Nematodes Vertebrate pests Molluscs Weeds & parasitic plants

1.2.2 Classification based on nature, intensity or frequency of occurrence Regular pest: A pest that occurs every crop season and causes yield losses. Sporadic pest/Occasional pest: A pest that is irregular, occurs here and there or at random but occasionally causes problems. Potential pest: A pest that could cause significant economic loss if allowed to establish. Major pest: A more common and important pest Minor pest: A pest that is not causing huge damage

1.3. Pest outbreak Factors affecting natural insect population The way that the numbers of an insect species change represents the balance of birth and deaths over a given time period.

Birth rate is influenced by 1.Weather and climate 2.the food quality received by the adults 3.the degree of crowding of the individuals. Crowding affects birth rate partly through affecting the quality of the food but also by more direct influences such as stimulating restlessness of individuals.

Death rate is influenced by 1.climate 2.natural enemies 3.diseases 4.crowding Crowding may lead to cannibalism or starvation. Moreover, crowding may also lead to emigration which, like death, leads to a reduction in the number of individuals in an area.

With natural enemies for own survival Directly Differential effect on organism and natural enemies Mortality e.g. Frost, storms Natality e.g. Fecundity, development rate Growth and condition of food plant Indirectly With other species e.g. For food, oviposition sites Within the species e.g. Overcrowding, starvation FACTORS AFFECTING INSECT POPULATION COMPETITION CLIMATE

Causes of pest outbreak 1.Favourable weather 2.Intensive cultivation and monoculture 3.Destruction of natural enemies 4.Destruction of natural habitat 5.Introduction of new crops 6.Accidental introduction of foreign pests 7.Use of pesticides

Favorable weather condition Direct effect: under favourable weather conditions, birth rate is high and death rate low. Indirect effect: a) through the improvement in the growth and condition of food plants. b) the differential effect. If the good weather and climate favors the pest more than it favours the natural enemies, it will increase the rate of growth of the pest population

Intensive cultivation and monoculture Intensive cultivation means making rigorous use of the cultivable land. Both intensive cultivation and monoculture favour insect pests. When land is intensively cultivated, pests get a continuously supply of food and natural enemies of pests are low or absent.

In an uncultivated land, individual plants of any one kind are often few and scattered. Arriving insects are met by a bewildering mixture of odour cues which makes it much harder for them to locate their host plants, and it is hard for overcrowded population on one plant to spread to another; large mortalities are likely to occur in the process.

In a monoculture system plants have been specially selected and managed so as to emerge, grow and mature in synchrony. Provided that the pest arrives at the right time, it will find a very large population of the plants at a high level of suitability.

In a monoculture system Fertilizers and thinning are among the management practices which maintain a high plant quality suitable also for the rapid development of pest infestations. Infestation spreads much more evenly, and average densities can be very high before emigration or other competition effects of crowding cause significant reductions.

Destruction of natural habitat Agriculture generally involves destruction of the natural areas including forests that are the natural habitat of insects and natural enemies. When the natural areas including forests are encroached upon for cultivation, habitation or industrial development, insects migrate to fields.

Accidental introduction of pests When planting materials are brought into the country, new pests may be accidentally introduced. These new pests may have been under suppression in its native place but in the new place where natural enemies are absent, it can multiply and buildup population.

Use of insecticides When insecticides are used excessively, not only the pests are killed but also the natural enemies of the pests are wiped out. In the absence of natural enemies, the surviving population of insect pests multiply rapidly and reach epidemic proportions. Indiscriminate use of pesticides also leads to development of resistance in pests. This occurs as a result of killing the susceptible genotypes and selecting the more resistant genotypes at every pesticide application event. After several years of using the same pesticide, there would come a time when that particular pesticide will have no effect on the pest because they have developed resistance to the pesticide.