the occurrences of pest
Estimates of the pest problem on a world scale suggest that, without insect pests, world food production could be increased by about a third. As this estimate represents the loss despite current control measures, it would clearly be catastrophic for mankind if control of insect pests were not attempted or should fail.
Obviously each insect individual has a fairly small food requirement. For example, a greenfly is unlikely to extract more than about 0.5 cm3 of sap from a plant in its lifespan, and even a voracious caterpillar is likely to consume only 50g of the fresh weight of its host plant.
Statistics calculated for such powers would not seem out of place in a science fiction novel. They are impressive, but wildly unreal, ignoring as they do any restraints on population increase of morality, food supply or a space to live, and in assuming optimum conditions for the whole twelve months of the year. Yet they do serve to illustrate the potential population explosion that pest control must aim to suppress. Thus, the potential of cabbage aphids to produce a new generation every two weeks, with 50 young per female with each generation, is more dramatically (if nonsensically) expressed as a potential in one year of one aphid mother to produce offspring weight 250 million tons, encircling the Equator nose to tail a million times. Equally startling is the notion that in one year a pair of house files could cover the earth to a depth of 15m with their offspring (200 billion individuals).
Factors affecting the occurrence of pest are influenced by many things, including the weather, the food quality received by the adults during development and also 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 mainly by climate and natural enemies or disease; 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.
These influences can classified under two headings:
'Climate and Competition. However, for the longer-term regulation of insect numbers at a particular level, more important than whether a factor is related to climate or competition is whether that factor varies in its impact with density of the insect population it is acting on. Because insect populations tend to increase geometrically, a typical population increase can be represented as a straight-line plot of the logarithm of density against time. If there is a restraint on birth rate, the line will have a shallower slope. If the restraint is very strong, the population may actually decline. Any factor which causes a simple change in the rate of increase of this kind will, if extrapolated, lead either to enormous populations or extinction.
This is clearly a limit to the size of any population if the increase rate is positive. At some point, density will reach a point where competition for space or food becomes intense and the increase rate will then slow
Factors affecting changes of insect abundance in the field
The increasing impact of their depredations as pest density rises is partly due to increasing numbers of natural enemies locating the prey source and remaining there to breed and partly due to their spending a higher proportion of their time feeding and less time searching for prey at high prey densities. Even crowding may exert a density-dependent relationship at a much lower density than would be expected. Encounters between individuals of the same species will lead to some cannibalism at quite low prey densities or stimulate the production of emigrant individuals long before overcrowding is apparent. Competition for a convention (conventional competition) rather than for an absolute food resource is therefore a widespread phenomenon in animals, including insects. This competition very often takes the form of a territorial behaviour; additionally, increasing contracts between individuals as density increases may reduce fecundity, promote cannibalism and induce emigration or an arrest in reproduction.
Density-dependent relationships (crowding effects);
In 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 make it much harder for them to locate their host plants, and it is hard for overcrowded populations on one plant to spread to another; large mortalities are likely to occur in the process. In the crop, infestation spreads much more evenly, and average densities can be very high before emigration or other competition effects of crowding cause significant reduction.
Insects are normally regulated by their natural enemies in the endemic situation, but any sudden change (especially in density independent factors such as climate and host plant nutrition) enables density to rise sharply so that the population escapes that regulatory restraint and then will build up irrevocably on the epidemic ridge until eventually the pest populations own responses to crowding slow the population increase at very high densities. These endemic beetles as scattered individuals on the eastern slopes, feeding on the weed Buffalo-bur (in the potato family). When (30 years later) settlers brought potatoes as a crop to the region, the beetle discovered a new nutritious and well-managed food source on a large scale (i.e. it escaped onto the epidemic ridge). From this time on, the beetle spread eastwards to cause famine at a speed of 140 km a year, and soon reached Europe.