NATURAL INSECT POPULATION GROWTH AND DYNAMICS

1. NATURAL INSECT POPULATION GROWTH AND DYNAMICS

2. Insect population growth • Dramatic changes in insect population numbers over relatively short periods of time, have been a major concern to humans especially since the advent of Agriculture • plagues of locusts, flies, aphids and ants that seem to emanate from nowhere but take over and destroy virtually all vegetation corroborate the earlier concern • This concern may however not be explainable by short generation times and high reproductive rates of insects alone • They have therefore been explained by various concepts such as “Acts of God” and “spontaneous generation” This concern unfortunately has been a major concern in entomology especially as it affects economic insects

3. Insect population Numbers • Insect populations number fluctuate dramatically over time. • If environmental conditions favors the insect, its population number will increase until secondary regulatory factors set in to reduce and finally stop population growth • Three general patterns are recognized based on amplitude and frequency, i.e. • Stable population pattern: fluctuate relatively little over time • Irruptive population pattern: sporadically increase to peak followed by a decline • Cyclic populations pattern: oscillate at regular wide intervals. Most interesting and difficult to understand

4. Factors affecting insect population numbers • These factors can be grouped into two: • Density-independent factors: these are factors that the proportion of organisms affected by it is a constant for all population density or the effect of the factor does not depend on population density, e.g. climate, disturbances (These are often responsible for cyclic and irruptive patterns) 2. Density-dependent factors: these are factors that the proportion of organisms affected varies with density or the effect of the factor depends on population density, e.g. space, food resource, etc (These are often responsible for stable patterns)

5. NATURAL INSECT POPULATION Like most other organisms, each insect species • has peculiar boundaries and unique characteristics such as: • derive mating and reproductive benefits from staying in groups • growth rates • Natality rate • mortality rate • age structure • optimal density • Dispersion rates • age distribution • spatial distribution • Time of activity • Active seasons • etc. Thus Insects enjoy dwelling together as a large society and they tend to derive maximum benefit from such associations.

6. Insect population number is informed by 1. the overall expression of each of the inherited characteristics (Intrinsic factors) are expressed genetically as biotic or reproductive potential 2. the influence of the environmental factors or resources within the locality (Extrinsic factors) such as climate, food quality and quantity, space, etc. This is usually negative and restrictive (environmental restraints) The actual number of insect population is therefore the algebraic sum of innate ability of the organism to reproduce and survive the restraints instituted by the environment, i.e. Biotic potential and environmental restraints, respectively.

7. Insect population number cont’d i.e. Actual Abundance = Biotic Potential – Environmental Potential Where: Nt= Number at the end of a period E = Emigration rate N0= Number at the beginning I = Immigration rate e = Base of natural Log(2.7183) b = Birth rate t = time d = Death rate Nt = N0 e(b-d)t – Et + It This expression is a general model that can be used to predict changes in number of a typical animal population assuming that the environmental variations are reasons for the immigration and emigration.

8. Insect population numbers over time • The number of insect population in at typical life system • Increases over time (Fig 2 : Insect population number against time) • The increase is initially slow and it picks up gradually • It subsequently reduces slowly with improved reduction rates • Eventually stops increasing and begin to reduce • Subsequently the number revolves around a climax population • These expressions are a informed by the intrinsic properties of the insect and the environmental resource available to them.

9. Insect Population Growth • Thus the number of insects occurring at a point in time in a typical system is actually determined by the sum total of all the various environmental factors influencing them. • Each of these factors either support or discourage population growth of the insect; hence the factors can be grouped into two broad categories, i.e. • Growth enhancing factors (GEF) • Growth suppressing factors (GSF) • When • GEF = GSF there is no population growth • GEF ˃ GSF there is positive growth • GEF ˂ GSF there is negative growth

10. Insect Population Growth cont’d • The interplay of these factors (GSF and GEF) therefore determines the spontaneous number of insect and the growth rate of the population at any point in time (Fig 2a). • When GSF is totally absent an exponential curve result but a logistic curve presents when GSF is present. (Fig 2b). • Note that the critical density is marked by a notable deflection that is informed by the advent of GSF and the peak population number occurs when population growth begins to have a negative value, i.e. GSF > GEF. Thus all insect populations are under a natural check that ensures that their numbers and rate of growth are kept at a level that is tolerable and dictated by the system they inhabit.

11. Life system implications • The life systems of some otherwise naturally existing ecosystems have been greatly improved to provide enhanced insect GEF or suppress GSF. • Unfortunately majority of these have emanated from human activities directed at solving human problems • A good example of such is agriculture: • It has modified most ecosystems into agroecosystems: A system created to encourage and maintain a single or handful of organisms to satisfy human needs. • the new system therefore • is composed of one or two species of organisms; a far cry from the situation in a natural ecosystem that has several species and therefore high species diversity • has an unnaturally high population density of the single species • has uniform vegetative and phonological structures and imbalances with respect to diseases and outbreaks • Has artificial temporal discontinuity as against a balanced out terrain of natural ecosystems • Has an serious vested human interest and commitment

12. Life system implications cont’d • The new system unfortunately provides inordinate GEF to specific insects that are pests of this selected crop • The Insects take advantage of the alterations since it empowers them to keep an abnormally high population size • Unfortunately, the only plausible solution, i.e. to remove the source of the said factor, disagrees with human interest • Other examples of the human alterations include: • Food storage • Poor hygiene, • Human mobility • Goods storage and packaging • Change in human behavior, • Monocroping or monoculture, • Use of fertilizer • Use of pesticides, etc.

13. Life system implications cont’d • When insects occur in abnormally high numbers and their activities contradicts ours, an average human being unfortunately • readily notice and abjure the insect • do not understand their reason to be at the location • believes the insects have a destructive mission • opines that such insects are likely to invite more colleagues • believes if unattended to the insects will exhaust the resource totally Human beings therefore control insects mercilessly, totally and on the spot despite the fact that that his reasons are baseless

14. Life system implications cont’d • The merciless and prompt control actions informs the choice of chemical method of control over others • Insects decided for a path of honour by • continuing to enjoy and derive maximum benefit and advantage from the human manipulation and interest • Struggling to adapt and develop resistance to the chemicals used by man Man may therefore need to reconsider his ways and utilize the naturally ordained methods that will not warrant resistance development and be more tolerable to insects