Life table and its importance in insect pest management
Dr. Faisal Hafeez, Ayesha Iftekhar, Muhammad Sohaib, Ayesha Arshad, Muhammad Arslan Ibrahim
Ayub agriculture Research Institute, Faisalabad, Pakistan
Insects:
Insects are arthropods whose body is divided into three regions, that is head, thorax and abdomen. Head contains sensory organs, thorax contains locomotory organs and abdomen contains all body visceral.
Worldwide population of insect:
It is estimated that there are some 10 quintillion (10,000,000,000,000,000,000) individual insects alive in whole world. But only less than 0.5 percentage of the total number of the known insect species are considered pests, and they cause damage to crop and people. The largest numbers of described species in the U.S. fall into four insect Orders: Coleoptera, diptera, hymenoptera and Lepidoptera.
Life table:
The table which is used to summarize key demographic variables which includes age-specific mortality, survivorship, and expectation of further life of a papulation. When we have compiled all such data, we can use them to investigate demographic patterns and processes, such as differences in the survival rate or life expectancy of different groups of organisms a papulation.
History of life table:
Life tables first were constructed in the third century A. D. by Trenerry in 1926, but they were not used for other animals until Pearl and Parker (1921) published life tables for
Drosophila melanogaster.
Types of life table:
There are 2 types of life table:
(1) ecological life tables for natural populations of animals.
(2) life tables for laboratory populations of animals.
They may be further categorized as horizontal or age-specific life
tables and vertical, or time-specific life tables.
- Cohort or age-specific or dynamic life tables:
Data collected by following a cohort throughout its life. A cohort is a group of individuals having same age and same parental origin for example larvae of pink bollworms emerging from same eggs at same time. This is rarely possible with natural populations of animals. The cohort life table is more frequently used as it is able to make any prediction of any expected changes in mortality rates of a population in the future. Cohort tables are also used analysed patterns in mortality rates that can be observed over time.
- Static or time-specific life tables: Age-distribution data of a population is collected at one time or during a short segment of time, such as through mortality data.
Why life table study is needed?
Life tables are needed in order to Understand the demographic processes and how these processes affect populations on a certain stage and conservation biologists. In ecologically, integrated pest management program, it is necessary to thoroughly understand the ecology of the pest. The life table generates an integrated and comprehensive description in details of development, survival, fecundity and life expectancy of a population, and is often used by scientists as a means of projecting the growth of populations.
Construction of A Life Table:
For making of life table, we consider life history of individuals, how many offspring are produced by each female, when each one dies and what is cause of mortality. For example, a hypothetical data is taken to understand the concept of life table. Suppose a female lays 400 eggs before her death. Half of these eggs are killed by predators , 90% larvae are consumed by parasites and three-fifth of pupa will freeze to death in winter.
The complete life table from proceeding data would be as follows.
| Development Stage | Number Alive | Mortality Factor(s) | Number Dying | Percent Mortality (%) |
| Egg | 400 | Predation | 200 | 50 |
| Larvae | 200 | Parasites | 180 | 90 |
| Pupa | 20 | Winter | 10 | 50 |
| Adult | 10 |
5 males and 5 females if sex ratio is 1:1
This number represents the maximum biotic potential of the species i.e. the greatest number of offspring that could be produced in one generation under ideal conditions.
The first line in life table tells us main causes of death, number dying and percent mortality during egg stage. In this example, only 400 eggs are produced in which 200 die due to predation. The second line of table mortality experience of these 200 larvae and 90% mortality of larvae occurs. The third line of table enlists mortality experience of 20 pupa and 50% mortality occurs due to freezing. Only 10 adults are left. If we assume a 1:1 sex ratio, then there are 5 males and 5 females to start the next generation.
In ecology, the symbol “R” (capital R) is known as the replacement rate. It is a way to measure the change in reproductive capacity from generation to generation. The value of “R” is simply the number of reproductive daughters that each female produces over her lifetime:
Number of daughters
R = ——————————-
Number of mothers
If the value of “R” is less than 1, the population is decreasing.
If the value of “R” is greater than 1, the population is increasing.
If the value of “R” is equal to 1, the population is stable.
Importance of Life Table:
- Life tables are made to estimate population trend regarding death rate, average expectation of life, migration rate etc.
- It is used to calculate expectation of life and comparison of mortality among communities.
- It evaluates average expectation of life based on age specific death rates.
- It is used to calculate net migration rate.
- Life tables can be used to compare population trend at national and international level.
- Life tables are particularly used for formulating family planning programmes relating to infant mortality, maternal deaths, health programmes.
- It is used to analyse data by using static- and cohort- composite life tables.
- We can understand the concept of age structure of a population and the possible effects upon population size.
- We can understand how factors such as change in medicine and environmental change; world wars and pandemics have affected human behaviour in last few years.
- To understand how human behaviour might change in the future.
- Population studies of insects can be conducted in laboratory with the help of life tables.
- Different methods have been developed to measure the reliability of life tables when used to predict the number of bollworms pupating or transforming to adulthood.
Note:
We can determine every aspect of insect population regarding migration, comparison in mortality rates, reason of mortality etc and how a population can perform in future. We can not only tell about insect population but life tables can also be used for any organism’s population.
