Insect Pests of Fruits, Plantation, Medicinal & Aromatic Crops

Factors that determine insect abundance

i) Biotic potential

• It is the innate ability of the population to reproduce and survive. It depends on the inherited properties of the insect i.e., reproduction and survival. Potential natality is the reproductive rate of the individuals in an optimal environment. Survival rate depends on the feeding habits and protection to young ones (Eg. Viviparity). Generally insects with high reproductive rate tend to have low survival rate and vice versa.
• Insect pests with high reproductive rate and low survival rate are called r strategists named after the statistical parameter r, the symbol for growth rate coefficient. Such pests succeed because of sheer numbers. Eg. Aphids.
• K strategists reproduce slowly but effectively compete for environmental resources and so their survival rate is high. (K letter denotes flattened portion of growth curve) Eg. Codling moth of apple.
• Birth rate or natality is measured as the total number of eggs laid per female per unit time. Factors determining birth rate are fecundity, fertility and sex ratio.
• Death rate or mortality denotes the number of insects dying over a period.
Example of High reproductive rate

• A single moth of Earias vitella (Bhendi fruit borer) lays about 200 eggs per female. Life cycle is completed in 1 month
After 1 month 200 adults
100 male + 100 female
100 x 200 = 20,000 eggs
After 2nd month 10,000 x 200 = 2,000,000 eggs
After 1 year 2,000,000,000,000,000,000,000,000 adults
(i.e., 2 followed by 24 zeroes)

• If a single moth can produce this much, they will cover 24.32 above earth surface in 1 year. But in reality only a fraction of progeny completes life cycle due to environmental resistance.
Environmental resistance is the physical and biological restraints that prevent a species from realizing its Biotic potential. Environmental resistance may be of 2 types.
1. Biotic factors - includes

a) Competition (interspecific and intraspecific)
b) Natural enemies (predators, parasites and pathogens)

2. Abiotic factors

a) Temperature
b) Light
c) Moisture and water
d) Substratum and medium

Bioresources in ecosystem

• Ecosystem comprises of biological communities and non-living environment. Eg. Agro ecosystem, pond ecosystem, etc.). Bioresources refers to the biodiversity of various organisms living in that ecosystem.
• Eg. The different pests of cotton, its natural enemies, hyperparasitoids, microbes, etc. are referred to the bioresources in cotton ecosystem.
• The ecosystem should have more bioresources. Such ecosystem will be more stable. Insecticides will deplete the bioresources in ecosystem and make it less stable and prone to pest outbreak.
• Natural control will be high when bioresources (e.g. Parasitoids and Predators) are more.
Population dynamics and role of biotic factors
Attributes of a population

i. Density: Population size per unit area
ii. Birth rate (Natality): Rate at which new individuals are added to the population by reproduction
iii. Death rate (Mortality): The rate at which individuals are lost by death.
iv. Dispersal: The rate at which individuals immigrate into and emigrate out of the population.
v. Dispersion: the way in which individuals are distributed in space. It may be of 3 types.

a) Random distribution
b) Uniform distribution
c) Clumped distribution

vi. Age distribution: the population of individuals of different ages in the group.
vii. Genetic characteristics: adaptive ness, reproductive fitness, persistence.
viii. Population growth form: the way in which population changes / grows as a result of natality, mortality, and dispersal.

Population dynamics

• Populations grow in two contrasting ways. They are
i. J- shaped growth form (Fig. 1a)
ii. S- Shaped or sigmoid growth form (Fig. 1b)
N K
Density
Time

• Fig. 1a. J- Shaped growth form Fig. 1b. S - Shaped growth form. In the J - shaped growth form, the population density increases in exponential or geometric fashion; for example 2,4,8,16,32 … and so on until the population runs out of some resource or encounters some limitation (limit N, Fig. 1a). Growth then comes to a more or less abrupt halt and density declines rapidly. Populations with this kind of growth form are unstable. Their reproductive rate is high and survival rate is less and so they are r strategists. A factor other than density regulates the population. (Eg. Aphids).
• In the S-shaped growth pattern (Fig. 2) the rate of increase of density decreases as the population increases and levels off at an upper asymptote level K, called the carrying capacity, or maximum sustainable density. Their reproductive rate is less and survival rate is more. So they are K strategists. This pattern has more stability since the population regulates itself. (Eg. Hymenopterans).
• The population growth rate or change is worked out using the formula,
Nt = N₀e^{(b-d) t} - E_t + I_t

Where

Nt = number at the end of a short time period
N₀ = number at the beginning of a short time period
e = base of natural logarithm = 2.7183
b= birth rate
d= death rate
t= time period
E= emigration
I = immigration.

Life table: Life tables are tabular statements showing the number of insects dying over a period of time and accounting for their deaths.