🐞 Sterility Methods and Sterile Insect Technique

This lesson explains sterility-based insect population control, especially the Sterile Insect Technique (SIT/SIRM), its principles, methods, and practical use conditions in integrated pest management.

Sterility method - Definition

Control of pest population achieved by releasing large number of sterilised male insects, which will compete with the normal males and reduce the insect population in subsequent generation.

It is usually referred as SIT (Sterile insect technique) or SIRM (Sterile insect release method)

Sterile insect release method is a genetic control method. This is also called Autocidal control since insects are used against members of their own species.

E.F. Knipling in 1937 in South East USA used the SIRM technique to control the screw wormfly ( Cochliomyia nominivorax ) a serious livestock pest.

The sterile to fertile male ratio, called S:F ratio is important, as the reduction in reproductive potential of natural population depends on S:F ratio.

The mating with the sterile males will produce inviable or sterile eggs.

Trend of hypothetical population subjected to SIRM Assumption

1. Female:Male ratio 1:1
2. 1 female produces 5 females as off spring in one generation

No.of sterile

males released

No.of

females releases(9:1)

Ratio sterile

to normal

males

No. of

fertile females

Generation

No.of females

without releases

1. 1,000,000 9,000,000 1,000,000 9:1 100,000

2. 5,000,000 9,000,000 500,000 18:1 26,316

3. 25,000,000 9,000,000 131,579 68:1 1,907

4. 125,000,000 9,000,000 9,535 944:1 10

5. 625,000,000 9,000,000 50 180,000:1 0

In suitable circumstances sterile male release method (SIRM) can be more effective, compared to insecticide application.

Comparison of SIRM with insecticide - Trend of hypothetical population

Generation No. of females with no treatment

No. of females with

sterile release (9:1)

No. of females with

insecticide (90% kill)

1. 1,000,000 1,000,000 1,000,000

2. 5,000,000 500,000 500,000

3. 25,000,000 131,579 250,000

4. 125,000,000 9,535 125,000

5. 625,000,000 50 62,500

6. 3,125,000,000 0 31,250

SIRM technique can also be used after insecticide application which will be more effective.

Circumstances for using this method

1. Against well established pest when their population density is low
2. Against newly introduced pest
3. Against isolated population as in island
4. Combined with cultural and chemical methods

Methods of sterilizatoin 1. Chemosterilants: Any chemical which interfere with the reproductive capacity of

an insect. a. Alkylating agents

They inhibit nucleic acid synthesis

inhibit gonad development produce mutagenic effect (e.g.) TEPA, Chloro ethylamine

b. Antimetabolites

Chemicals having structural similarity to biologically active substances. They interfere with nucleic acid synthesis. e.g. 5-Fluororacil, Amithopterin

Methods of sterilization - continued II. Irradiation

Irradiation done by exposing insects to , , radiations, X rays and neutrons. Of these, -radiation by [60] CO (cobalt) with its half-life of 60 years is the most common method.

Irradiation causes following sterility effects in insects Infecundity Aspermia Inability to male Dominant lethal mutation

Radiation dose required for different species and stages for sterilization (expressed as rads - radiation absorbed dose).

Insect Stage Dose

Housefly

Screw worm

2-3 day pupae

5 day pupae

1 day adult

3000 rads

2500 rads

5000 rads

Sterilizing natural population

In this method, instead of releasing sterilised males into the field, a chemosterilant is sprayed in field like insecticide. The chemosterilant sterilizes both male and female. These do not produce offspring-equivalent to killing them.

Bonus effect: The bonus effect of this method is that the sterilized males mate with normal females and reduce their reproductive capacity.

Chemosterilants used are TEPA, HEMPA, BISULFAN, etc.

Requirements for SIRM

1. A method inducing sterility without impairing sexual behaviour of insects.
2. Mass rearing of the insects
3. Information on population density and its rate of increase
4. The released insects must not cause damage to the crops, livestock or human beings
5. Good intermingling of released and natural population
6. Releasing sterilized insects when the wild population is abundant
7. This method is effective against newly introduced pest or isolated insect

population as in island. 8. There should be high sterile to fertile (S:F) ratio for quicker control.

Limitations of SIRM

1. Not effective against insects which are prolific breeders
2. Sterilizing and mutagenic effect of chemosterilants and irradiation cause problem in

higher animals and man (Carcinogenic and mutagenic)

Summary Cheat Sheet

• Sterility methods suppress pest populations by releasing large numbers of sterilized male insects.

• SIT or SIRM is a genetic control method and is also described as autocidal control.

• Success depends heavily on the sterile-to-fertile male ratio and repeated release strategy.

• Sterile mating reduces viable offspring generation after generation, causing population decline.

• Under suitable conditions, sterile insect release can outperform repeated insecticide use for target pests.

• Review core concepts, definitions, and field-level application points from this lesson.

• Prioritize economic threshold-based decisions and integrated management logic where relevant.