Insect sterility methods in pest management

1. INSECT STERILITY METHODS
HISTORY:
Controlling insects by sterilization was first conceived by Raymond Bushland
and E. F. Knipling in 1937, while he was studying the biology of the screwworm
fly (Cochliomyia hominivorax) a serious livestock pest in south- east USA. The
larvae of these flies invade open wounds and eat into animal flesh, killing infected
cattle within 10 days. The female of this insect mated only once, if the male of this
could be sterilized without impairing their ability to mate, it is possible to
eradicate the isolated population of this pest.
INTRODUCTION:
The sterile insect technique (SIT) is a method of biological insect control,
whereby overwhelming numbers of sterile insects are released into the wild. The
released insects are preferably male, as this is more cost-effective and
the females may in some situations cause damage by laying eggs in the crop, or, in
the case of mosquitoes, taking blood from humans. The sterile males compete with
wild males to mate with the females. Females that mate with a sterile male
produce no offspring, thus reducing the next generation's population.
Sterile insects are not self-replicating and, therefore, cannot become established
in the environment. Repeated release of sterile males over low population
densities can further reduce and in cases of isolation eliminate pest populations,
although cost-effective control with dense target populations is subjected to
population suppression prior to the release of the sterile males.

2. The technique has successfully been used to eradicate the screw-worm
fly (Cochliomyia hominivorax) from North and Central America. Many successes
have been achieved for control of fruit fly pests, most particularly
the Mediterranean fruit fly (Ceratitis capitata) and the Mexican fruit
fly (Anastrepha ludens).
Sterilization is induced through the effects of irradiation on the reproductive
cells of the insects. SIT does not involve the release of insects modified through
transgenic (genetic engineering) processes. Moreover, SIT does not introduce non-
native species into an ecosystem.
METHODS:
There are two systems involved in this technique.
1. Sterilemale technique:
According to this technique, if fully competitive sterile males are released in the
natural populations, it will reduce the reproductive potential of the natural
populations in the same ratio as sterile to fertile. If, the sterile to fertile ratio is 1:1,
then according to law of chances, both sterile and fertile males will have an equal
chance of mating with the wild females. The mating with the sterile males will be
infructuous, which mean that the reproductive capacity of the natural population
is reduced by 50%. Thus if the S: F ratio is increased to 9:1, then the control will be
90%.
2. Sterile insect release method(SIRM) :

3. Both males and females, mass reared and sterilized in the laboratory and are
released at every generation until the pest is eradicated or controlled.
This technique involves sterilization of a portion of the wild population itself
comprising of both males and females by chemosterilants. The effect then will be
two fold. The percentage sterilized cannot reproduce and in effect, will amount to
killing the same proportion of insects. The sterilized males and females of the wild
population will, in turn, nullifythe reproductive capacity of a proportionate
number of the fertile individuals in the population by competing with them.
This method combines chemical and biological control methods. Thus if 90% of
the given population is sterilized, which cannot reproduce will amount to 90%
killed (first effect). Of the remaining fertile 10%, only 1% can be expected to have
fertile mating because of the existence of 90% sterilized insects in the population
(bonus effect), which will amount to 99% control. An added effect produced by
the released insects the natural population could be competing with them for food
and shelter.
METHODS OF STERILIZATION:
1. Chemosterilants :
Any chemical that can inhibit the growth of gonads or interfere with the
reproductive capacity of an insect in any other way.
Alkylating agents: they act in the following way
 Inhibiting nucleic acid (DNA, RNA) synthesis
 Inhibiting gonadial developments
 By producing mutagenic effects

4. E.g. TEPA, Chloroethylamine
Antimetabolites :
Chemicals with structural similarity as biologically active substances, as a
result biological system fails to distinguish them from their own natural
substances and utilize them. They interfere with the synthesis of nucleic acids.
E. g. 5- Fluorouracil, Amithopterin, Folic acid, Leucine, Colchicine, Thiourea
2. Irradiation :
This can be done by exposing insects to g, b, l radiations, X- rays and neutrons.
Of this g- radiation by 60
CO with is half life 6 years is the most common method.
It can produce the following types of sterility in insects,
 Infecundity
 Aspermia
 Inability to mate
 Dominant lethal mutation
Radiation doses differ for different species and their stages, expressed as rad
(radiation absorbed dose)
Insect Stage Dose
House fly
Screw worm
Mosquito (Culex)
Honey bee
2 to 3 day pupae
5 day pupae
1 day adult
Pupae
Adult
3000 rads
2500 rads
5000- 7000 rads
7700 rads
7000- 11000 rads

5. BASICREQUIREMENTS:
 A method inducing sterility without impairing sexual behavior of insects
 A practical method of mass rearing of insects to be controlled
 A quantitative information on the natural population density
 Information on the rate of population increase
 Released insect must not cause damage to the crops, livestock and man
 Good intermingling of released and natural populations
 Good coincidence between the emergence of released and wild insects from
their pupae
 Releasing of sterilized insects when the wild population is abundant
SUCCESSFUL PROGRAMMES:
 The screw-worm fly (Cochliomyia hominivorax) was eradicated from the
United States, Mexico, Central America, Puerto Rico and Libya
 The Mexican fruit fly (Anastrepha ludens) was eradicated from most of
northern Mexico
 The tsetse fly was eradicated from Zanzibar
 The Mediterranean fruit fly (Ceratitis capitata) was eradicated from the
northern part of Chile and southern parts of Argentina, Peru and Mexico. It
is being suppressed by SIT in fruit-producing areas of Croatia, Israel, South
Africa and Spain
 The codling moth (Cydia pomonella) is being effectively suppressed in parts
of in British Columbia, Canada
 The pink bollworm (Pectinophora gossypiella) eradicated from southwestern
USA and northwestern Mexico
 The false codling moth (Thaumatotibia leucotreta) is being effectively
suppressed in parts of South Africa

6.  The cactus moth (Cactoblastis cactorum) was eradicated from an outbreak in
Yucatan, Mexico
 The melon fly (Bactrocera cucurbitae) was eradicated from Okinawa
 The onion fly (Delia antiqua) managed in onion production areas in the
Netherlands
ADVANTAGES OF SIRM:
 Release of sterile insects achieves an increasingly higher sterile to fertile ratio
and thus becomes progressively more efficient, which is an advantage over
the insecticides, where the kill in each generation remains constant.
 Eradication of tsetse has resulted in major socioeconomic benefits for
Zanzibar.
 When implemented on an area-wide basis and a scaled rearing process, SIT
is cost-competitive with conventional control, in addition to its
environmental benefits.
LIMITATIONS:
 Not effective against insects that are prolific breeders
 Sterilizing and mutagenic effects of the chemosterilants and irradiation
extend to higher animals including man (Carcinogenic and Phytotoxic)
 Naturally low population periods or repeated pesticide treatment are
sometimes required to suppress populations before the use of sterile insects
 Sex separation can be difficult, though this can be easily performed on a
large scale where genetic sexing systems have been developed as for the
Mediterranean fruit fly
 Radiation, transport and release treatments can reduce male mating fitness

7.  The technique is species-specific. For instance, the technique must be
implemented separately for each of the 6 economically important tsetse fly
species.
 Mass rearing and irradiation require precision processes. Failures have
occurred when unexpectedly fertile breeding males were released.
 Area-wide approach is more effective, as migration of wild insects from
outside the control area could recreate the problem.
 The cost of producing sufficient sterile insects can be prohibitive in some
locations but decreases with economies of scale
REFERENCES:
 Principles of Applied Entomology/ 2003/ N. K. Ragumoorthi, M. R.
Srinivasan, V. Balasubramani, N. Natarajan/ A. E. Publications, Coimbatore-
41.
 Entomology and Pest Management/ Larry P. Pedigo/ published by Prentice-
Hall of India Private Limited, New Delhi- 110 001.
 en.m.wikipedia.org