It is rightly being viewed as making the start of a
new phase in the chequered history Pak-US ties
By
S.M. ALAM NIA,
Tando Jam.
Feb-18 24, 2002
The term biological control is a subject of
entomology, which deals with the study of insects and related animals
found in the ecological environments. Applied entomology uses the
knowledge gained through basic research to solve problems in
agriculture, medicine, health, criminology, ecology, forestry and
more. Agricultural crops, turf, ornamental plants and trees are all
attacked and injured by insects and many entomologists are engaged in
activities to minimize this destruction carried over by these insects
all over the world. The concept of biological control is very old. Use
of cat to control mouse is a very simple example. Biological controls
nothing but the utilization of parasitoids, predators and pathogens to
control pests. These are known as natural enemies of insect pests.
Parasitoids are species whose immature stage develops on or within a
single insect host, ultimately killing the host. Many species of wasps
and several flies are parasitoids, e.g. Trichogramma spp.
Predators: A predator is an organism that
requires the consumption of more than one prey to develop to maturity.
Predation is generally density-dependent in its action and therefore
is one of the key factors in regulating the population density of prey
species. The occurrence of Paederus alfierii adult beetles in
maize, cotton and clover field seemed to follow a certain seasonal
succession, which seems to be related to the seasonal abundance of
these different preys. The anthocorid predator Xylocaris flavipes
was found associated with some insect pests on stored barley and Sorghum.
Promising natural enemy candidates for the control of native pests or
in new associations would be required to have the same attributes as
those used against introduced pests such as ecological compatibility,
high searching capacity and fecundity, short developmental period, and
should not be monophagous as otherwise they could not attack the new
host. Prior to release, laboratory tests will have to show whether the
intended new host is accepted and whether it is physiologically
suitable for normal development of the predator or the parasite.
The effect of different insecticides was widely
studied on natural enemies by many researchers. Recently, the
laboratory tests revealed that some pesticides, which were used to
control white flies and aphids adversely, affected the two perdators Chrysoperla
carnea and Corcinella undecimpunctata and the two parasites
Eretmocerus mundus and Diaeretiella rapae. In addition
to killing these beneficial insects by high doses of the used
insecticides (primiphos methyl and carbosulfan), low doses affected
markedly the viability of those biological agents. Insecticidal
treatments against white-flies and aphids can be used when parasitoids
are absent or else their few numbers can not stop or decrease the
numbers of both pests. Augmentation involves the supplemental release
of natural enemies. Relatively few natural may be released at a
critical time of the season (inoculative release) or literally
millions may be released (inundative release).
Sugarcane and cotton are two main crops where
biological control system has been used in large extent in the
country. Cotton has very important pest problems . The American Boll
worm, the Spotted Boll worm and the Pink Boll worm are the pests which
are difficult to control with chemical pesticides. Trichogramma
spp. Have been developed for the control of sugarcane borer.
Biological control is an indispensable component of an integrated pest
management strategy. It is defined as the reduction of pest population
by naturalenemies and typically involves as an active human role. The
fact is, all insect species are also suppressed by naturally occurring
organisms and environmental factors, with no human input and this is
referred as natural control.
One recent development in the laboratory, is the
molecular monitoring techniques used in ecological research and
agro-practice are also being studied. Natural enemies from Trichogramma
such as T. dendrolimi, Tchilonis and T. ostriniae are
being applied on farms in many countries of the world. Biological
control as a major factor towards IPM relies much on parasitoids and Trichogramma
is closely connected to IPM system. Trichogramma as microhymnoptera,
is the most widely distribute natural enemy against major crop pests.
Its identification at species level depends mainly on male external
genitalia. The problem is, only females can be found from many species
under natural control conditions. Trichogramma is minute and
indistinguishable morphologically, further, the environment factors
influence its morphology and physiology significantly. There are
numerous species of Trichogramma wasps that attack the eggs of
over 200 species of moths and butterflies. These almost microscopic
wasps (0,5 mill; 1/64 inch) are very important in preventing crop
damage because they kill their hosts before the insects can cause
plant damage.
The female Trichogramma lays an egg within a
recently laid host egg, and as the wasp larva develops, the host egg
turns black. Each female parasitizes about 100 eggs and may also
destroy additional eggs by host feeding. The short life cycle of 8-10
days allows the wasp population to increase rapidly. These wasps are
harmless to people, animals, and plants. Trichogramma are
readily available in large quantities from commercial suppliers. There
are several species and strains of Trichogramma, which
considerably in their ability to control different insects and in
their adaptation to different environmental conditions and crops.
Determining the best species or strain to release may be difficult.
Most suppliers provide detailed instructions for the strain selection,
release, and use of Trichogramma, but their recommendations may
not always be accurate. The most suitable species commercially
available for release in field and vegetable crops is probably T.
pretiosum, but again, results may vary considerably.
The wasps are shipped as immature inside moth eggs
glued to small white cards that can be attached by hand to infested
plants. Renal application is possible for large acreages. The related
cards should be kept in a warm, humid place out of direct sunlight
until the emerging adults can be seen as small dots moving around in
the close glass jar container. A few tiny caterpillars may also be
found in the container because it is very difficult to obtain 100 per
cent parasitization of the moth eggs, but these are harmless in crops.
When most of the adults have emerged, place the containers in a shaded
spot upwind of the areas where moths are suspected or egg laying is
occurring. The adult wasps will fly onto the plants in search of new
host eggs to attack the cards. The card should not be put out before
the wasps have emerged because ants and other predators may eat them.
The emerging wasps will have the best chance of finding and
parasitizing eggs when the weather is moderate. The best time to
release is early morning or evening when direct sunlight will not hit
the cards. Avoid making releases under extremely hot, cold, rainy, or
windy conditions.
Whenever possible, releases should begin at the
time of the first moth flight, before the pest population has built
up. Pheromone traps for diamondback moth and cabbage looper, black
light traps for cabbage looper, or visual inspection for imported
cabbage worm butterflies are useful for monitoring adult flight.
Regular scouting to determine the appearance of caterpillar eggs is a
more accurate method to determine when hosts for Trichogramma
are available. Frequent release made over several weeks result in
better parasitism and control than a single release. Releases of 5,000
Trichogramma made at weekly intervals over 3-6 weeks will
usually be sufficient for the home garden of up to 2,000 square feet
(186 square meters). For each additional 2,000 square feet, release
5,000 more Trichogramma.
Utilization of Trichogramma spp. (Egg Parasitoids):
For biological control to become a success it has to be a national
movement. There should be a national policy that biocontrol agents
alone will be used for specific pests. The Trichogramma spp.
are egg parasitoids which present one of the most important biological
agents. The life cycle is very short (7 days at 30°C). Longevity of
the adults reaches 30 days and fecundity is very high. These
parasitoids may be used in several cultivations, attacking a very wide
range of insect pests' eggs used in fields in several countries like
China, Egypt, France, Germany, Indonesia. Malaysia, Mexico, Pakistan,
Russia ,USA etc. In China for the last 20 to 25 years for the control
of sugarcane borers only Trichogramma parasitoids are beig used
. The same is true with the paddy leaf roller. In Malaysia and
Philippines also they have completely done away with pesticides in
rice.
Trichogramma may be mass produced easily in the
laboratory. A technique of mass production of Ephestia eggs was
adopted in France, to be used as a good host. In Germany, a
semi-automatic production unit for the host Sitotroga cerealella
as well as a rational mass rearing technique for the parasite were
developed. The successful use of Trichogramma to control the European
corn borer (Ostrinia nubilalis) was repeatedly demonstrated in field
experiments. The release of about 135,000 parasitoid/ha reduced the
population of the European corn borer Ostrinia about 75 %. The
release of Trichogramma at 300,000/ha at the beginning of the
appearance of the insect pest gave the best results. Two-four releases
gave 80 to 97 % reduction in the infestation by the corn borer. The
necessary number of releases depends on the weather conditions, the
duration of the Ostrinia, the infestation and habit of the
crop.
In Portugal, Trichogramma was used
successfully to control Helioverpa armigera in tomato field
(about 20 % reduction was obtained). Release experiments were done in
a cabbage crop with several Trichogrammaspecies. The number of
some pest species were significantly reduced after Trichogramma
release. Further the distribution patterns (single eggs per leaf
clusters of the hosts) differ a lot and these differences strongly
influence the number of hosts parasitized per unit time and the number
of eggs laid per host. Field studies have shown that chemical
mediators emanating from the host can be used to increase the
parasitization rates by Trichogramma.
The yield of sugarcane increased in south India by
releasing Trichogramma over seven years at the rate of
40,000/hectare/week for 12 weeks following crop germination. Release
of Trichogramma also resulted in effective control of stem and
root borers. A practical and reliable proposal for the management of
sugarcane borers in India is summarized in the following: More
efficient species/strains; improved stock and cheap production; field
release systems; assessment of impact of release. Trichogramma
seems to be clearly useful in sugarcane internode borer control. It is
important to reevaluate the Trichogramma method across chosen
regions and also against particular population levels to obtain the
most stable and economic impact of their release. Eggs of Ephestia
were treated by ultraviolet radiations. No changes in the biotic
potential of Trichogramma reared on these eggs. Parasitized eggs were
stored at 4°C for 60 days. Field treatment by releasing Trichogramma
evanescens was carried out once/year. About 20,000 parasitoid/fed
were released between the end of May and first of July.
A total of 29,919 fed of sugarcane fields in Egypt
have been treated against Chilo agamemnon. The results show
that infestation decreased from 16.3 to 4.1 %. The native species Trichogramma
pretiosum, released periodically on commercial crops as cotton,
soybean, cassava, tomato, corn, sorghum and bean makes a very
satisfactory biological control on Heliothis spp., Alabema
argiliacca, Anticaria genmatalis, Omiodes indicata and other Lepidopterous
pests. The results with egg parasitoids justify the continuation of
species diversification programmes, development of massive rearing
techniques and tests to assess effectiveness in the field of Columbia.
Trichogramma plaseyensis is the most important
parasite, with egg mass parasitism of 70-100 against the Asian Corn
Stem Borer, Ostrinia furnacalis. An early release of 140,000 wasps per
hectare reduced the larval population and gave higher yield in New
Guinea. When Trichogramma galloi was released against second
and third generation of Diatraea saccharalis on sugarcane, 44 %
decrease in the pest intensity was recorded. Released T. chilonis
decreased the great numbers of the jute green looper, Anonis flava
in Vietnam. One larva of the red ladybird beetle can eat 24-30 eggs
and 12-15 1st. instar larvae of Anomis. T. japonicum
utilization in Vietnam, increased the parasitized eggs of the rice
leaf folder, Cnaphalocrosis medinalis. The parasitism capacity
of T. chilonis on the sugarcane stem borers Sirpophaga
nivella and Chilo sacchariphagus inducus was 40 and 79 %.
Initial inundative releases of Trichogramma spp. were
encouraged with levels of Heliothis egg parasitism being 33 and
76 % in non-release and release sites in Australian rain-grown cotton.
Release Components: a) As regards the
characteristics of Trichogramma, these depend upon:- The
species used; the type of development presented by the parasitoid in
laboratory, i.e. before the field release (continuous or interrupted
development); the stage during which the insect is deposited in the
field (end of last larval stage, nymph, emerged adult); the
arrangement of parasitized eggs (naked or protected in a container).
b) As regards the conditions for release characteristics, those depend
upon: The number of insects released per ha and per release; the
number of release operations needed to cover the whole oviposition
period; the date of the first release as compared to that of the
beginning of the oviposition period; the distribution of parasitoids
on and between rows; the time interval between two releases.
Trichogramma spp. with a wide forging spectrum
is a noted genus that has been used for biocontrol in field for many
years, but many closely-related species are morphologically-
indistinguishable owing to their minuteness ( smaller than o.4mm) and
evolutionary homology. Meanwhile, Trichogramma can provide a
fine model system for studies of population interactions, behavioral
ecology, sex ecology and molecular evolution etc. Pragmatism and
theory both call for a powerful tool, in particular molecular tools,
to cope with such problems as identification, breeding, genetics,
behavioral studies and molecular monitor. Genetic marker technique can
play a big role in that molecular markers can realize molecular
monitoring of the microhymnoptera under experimental conditions
and in fields.
The establishment of a monitoring system is helpful
for practical use of this wasp. In this system, the environmental
hazard does not exist. A lot of researches have been done in Pakistan
on biological control at NIA, Tando Jam. Major pests have been
surveyed and their natural enemies identified. In fact in some cases
use of natural enemies has also been recommended as an alternative to
pesticides. Among the alternative of insecticides, biological control
holds great promise. The advantages of this method are that it is
non-polluting and long lasting in reducing the pest population.
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