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Environment

Taking biological control to farmers

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 30C). 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 4C 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.