Yellow Stem Borer (Tryporyza incertulas)

Introduction

Any species becomes a pest when it crosses the threshold of abundance and this lime it goes against the interest of the human and is regarded as pest. The science which deals with the life, ecology, damage and control of the pest, is known as pestology. Due to loss of ecological balance and due to cultivation of large areas for crop the insects cross their threshold levels and become the pest. They then leave then host plants and attack the crops as the crops appear to be an immense wealth of food to them. During the act of food procuring they damage the crops in diverse ways. Some deveour on the roots and leaves, some on stems some on seeds etc. as a result the yield falls down sharply. About 13-14% or one crores tons of food crops are damaged by insects.

Paddy is the most staple crop of our country and many insect pests cause great damage to this crop by attacking the roots, stem, leaves and even the young grains when they are in milkstage. Rice (Oryza sativa L.) is one of the world's most important crops, providing a staple food for nearly half of the global population (FAO, 2004). Almost 90% of the rice is grown and consumed in Asia (Khush and Brar, 2002). It feeds more than two billion people in the developing countries of Asia (FAO, 1995). Among the biotic stresses, insect pests continue to be a major threat for increased rice production. Stemborers cause the most destructive damage to rice crops the world over (Shu et al., 2000).

Insect pests are one of major factor responsible for low yield of rice crop. As many as 54 different species of insect pests have been reported attacking the rice crop    (Inayatullah et. al., 1986). It has been estimated that each year the insect pests alone caused about 25-30% losses in yield (Ashraf et. al., 1986). Besides, other insect pests of rice, stem borers are the most serious ones, which take heavy toll of the crop. About 20 species of borers damage rice plant but only two of them (Tryporyza incertulas Walker and Tryporyza innotata Walker) are the most important (Nazir et. al., 1994). In South East Asia the average estimated yield loss due to Tryporyza incertulas and Tryporyza innotata were about 17% (Islam, 1990). Integrated pest management IPM practices can combat the pest very effectively. Natural resistance against insect pest is one of the very important components of IPM. The resistance phenomena can be of non-preference, variety potential of high yield, and presence of antibiotic chemicals in the variety. Knowledge of resistance level of a variety is also very important for planning a crop and breeding. In the present studies some of the cultivated varieties were evaluated for their resistance under the natural infestation conditions.

Rice stem borers are key group of insect pests damaging the rice crop. These are pale yellow coloured caterpillars that live in rice stems. There are five species of stem borers distributed throughout India. Among these, yellow stem borer, S. incertulas is the most widespread, dominant, monophagous and destructive.

Tryporyza incertulas is a major rice pests in the tropic and variable zone of Asia.The male and female adults have different colors and markings. Scirpopaga incertulas also called Tryporyza incertulas known as yellow stem borer, because this moth have brownish yellow. This moth also has a black spot on the back of the front wing. For the female moth has a larger black spot than male moth. Previously, these pests are known as pests that exist in a good irrigation where moth does not experience periods of fasting. However, this pest has now spread in an area that actually grow rice twice a year.

The yellow stem borer or YSB (Scirpophaga incertulas) is the most damaging rice stem borer in tropical Asia. The caterpillars (larvae) bore into the rice stem and hollow out the stem completely causing it to break. In Indonesia, stem borers are still a major threat especially in large-scale paddy fields, despite the Indonesians’ use of integrated pest management (IPM) technologies.

Yellow stem borer (YSB; Scirpophaga incertulas), a monophagous pest, is the most destructive because of its ubiquitous distribution and chronic pattern of infestation. It causes more loss than any other insect pest of rice causing 3 to 95% yield losses in India (Senapati and Panda, 1999) and accounts for 50% of all insecticides used in rice field (Huesing and English, 2004). YSB attacks the crop from the seedling up to the harvesting stage and thus causes complete loss of affected tillers (Salim and Masir, 1987). This could cause more damage if sowing/transplanting spread over longer period of time and especially infest the late sown crop more severely. Bhambhro noted that a month’s delay in planting resulted in heavy rice stem borer infestations, because the late-planted crop reached its maximum susceptibility to borers when the neighboring crops were nearing maturity and were not highly susceptible to fresh infestations.

Description of Yellow Stem Borer

Tryporyza incertulus is commonly known as ‘majrapoka’ to the cultivators. It is a moth and belongs to the phylum-Arthropoda, Sub-phylum-Mandibulata, Class-Insecta, Sub-class-Pterygota, order-Lepidoptera, Family-Pyralidae, Genus-Tryporyza, species-incertulus. They are distributed all over India. They are popularly known as yellow stem borers.

Tryporyza is a monophagus pest and only attacks the paddy plants and devours the stem tissue. As adult moths they are harmless but very dangerous at larval stage. Actually the larva is called ‘Majrapoka’ by the farmers. The larvae bore into the stem and consume the central shoot portion. The shoot gradually becomes yellow and ultimately no grain formation results. Characteristics:

- Group of eggs laid on the leaves of the tip.

- Just a larva in a single stem.

- Pupa is in the base of the shoots below the soil surface.

Yellow stem borer is a lepidopterian insect. The female fly which has a pair of black dot on its wings is mostly hazardous. Its newly hatched larva causes dead heart at seedling stage and white ear head at flowering stage. It has 6 brooding stages.

1.    3rd week of February

2.    2nd week of April

3.    4th week of May

4.    1st week of July

5.    3rd week of August

6.    1st week of October

Life Cycle of Yellow Stem Borer

The adult male and female moths can easily be distinguished. They possess yellow wing but the female is larger comparatively. The anterior pair of wings of female only possess a black spot each. After copulation the female lays eggs in phases and the eggs numbering more than 600, remain under the lower surface of the leaves. The eggs are placed in groups and in each group there are 40-100 eggs. The egg mass in a group is covered by buff coloured hair. If an infected paddy leaf is examined the egg masses become clearly visible.

Adults show sexual dimorphism. Females are yellowish brown to orange yellow in colour, with wing span of 3.0 cm, one black spot in the middle of fore wing and tip of abdomen bearing yellowish to buff coloured anal tuft of hairs. Males are slightly smaller in size; have yellowish forewings containing 6-7 small blackish spots.  Eggs are oval, flattened and whitish in colour. Incubation period of eggs is 5-8 days. Larvae are pale yellow in colour with a brownish head, 20 mm long and bore into the stem near the node. They usually feed on the lower part of the stem and migrate from one plant to another to mature. There are 6 larval instars and larval development takes 20-27 days. Pupation takes place inside a whitish silken cocoon within the rice stem, near the root-stem joint. Before pupation larva cuts an exit hole on the stem, above the surface of water and covers it with a silken web as an exit for emerge. Pupal period is 9-10 days or longer in cold weather. Total life cycle takes about 45 days and 4-5 generations can be completed from April to October. 

Ø Moth / Imago

After 10 days of pupation the pupa is converted into adult or imago moth which makes its exit through the hole prepared by the larva. The moths are attracted by the ligth and prepare to give rise to new generation. 45 day’s time is required to complete the life cycle of Majra Poka (Tryporyza). It may he noted that after harvesting the stubbles left on the field may harbour larvae or pupa which pass undisturbed throughout the winter season without an metamorphosis. They metamorphose only when favourable conditions appear in the next rainy season.

- The male moth is yellow and has dark spots on the front wings.

- The moth females yellow with a black dot in the center of the wings.

- Length 14 mm male moths and moth females 17 mm.

- Moths are active at night and attracted light.

- Reach fly 5-10 miles.

- Long live moths 5-10 days with the life cycle of 39-58 days

Ø Eggs

- Flocking 50-150 grains / group.

- Closed yellowish brown fuzz.

- Placed on the leaves at night at 19:00 to 22:00 during the 3-5

    night since the first night.

Ø Larva / Caterpillar

The eggs hatch within 6-8 days time and hatching of eggs depend upon the temperature and moisture. The larvae are covered with very fine hair and hence they are known as caterpillars. The baby caterpillars begin to eat the tissue of the leaves and advance towards the leaf apices. Some of the caterpillars hang by their saliva threads from the leaf apices and swing in air and they are driven to another fresh plant and infect them. Some; caterpillars crawl down the plants and reach the central stem, make small bores and migrate within the central tissue. That is why Tryporyza is called a stem borer. The larvae confined within the stem tissue, eat, the tissue and attain a length of 2 centimeters. The larval period extends from 4-5 weeks. A fullgrown larva has a faint yellow coloured body while the head is yellowish orange. The body is with chitinous head, 3 thoracic and 12 abdominal segments. Each thoracic and abdominal segment bears a black spot. The maximum length of 25 mm.

Ø Pupa / Cocoon

The matured larvae cease feeding and prepare themselves to be metamorphosed into pupa. But before that, they make holes on the stem for the exit of the imago. The larva form cocoons by their saliva and within the cocoon the larva pupate. Time for complete pupation is 10 days.

- Color yellowish or slightly white.

- Cocoon form membranes thread, white.

- The length of 12-15 mm.

Nature of Damage

Tryporyza damages the crop in the larval stage. Due to the effect of baring and consumption of stemtissue the plant gradually turns yellow and ultimately dies. Sometimes they attack the spikelet and thus grain formation is prevented. They are regarded as major pest. The yellow stem borer damages the paddy plants in all stages of its growth. If the seedling is attacked then the seedling do not grow and gradually die. But before the arrival of spikelet if it is attacked by stem borer then the stem of the plant becomes yellow; a condition know as dead heart. But if the stem is attacked before the arrival of the spikelets then the spikelets become white and the grains become chauffy; a condition known as white ear head.

Dead heart is produced when the insects attack at vegetative stage while white head occurs when the stemborers attack at the time of ear development. It is mostly found in aquatic environments where there is continuous flooding. Second instar larvae enclose themselves in body leaf wrappings to make tubes and detach themselves from the leaf and falls onto the water surface. They attach themselves to the tiller and bore into the stem.

·       Caterpillars bore into the central shoot of seedlings and tillers leading to death of central shoot.

·       Caterpillars bore at the base of earhead and cause chaffy earheads.

Symptoms of Damage

Larva bores into rice stem, feed on tissue and make gallery causing “deadhearts” in younger plants. At seedling and tillering stage, tender leaves can be injured by the feeding of larva and roll, appear yellowish to bluish white discoloration, looks like onion-shoots, called “deadhearts”. Rice seedling of deadhearts turn yellow and withered. Damaged holes are small, and there are not feces outside holes, but white particle of feces inside the stems.

·       At vegetative stage, the central leaf whorl does not unfold, turns brownish and dries out although the lower leaves remain green and healthy. The dried leaf can be easily pulled out. This characteristic damage is known as "Dead Heart"

·       At reproductive stage, panicle turn whitish, erect with chaffy spikelets and can be easily pulled out, is known as "white ears".

Field identification

The newly hatched larva is pale white colour and fully developed larva is yellowish white, usually one 1 larva is found inside a stem but occasionally 2 to 4 larvae may also be noticed. The moth is white-yellow to pale-orange wings of the female span about 24-36 mm (0.93-1.4 inch) and bear a clear black spot in the middle of the forewing. The male's wingspan is about 20-30 mm (0.78-1.18 inch).

  After hatching, the larvae of S. incertulas bore into the rice leaf sheath (where their feeding produces longitudinal yellowish stripes) and shortly afterwards into the stem, to feed on its inner wall, hollowing it out completely. In many cases, there are no visible symptoms of infestation at this stage. However, if the growing point is killed, the central shoot will dry up (a symptom called "dead heart"). The dead heart comes out easily when pulled and emits foul smell. While the rice plant can compensate for losses to a considerable extent by developing new tillers, these are smaller and produce fewer grains, so there is still some damage. In the vegetative stage, however, this is only moderate: larval feeding during the time of panicle formation and grain-filling is much more damaging, as it leads to the production of white, empty panicles ("white ears") and may result in significant yield reduction.

How to Manage

The chemical control of YSB is neither economical nor eco-friendly. It has proven to be ineffective because the insect larvae feed inside the stem pith and remain out of the reach of the pesticide. Integrated pest management has historically placed great hopes on host plant resistance. There are two potential sources for increasing the level of host plant resistance against insect pests; one is the natural resistance systems that may exist in germplasm of host plant species and their wild relatives, and the other is the potentially utilizable heterologous resistance systems that are often found in organisms like bacteria (Sharma et al., 2003). However, conventional host plant resistance to insects involves quantitative traits at several loci. Progress on the development of rice varieties with resistance to stemborer has been slow due to the lack of suitable germplasm, screening techniques and poor understanding of the genetics of resistance. On the other hand, a good level of resistance against the widespread yellowstemborer has been rare in the rice germplasm (Bhattacharya et al., 2006). The lack of a high level of resistance against the yellowstemborer had stalled development of suitable varieties in the past (Bentur, 2006).

Most breeding programs are still based on visual and phenotypic selection according to breeders experience and most resistance breeding to date has focused on vertical resistance wherein resistance is based on a single gene. The recent development of a molecular design for breeding provides opportunities to study dynamic behaviors from multiple levels among all components contained in a plant, and their interactions with environments during development (Cheng et al., 2006). With advances in biotechnology, breeding of horizontal resistance, whereby resistance is based on many genes, along with genetically enhanced sustainable pest resistance with fusion genes, is becoming more popular (Wan, 2006). Recent progress in rice transformation technologies has made it possible to produce genetically modified (GM) new rice cultivars with improved resistance to insect pests by genetic engineering.

Any pestisides/ insectisides which contain chloropyriphos as ingredient. It is recommended that all the pestisides/ insectisides should be applied in alternate manners. Collection and destruction of egg masses and plants showing dead hearts helps to reduce pest population, particularly in nurseries. Burning of stubbles or ploughing the field after harvesting is a good cultural practice. Early or late planting is recommended in areas of heavy infestation.

Spraying of the following insecticides: fenthion 0.05%, endosulfan 0.035%, endrin 0.025%, parathion 0.04% @ about 400 litres per hectare 2-3 weeks after transplanting and then every 20 days. Application of granules in the root zone of the following insecticides: Lindane 10%, diazinon 5%, carbofuran 5%, quinalphos 5% @ 2-2.5 kg a.i./ha at 20 days interval gives satisfactory  control of this pest. The following natural enemies keep the pest population under check and must be conserved: Egg Parasites: Tetrastichus schoenobii, Telenomus beneficiens, Trichogramma sp.   Larval parasites: Amauromorpha accepta schoenobii; Isotima javensis. Larval-pupal parasite: Trichomma sp.

Stem borers are difficult to control with insecticides because the larvae and pupae are inside the stem and they have overlapping populations in the field. Proper timing of insecticide application is critical to stem borer control.

1. Know the population peak of yellow stem borer in your place and avoid planting when stem borer population is high. At PhilRice CES, for example, the population peaks of stem borers are from April to May and from October to December. Hence, planting should be done in December-January for the dry season and June-July for the wet season so that the crop will be harvested before the peak of stem borer population.

2. Maximize the use of biological control agents like parasitoids, predators, and microbial agents.

3. Rice plants can compensate for stem borer damage at vegetative stage by producing more tillers; hence, insecticide application may not be necessary during vegetative stage.

4. Harvest the plants at ground level to remove stem borer habitat.

5. Plow and flood the field immediately after harvest to kill larvae and pupae inside the stubbles.

6. Raise the level of irrigation water periodically to submerge the eggs deposited on the lower parts of the plant.

7. Apply N fertilizer in two splits, following recommended rate and time of application. High N rate increases crop duration and susceptibility to stem borers.

8. Stem borers are difficult to control with insecticides because the larvae and pupae are inside the stem and they have overlapping populations in the field. Proper timing of insecticide application is critical to stem borer control.

9. To determine if insecticide is needed, check the population of adults and egg masses in the field.

- Observe the abundance of adults attracted to lights before and after transplanting.

- Collect egg masses from the field. Place these in covered vials or glass jars with a moistened paper. If 30% of the egg masses are parasitized, i.e., parasitoids emerge from the eggs, no insecticide is needed because the parasitoids can control the pest. If 70% of the eggs hatch as larvae, apply insecticide 1-2 days after the collected eggs have hatched.

Ø Preventive measures

• Use of resistant cultivars : Ratna, Sasyasree, Vikas (DRR), IR 36, IR 32, IR 66 and  IR 77 (IRRI)
• Clean cultivation and destruction of stubbles
• Clipping of leaf tips of the seedlings at the time of transplanting
• Installation of  pheromone traps with 5 mg lure @ 8 traps per ha for pest or 20 traps per ha for direct control by mass trapping
• Setting up of light trap as these moths are highly phototrophic.

Ø Biological control

ö   Egg parasitoid

Inundative release of egg parasitoid, Trichogramma japonicum Ashmead five to six times @ 1,00,000 adults per ha starting from 15 days after transplanting.   

þ  Two important egg predators are:  

·       Meadow grasshoppers - Conocephalus longipennis (de Haan) (Orthoptera: Tettigoniidae); and

·       Crickets, Metioche vittataicollis (Stal) (Orthoptera: Gryllidae)

þ  Larval and pupal parasitoids:

·       Cotesia flavipes Cameron (Hymenoptera: Braconidae)

·       Temelucha philippinensis (Ashmead) (Hymenoptera: Ichneumonidae)

·       Stenobracon nicevillei (Bingham) (Hymenoptera: Braconidae)

·       Bracon chinensis Szepligeti (Hymenoptera: Braconidae)

·       Tropobracon schoenobii (Viereck) (Hymenoptera: Braconidae)

·       Xanthopimpla stemmator (Thunberg) (Hymenoptera: Ichneumonidae)

·       Tetrastichus ayyari Rohwer (Hymenoptera: Eulophidae)

þ Important larval predators are:

·       Lady beetles, Micraspis spp. (Coleoptera: Coccinellidae)

·       Carabid beetles, Ophionea spp. (Coleoptera: Carabidae)

·       Rove beetle, Paederus fuscipes Curtis (Coleoptera: Staphylinidae)

·       Water bug, Microvelia douglasi atrolineata Bergroth (Hemiptera: Veliidae)

·       Water bug, Mesovelia vittigera (Horvath) (Hemiptera: Mesoveliidae)

·       Water bug, Limnogonus fossarum (F.) (Hemiptera: Gerridae)

·       Ants (Hymenoptera: Formicidae)

ö   Adults

þ  The important predators feed on adult stem borer

·       Anthocorid bug - Euspudaeus sp., (Hemiptera: Anthocoridae), Wolf spider - Lycosa pseudoannulata (Boesenberg and Strand) (Araneae: Lycosidae) feed on nymphs of GLH , BPH and adults of stemborer

·       Black drongo - Dicrurus adsimilis (Bechstein)(Dicruridae)

ö   Diseases:

Beauveria, Cordyceps and Nomuraea are white fungi that infect stem borers. Important pathogens are:

·       Beauveria bassiana (Balsamo) Vuillemin (Moniliales: Moniliaceae)

·       Nomuraea rileyi (Farlow) Samson (Moniliales: Moniliaceae)

·       Cordyceps sp. (Entomophthoraceae)

·       Bacillus thuringiensis Berliner (Bacteria)

·       An unidentified nuclear polyhedrosis virus

Chemical control of stem borers is often not economic. The caterpillars are only vulnerable stage to many of the foliar sprays in a short time between hatching from the egg and entering a stem. Systemic insecticides, which go inside the plant, are the only reliable form of chemical control for stem borers after the borers have entered the stem, but by then it is generally too late to save the rice stem anyway.

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