Among the stem borers infesting rice, the yellow stem borer, Scirpophaga incertulas (Walker) (Lepidoptera: Pyralidae), is an important pest of rice throughout tropical South and Southeast Asia. Its incidence is most predominant in tropical lowland rice and deep-water rice. The pest attacks all stages of the crop. Larval damage to tillers during the vegetative stage results in ‘dead heart’ symptom (drying up of central shoot) and damage during the panicle initiation stage results in ‘white ear’ (chaffy, unfilled grains). Stem borer adults are strongly attracted to light and signal the initiation of a fresh brood. Several factors strongly influence the relative abundance of stem borer populations like rainfall and humidity. However, development of stem borer life stages is strongly driven by temperature. Cooler temperatures and day length changes induce diapause or temporary arrest in development in mature larvae. One can can easily observe adult moths attracted to light sources near rice fields during the season. This information can be used to forecast the timing of generations under field conditions by providing current weather data for the location and date of observation (BioFix).
The yellow stem borer (YSB), Scirpophaga incertulas (Walker) (Lepidoptera: Pyralidae), is an important pest of rice, Oryza sativa L., throughout tropical Southand Southeast Asia[1]. It is the most abundant stem borer in tropical lowland rice anddeepwater rice [2,3] and is a damaging pest as it attacks allthe stages of the rice crop [4]. While yield loss due to S. incertulas is typically 5 to10% [5-7], it can be as high as 20% in certain regions and on particular varieties [8,9]. Larval damage to tillers during the vegetative stage results in “dead hearts”, whereas damage during the booting stage results in “white heads” or unfilled panicle grains. The boring behavior of the larvae limits the range of management options.
It is found in Afghanistan, Nepal, north-eastern India, Sri Lanka, Bangladesh, Burma, Vietnam, Thailand, Malaysia, Singapore, Sumatra, Java, Borneo, Sumba, Sulawesi, the Philippines, Taiwan, China and Japan.
Multivoltine S. incertulas is monophagous on rice. The number of generations depends on environmental conditions, primarily temperature.The life cycle of YSB comprises of four stages i.e., egg, larva, pupa and adult stages. In cooler climates, diapause occurs in larval stage.
Adult:
Adults exhibit remarkable sexual dimorphism. Male moths are smaller than the female moth with 8-9 small dark spots near the tip of the forewing and five along the sub-terminal area. Abdomen is slender, anal end has a thin hairy structure covering dorsally. The female moth is straw coloured with a conspicuous single black spot at the centre towards lower angle of each of the fore wings while hind wings are pale straw coloured. The tip of the abdomen of female is covered with tufts of yellowish silken hairs forming a circle around a ventral opening. Yellow stem borer undergoes three or more generations in a season. Most borer species are capable of flying only a short distance; however, they can travel 8-16 km if carried by wind [10]. YSB adult females emerge near the time that rice is tillering and live for 7 to 10 days.
YSB Female moth
Egg:
Adult female moth lays between 150 to 600 eggs deposited in three or four oval masses of 50 to 200 eggs each. Eggmasses are covered with brownish velvety hair obtained from the hair on the abdomen. Egg incubation takes between 5 to 9 days at optimum temperatures of 24 to 29°C [11-13]. In healthy egg masses, all the eggs hatch within a few minutes of each other, usually in the early or midmorning.
YSB Eggmass
Larva:
The larvae usually undergo four to seven larval instar stages to become full grown. Most larvae undergo five instars when reared at 23 - 29°C, but only four at 29-35°C. The number of moults decreases in larvae feeding on maturing plants compared with those feeding on tillering plants. Moults increase where few host plants are available. Within 2 h of egg hatch, almost all the newly hatched larvae will have either entered the leaf sheaths of the plant on which they enclosed or will have dispersed to other plants. After1 week, some larvae move to other plants or other tillers of the same plant. Larval development takes about 17 to 46 days. At optimum temperatures, larvae complete development in 17 to 27 days [14].
YSB first instar larvae
Pupa:
The pupa is yellowish white with a tinge of green. It turns dark brown just before adult emergence. Pupation occurs in the rice stems or stubble and lasts about 9 to 12 days [11-13].
Overwintering of YSB occurs as mature diapausing larvae in rice stubble [14].Drought during the larval period can induce a temporary slowing down of body metabolism to prolong the developmental period. A more profound physiological change that enables stem borers to live for months in suspended development is called diapause. Diapause can be either hibernation (overwintering in temperate climates) or aestivation (dry season dormancy in the tropics). Hibernation is broken by warm weather and longer day lengths and aestivation is broken by rainfall or flooding.
Plants are most prone to stemborer infestation at the tillering and flowering stages [15]. In a transplanted crop, stem borer larvae cut off the growing points of tillers causing them to die, a condition commonly known as ‘dead heart’. When the plants are attacked later, during the flowering stage, larvae feed on the spikelets, panicle axis, and panicle neck causing empty, whitish-looking panicles called ‘white ears or earheads’. In the infested fields these white earheads stand erect and contain empty and unfilled glumes.
S. incertulas larvae kill both vegetative tillers and emerging panicles [12,17-20]. Damage to tillers is compensated to a large extent. However, lowest yields result from whitehead damage when infested at or just after the pre-booting stage [4].In irrigated production systems, 1% dead heart resulted in 0.3% or 12 kg/ha loss whereas a 1% white earhead damage level caused 4.2% or 183 kg/ha loss ingrain yield; the loss due to 1% infestation in both crop growth stages was estimated at 4.6% or 201 kg/ha [16].S. incertulasis often more abundant than all rice borers combined and attacks rice throughout the growing season [21,22]. In China, India, and Southeast Asia, annual losses to rice borers approximately 5-10%, but losses in individual fields may reach 50-60% [12,17,23 & 24]. Recovery or prevention of 5% of the losses to stem borers could feed approximately 140 million people for 1 yr [23].
A minimum temperature of 16°C is required for larval development, and at 12°C, development of the second and third instars stopped [11,13,25 &26]. At lower temperatures, larvae pass through more instars than at higher temperatures. At 23 to 29°C, most larvae pass through five instars. However, between 29 to 35°C, they pass through only four instars.
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