|Scientific Name||Scirpophaga incertulas Walker, 1863|
|Synonyms||Schoenobius bipunctifer (Wlk.)|
|Common Names||English: Yellow stem borer; German: Gelber Reisstengelbohrer; Spanish: Taladrador del arroz amarillo; French: Pyrale jaune du riz; Italian: Piralide del riso|
Scirpophaga bipunctifer shows a rather pronounced sexual dimorphism. In the past, the two sexes were even described as different species. The 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). It has several small, black spots at the tips of its forewings, which are otherwise dull brown.
The larvae are up to 20 mm (0.78 inch) long. Their normally yellowish-white color mostly changes to greenish when they fall into diapause.
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. They move from one internode to the next, perforating the septa. 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“). Each larva may destroy several tillers this way. 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 head“) and may result in significant yield reduction.
The female deposits 2-5 clusters of 50-150 eggs each (mostly several centimetres from the tips of leaf blades) and covers them with brownish scales from its anal tufts. The larvae hatch 5-10 days after oviposition, usually in the morning, with all of the individuals in one egg mass hatching almost synchronously. Within 1-2 hours, they bore into a rice plant. During a 3-5 week period, they pass through 5 or 6 instars: the caterpillars (especially the third instar) also migrate between plants. They are able to float on water and are often transported on plant parts by currents, but otherwise, they are spread mostly by the wind transport, which is facilitated by their tendency to hang from a leaf tip on a silken thread.
Pupation occurs inside the rice stem. Pupation occurs inside the rice stem. The mature larva cuts an exit hole through the stem wall, closes it temporarily with silk, and connects it with the cocoon by a silken tunnel. After 4-12 days, usually in the early evening, the adult will crawl through this tunnel and push open the temporary lid. It is able to emerge successfully even if the cocoon or the exit hole are situated below the water surface in the meanwhile. The moth does not feed, and lives for about 3-5 days. Males mostly remain hidden, but the females can often be seen on the plants during the daytime. Oviposition mainly takes place in the late evening.
Scirpophaga incertulas requires temperatures above 15°C, 59°F (optimum 25-30°C, 77 - 86°F) and high humidity for development. The larvae will survive between seasons in stubble, ratoon crop or volunteer plants. When conditions are unfavorable, they can diapause and the life cycle may extend to 4-6 months, but in the tropics, 4-6 largely overlapping generations per year are possible, depending on the number of rice crops.
S. incertulas is found throughout the rice-growing countries of tropical and subtropical Asia and Oceania. It is considered one of the most serious pests of rice. Losses mostly range from 1-20%, but during severe outbreaks, they may reach 60%.
This species commonly occurs together with other stem borers, but it is certainly the most important among them.
Useful non-chemical contribution to Integrated Weed Management
During transplanting, leaves with egg masses should be clipped off and destroyed. Removing harvest residues (e.g. by collecting and destroying stubble or by thorough tillage and subsequent flooding), will significantly reduce the number of surviving larvae. This is more effective if carried out area-wide in a coordinated community effort.
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