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Plantwise Technical Factsheet

purple stem borer (Sesamia inferens)

Host plants / species affected
Avena sativa (oats)
Coix lacryma-jobi (Job's-tears)
Cymbopogon citratus (citronella grass)
Cyperus rotundus (purple nutsedge)
Echinochloa colona (junglerice)
Echinochloa crus-galli (barnyard grass)
Echinochloa frumentacea (Japanese millet)
Eleusine coracana (finger millet)
Eleusine indica (goose grass)
Eriochloa procera (tropical cupgrass)
Hordeum vulgare (barley)
Ischaemum rugosum (saramollagrass)
Megathyrsus maximus (Guinea grass)
Miscanthus sinensis (eulalia)
Oryza sativa (rice)
Panicum miliaceum (millet)
Panicum repens (torpedo grass)
Paspalum scrobiculatum (ricegrass paspalum)
Pennisetum glaucum (pearl millet)
Phragmites (reed)
Phragmites australis (common reed)
Saccharum officinarum (sugarcane)
Saccharum spontaneum (wild sugarcane)
Scirpus maritimus (saltmarsh bulrush)
Setaria italica (foxtail millet)
Setaria pumila (yellow foxtail)
Sorghum bicolor (sorghum)
Sorghum halepense (Johnson grass)
Sorghum sudanense (Sudan grass)
Triticum (wheat)
Triticum aestivum (wheat)
Typha angustata
Zea mays (maize)
List of symptoms/signs
Inflorescence  -  wilt
Stems  -  dead heart
Stems  -  internal feeding
Stems  -  wilt
Whole plant  -  dead heart
Symptoms
Feeding occurs within the rice stem or base of the panicle. When a stem is severed it wilts causing a deadheart. Feeding at the base of the panicles often causes the panicle to be cut leading to a wilted panicle called a whitehead. Exit holes along the tillers may be visible at close inspection. These symptoms are common for most rice stem borers and not unique to S. inferens.
Prevention and control

Cultural Control

Cultural practices have a profound effect on S. inferens. Some methods are only effective if carried out through community-wide co-operation, others are effective on a single field (Litsinger, 1994). The community-wide practices act to prevent colonization and build-up, and have the greatest potential to minimize infestation. These include planting cultivars each season with the same maturity class and planting synchronously between fields. Rice cropping intensity is limited to two crops per year over the area and if irrigation allows a third crop it should be a non-host species such as field legume. Harvesting has a devastating effect on stem borers particularly if the straw is destroyed and the remaining stubble ploughed under.

Individual farmers can also adopt effective cultural control measures. Early planting within a contiguous area such as an irrigation turnout generally escapes damage (Shafique and Anwar, 1986). Increasing plant density ensures ample tiller densities to allow maximum compensation from damage (Litsinger, 1993). Using well timed and optimal levels of nitrogen and balanced fertilizer also aides a crop to compensate from damage even though it may increase the overall stem borer density. Good crop husbandry such as thorough land preparation, prompt weeding, and vigilant water management ensure vigorous crop growth and ability to tolerate stem borer damage. Early planting will usually cause the crop to escape the period of peak population build-up. The more farmers who plant early the greater the stem borer suppression. Optimally large contiguous areas would be planted within one generation of the stem borer (usually 5-6 weeks). Whatever the planting period, individual farmers who plant earlier than their neighbours usually will have fewer insect pest problems.

Early maturity is a mechanism to escape population build-up. Photoperiod sensitive cultivars often suffer heavy infestations from stem borers as they allow more generations to build up on the crop. In many locations where modern cultivars were introduced, two early maturing rice crops replaced single-crop, photoperiod-sensitive plant types. Each of the double crops normally suffers far less damage than the longer maturing single crop. This may be due to stem-borer mortality due to harvest occurring twice in the double cropping system.


Mechanical and Physical Control

The traditional and more labour intensive practices used against stem borers (Litsinger, 1994) are not particularly effective against S. inferens. Hand picking of egg masses is conceivable if labour is cheap, against Scirpophaga rice borers which lay egg masses in the open but is impractical against S. inferens which oviposits behind leaf sheaths. Roguing deadhearts is time consuming and often the larvae have already left the affected tiller.

Biological Control

Every life stage of S. inferens is vulnerable to attack by natural enemies. Once the eggs have been laid behind the leaf sheaths, the factors that will most reduce the stem borer population are natural enemies. Because of their fecundity, S. inferens populations will increase even if 90% of the population dies during the crop season. For a population to decline, more than 99% of the eggs laid must fail to reach the reproductive stage.

Classical biocontrol has been carried out in Taiwan and the Philippines with the introduction of the tachinid parasitoid Sturmiopsis inferens from India (Kamran and Raros, 1971). Even though the species became established, this method has not been highly successful as studies have shown that there are already many natural enemies of S. inferens in paddy fields and the effect of one more, even as effective as Sturmiopsis, will not be noticed against this indigenous pest. Conservation of natural enemies is the most practical tactic. For example, Sturmiopsis populations were hard to establish in areas where prophylactic insecticide applications occurred (Kamran and Raros, 1971).

Host-Plant Resistance

Breeding programs have screened for resistance to rice stem borers but have not focused on S. inferens. Accessions and cultivars have been screened in the field and a number of lines have been listed by Khan et al. (1991) as being resistant to S. inferens from the results of field evaluations (Pathak, 1971; Bhatt et al., 1984; Garg, 1984). The levels of resistance, however, are only quantitative and high levels of resistance have not been found. Plant type, however, can have a broad effect on all stem borers. Short-stature, high tillering, early-maturing (110-125 days) plant types offer the greatest reduction to population build-up and damage. Short-stature cultivars reduce the highly susceptible periods of tiller and panicle elongation. Deepwater rice cultivars which elongate several meters are therefore highly susceptible. First-instar larvae need to bore into tillers and panicles in order to survive and they have the greatest chance during elongation when silica, the plant's natural defence, is less densely packed. The longer it takes first-instar larvae to bore into a tiller the more chance natural enemies have in finding it. The larvae may also die of starvation if it cannot enter. First-instar larvae of S. inferens are a little larger than other stem borers and therefore take a shorter time to bore into a tiller. If one panicle is severed, the assimilate can be diverted to other panicles which still have unfilled grains.

Recent advances in biotechnology have raised hopes for the development of a highly resistant transgenic rice variety against stem borers. The incorporation of an insecticidal proteinase inhibitor gene into rice plants has been used for this purpose. The fifth generation of transgenic japonica rice plants containing the potato proteinase inhibitor II gene (pin2) showed increased resistance against S. inferens (Duan et al., 1996). Small scale field tests also showed that transgenic rice plants expressing the cowpea trypsin inhibitor (CPTi) gene had increased resistance against S. inferens and Chilo suppressalis (Xu et al., 1996).

Chemical Control

Stem borers are difficult to control with insecticides. After hatching, the larvae are only exposed for a few hours before penetrating a tiller. Successful control involves repeated foliar applications with spray volumes of more than 400 l/ha. In temperate climates, stem-borer populations are more synchronized and well-timed applications have a greater degree of control than in the tropics where generations overlap. The decline in stem-borer abundance in Japan and Korea is attributed to the frequent use of insecticides over many years, even though the stem borers have developed insecticide resistance. Foliar sprays act on the larvae, eggs and adults, but sprays also come into greater contact with natural enemies. Cases of stem-borer resurgence are not evident, although secondary pest outbreaks have been reported in areas where heavy insecticide usage is directed against stem borers. Granular formulations give higher control than foliar sprays or dusts, particularly in high rainfall environments. Granules broadcast into the paddy water are particularly effective in preventing deadhearts in a young crop. Insecticide is partly dissolved in the paddy water and moves by capillary action between the leaf sheath and stem to contact with young larvae; the non systemic insecticide granules act as though they were systemic (Sethunathan et al., 1971). The limitation to using granules is cost; they are more expensive to transport. Stable water supply and deep water levels also are necessary for high levels of control (Bandong and Litsinger, 1979). As the water level falls, the capillary activity progressively declines. If the field dries out, insecticide efficacy ceases. Flooding from heavy rains also washes granules away. Systemic granules have an advantage in that the chemical can enter the plant even with low water levels. The dissolved chemical percolates into the soil and is taken up by the roots. From the roots, the chemical is transmitted through the xylem tissue into the stems and eventually to the tips of the leaves. If systemic granules are broadcast into the paddy water, high dosages are needed because much of the chemical is absorbed into the soil. The dosage needed is also higher as plant biomass increases. If systemic granules are broadcast during the last harrowing or levelling operation before planting, dosages can be cut in half (dela Cruz et al., 1981). Efficacy lasts more than a month because the granule is protected from rapid degradation by hydrolysis or bacteria.

Economic thresholds based on deadhearts or whiteheads use characters that are too late to predict timely corrective action. Egg masses are a more reasonable character and field surveillance should be intensified for several weeks before each of the two periods of highest crop vulnerability (stem elongation that leads to deadhearts and panicle extension that leads to whiteheads). Threshold values should be locally determined but would range around 1-2 egg masses per m². As egg masses of pink stem borer are difficult to find and as S. inferens most commonly occurs with other more abundant stem borers the threshold can be determined from combining egg masses from all species. Those from Scirpophaga are usually more abundant and easier to locate. But egg masses from all species could be collected and held in jars to determine the extent of parasitization. If the parasitization is high then there is no need to apply a corrective insecticide.

Pheromonal Control

The sex attractant pheromone produced by the female has been identified (Nesbitt et al. 1976; Zhu et al., 1987) but has not been used as a control method.

IPM

Management of S. inferens and other rice stem borers should first address preventative measures. These include a combination of cultural practices and selection of a tolerant plant type which will have the greatest effect in bolstering the crop's ability to tolerate normal levels of damage. A vigorously growing rice crop planted to a high tillering variety has been shown to tolerate up to 20% deadhearts and 10% whiteheads with no yield loss (Litsinger, 1993). Stressing non-chemical preventative measures also ensures conservation of natural enemies which will further protect the crop. Chemical control is costly and disruptive to parasitoids and predators and should be contemplated as a last resort.

Phytosanitary Measures

Geographic areas that need to be particularly vigilant against the introduction of S. inferens are Australia and Oceania. The greatest risk of entry would be through importation of stalks of rice, sugarcane, maize or other cereal crops, grass or sedge that are infested with larvae or pupae.

Impact
S. inferens is probably the least destructive borer pest among the rice stem borers; this may be due to its extreme polyphagy. Outbreaks in rice usually result from a population overspill from adjacent sugarcane fields or other alternative hosts (Dale, 1994). Its incidence in rice seems to have decreased with the intensification of rice cultivation during the past 30-40 years (Islam, 1991). S. inferens is generally more prevalent in areas where its alternative hosts are cropped such as surrounding sugarcane or maize or in rotation with wheat (Litsinger and Barrion, 1988). It also generally attacks the rice crop during its intermediate or late growth phases.

Stem borers are ubiquitous in rice fields. S. inferens is usually most often found in association with other stem borers attacking a rice crop. It belongs to a different family than other pyralid stem borers and is larger in size. However, modern rices are thinner stemmed and are preferred by Sciripophaga species. Wider stemmed rices favour Chilo species and S. inferens. All the stem borer species attacking a crop must be considered together and their damage as additive, as there is little or no difference in damage potential between species. Whiteheads are a greater contributor than deadhearts to yield loss. Hybrid or other high-tillering rices can tolerate high infestation rates if the crop is well managed and there is a lack of other stresses (Litsinger, 1993). The tunnelling caused by the larvae severs the vascular tissue but there are many conduits in a stem and reduction in nutrient and assimilate flow can normally be shunted to undamaged vascular bundles unless the stem is completely severed. Stem borer tunnelling has a greater impact on yield loss than defoliation and damage can act synergistically with other stresses. Therefore the significance of stem borer damage must be viewed in relation to other crop stresses from all sources. As a general rule the greater the number of stresses the less the crop can compensate from injury.
Related treatment support
Plantwise Factsheets for Farmers
Choudhury, R.; Mallik, A. H.; Karanja, L.; CABI, 2011, English language
CABI; CABI, Swahili language
Thailand, Bureau of Rice Research and Development; CABI, Thai language
Shrestha, T. K.; CABI, 2012, English language
Kafle, K. R.; CABI, 2013, English language
 
Pest Management Decision Guides
Thailand, Bureau of Rice Research and Development, Rice Department; CABI, 2013, English language
Bhandari, N. R.; Thapa, R. B.; CABI, 2015, English language
 
External factsheets
TNAU Agritech Portal Crop Protection Factsheets, Tamil Nadu Agricultural University, English language
TNAU Agritech Portal Crop Protection Factsheets, Tamil Nadu Agricultural University, Tamil language
TNAU Agritech Portal Crop Protection Factsheets, Tamil Nadu Agricultural University, English language
TNAU Agritech Portal Crop Protection Factsheets, Tamil Nadu Agricultural University, Tamil language
IRRI Rice Factsheets: Crop Health, International Rice Research Institute (IRRI), English language
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