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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.
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.
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).
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).
Due to the variable regulations around (de-)registration of pesticides, we are for the moment not including any specific chemical control recommendations. For further information, we recommend you visit the following resources:
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.