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Plantwise Technical Factsheet

rice green semilooper (Naranga diffusa)

Host plants / species affected
Echinochloa colona (junglerice)
Echinochloa crus-galli (barnyard grass)
Eleusine indica (goose grass)
Leersia hexandra (southern cut grass)
Leptochloa chinensis (Chinese sprangletop)
Oryza sativa (rice)
Paspalum conjugatum (sour paspalum)
Paspalum distichum (knotgrass)
Sorghum bicolor (sorghum)
List of symptoms/signs
Leaves  -  external feeding
Symptoms
The defoliation caused by N. diffusa is similar to that caused by other early season leaf-feeding larvae such as Rivula atimeta. First- and second-instar larvae scrape leaf tissue from the leaf blades, causing them to appear whitish when viewed from a distance.
Prevention and control

Cultural Control

Cultural methods of controlling N. diffusa have been reported by Okamoto and Abe (1960), Koyama et al., (1968), Loevinsohn (1984) and Litsinger (1994). The best insurance measure against losses caused by green semiloopers and other defoliating pests is for the farmer to practice good crop husbandry by ensuring the crop is well weeded, properly fertilized and adequately irrigated. Planting closely or at high seeding rates gives the crop a better opportunity to tolerate defoliation (Litsinger, 1993). Populations of green semiloopers tend to build up in areas of staggered planting (Loevinsohn, 1984), thus early planting is a good strategy to escape damage from N. diffusa and most other pests. It is best to time crop establishment within the first several weeks of the planting season. Ichimaru and Suenaga (1955) reported greater green semilooper numbers in later plantings than in earlier ones.

Mechanical and Physical Control

Litsinger (1994) reported several mechanical and physical methods of control which can be used in areas where field labour is inexpensive. Farmers can handpick pupae, net larvae from seedbeds, and use light traps to attract and kill moths.

Biological Control

Natural enemies normally reduce green semilooper numbers to insignificant levels. However, as semiloopers rapidly colonize a young crop they often build up more rapidly than the beneficial arthropods during the first 3 weeks. The crop can tolerate moderate levels of defoliation more easily at this stage. However, natural enemy build-up can be set back by the use of broad spectrum insecticides giving advantage to N. diffusa (van den Berg et al., 1988).

Conservation of natural enemies is very importance in the early growth stages. Thus, if insecticides must be used, the choice of chemical must focus on those which least upset the natural enemy balance.

Studies have shown that green semiloopers are highly susceptible to most strains of Bacillus thuringiensis (Tryon and Litsinger, 1988) and green semiloopers should be suppressed if rice varieties are genetically engineered with Bt genes.

Host-plant Resistance

Preliminary work at the International Rice Research Institute (IRRI), where several hundred rice accessions from the world collection have been screened, has not reveal any resistant lines (Ma and Heinrichs, IRRI, Los Banos, Phillipines, personal communication, 1983); therefore, the probability of finding resistance is low. Work should focus on identifying tolerant accessions, as studies have shown that high tillering and rapidly growing rices can tolerate high levels of defoliation. When appropriate, farmers should chose high-tillering over low-tillering plant types.


Chemical Control

Green semiloopers are highly susceptible to insecticides. Populations in Japan declined to very low levels nationwide under heavy insecticide usage in rice (Miyahara, 1972). If the use of insecticides is necessary, then those products which least disrupt natural enemies (such as microbial insecticides e.g. Bacillus thuringiensis) should be used. Botanical insecticides such as neem were found to be effective in China (Chen, 1937). Selective insecticides such as BPMC, etofenprox and MIPC have also been used.


Pheromonal Control

A chemical sex attractant has been identified from green semilooper females to attract males (Tamaki, 1985) but no attempts have been made to control it using confusion methods. Green semiloopers are a minor pest and control with pheromones would only be justified in areas with perennial problems.

Integrated Pest Management

The best strategy to minimize the impact of green semiloopers is to ensure that the crop is well managed agronomically and monitored for the presence of pests on a weekly basis. An early warning is the presence of moths in the field, but one should not attempt control methods on their presence alone as natural enemies, particularly egg predators, often make control unnecessary. Defoliation from green semiloopers should be assessed in terms of other stresses affecting the crop. If several stresses occur at the same time, corrective actions need not be taken against all of them and the farmer has the option of targeting the easiest and least costly to control. Semiloopers would be a first choice as they are relatively simple to control (easier than for example, stem borers or whorl maggots, which often occur together) with an application of a selective insecticide such as Bacillus thuringiensis. Forecasting has been attempted in temperate regions where semilooper numbers are associated with those of the striped stem borer, Chilo suppressalis (Eiri and Koyama, 1958).

Impact
Crop loss due to N. diffusa infestation was studied by Ishikura (1955), Miyashita (1963) and Litsinger et al. (1987). Frequent outbreaks have been recorded in Japan, even before 1900, and are associated with overcast skies, low temperatures and increased use of nitrogen fertilizer (Miyashita, 1963). Outbreaks tend to occur in the same generation in a given location. Damage is restricted to the seedbed and vegetative stage. Fortunately, a vigorously growing rice crop can tolerate high levels of defoliation (25-50% loss of leaf area). Defoliation of a young crop from other pests (caseworm, armyworm, green hairy caterpillar) is additive as several defoliators often damage the crop at the same time. The degree of tolerance to damage declines with crop age. Defoliation generally occurs in patches in the field. Defoliation from green semiloopers and other pests that attack the vegetative stage are of greater significance in terms of yield loss if the crop is also affected by other insect pests, diseases, weeds or soil problems, as many of these stresses act synergistically to increase crop loss (Litsinger, 1993). The growth of the crop and other stresses should be considered when assessing the potential significance of defoliation during the vegetative phase; the potential for yield loss is greater with the number, and severity, of stresses affecting the crop.

Economic thresholds, based on levels of defoliation and confirmed by the presence of N. diffusa larvae, have been developed by Bandong and Litsinger (1988). These levels are 0.5-1 larvae/hill or 10-15% of leaves showing damage and larvae present. However, thresholds should be determined locally through experience.
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