Rice stem borers
Stem borers are the serious pest of rice, maize and wheat. These are chronic insect pests which are present in almost all rice fields. Stem borers cause damage mainly in warmer area in large scale. Larval feeding at vegetative stage of plant cause the death of central whorl and symptom is known as “Dead heart”, damage at reproductive stage of plant cause the death of emerging panicle known as “white head”. Rice plant compensates the dead heart damage by production of additional tillers but suffers yield losses due to compensatory tillers bearing smaller panicles.
a) Yellow stem borer, Scirpophaga incertulas; Lepidoptera: Pyralidae
Yellow stem borers are specific pest of rice. Adults are yellowish white moth with orange yellow forewing. Body length is 13-16mm with wing expansion of 25-45mm. Females have one dark spot in the center of yellow forewings whereas male has series of small dark spots on brown forewings.
Caterpillars of yellow stem borer are destructive in nature causing direct damage to plant. Eggs are laid on the tip of the leaves. After hatching caterpillar enter into the rice stem after wandering 1-2 hours on rice plant. It feeds on soft part of stem for about a week and hollows out stem completely. Damage at vegetative stage causes death of central whorl and symptom is known as “Dead heart”, while damage at reproductive stage of plant cause the death of emerging panicle known as “White head”. This could be easily pulled out. Peak infestation occurs in October where deep water rice affected more as compared to upland rice.
Eggs of yellow and striped borers are usually laid on the upper half of a leaf, those of the pink borer and dark headed borer on the inner side of the leaf sheath. The larvae of yellow borer are yellowish white with brown head whereas stripes on the body and dark head in striped borer while in pink borer it is pinkish. Larvae and pupae are usually found in the leaf sheaths on culm at any height from below ground to the panicles.
Eggs 5-7 days Larva 3-6 weeks Pupa 8-14 days Adult
Eggs are laid on tip margin leaf part in dorsal side. The egg mass is covered by brownish hairs. The pupation takes place inside the stem by forming silken cocoon. In the harvested paddy straw, it hides inside the ratoon tillers from where adults are emerge out.
b) Pink borer (Sesamia inferens; Lepidoptera: Noctuidae
Host: rice, maize
Pink borer is polyphagous insect. It is 20-26mm in body length with forewing brown in color. Three black distal dots and intermediate brown stripe is present on forewing. The caterpillars are pinkish brown dorsally and white ventrally.
Eggs 1week Larva 25-54 days Pupa 8-36 days Adult
Damage: Caterpillars alone are destructive causing direct damage on plant. They feed within the plant stem. Damage at vegetative stage is called “dead heart” killing central shoots.
1. FIELD SANITATION:
- Field should be neat and clean.
- Remove the plant having symptoms of dead heart, destroy egg masses and remove infested seedlings from nursery to prevent the population buildup of rice borers in further generation.
2. CULTURAL PRACTICES:
- Collect and destroy all the rice stubbles at the time of first plowing after harvesting to prevent incidence of insects to the next year.
- Dip all rice stubbles after harvest in the field stagnating water for 5-7 days to kill population of insects in a particular crop field.
- Increase seed rate of maize by 50% for sowing in the borer endemic field to compensate plant population after borer infestation.
- Napier grass is an effective catch crop for controlling stem borers. When Napier or elephant grass is planted round maize or sorghum fields, the stem borers are attracted to the grass and damage to the crop is minimized. The Napier however can still be cut and fed to livestock. Plant a row of Napier grass all the way round maize fields.
- Generally early planting of maize and rice escape from borer infestation.
- Sudan grass, also a commercial fodder grasses, can provide natural control to stemborers by acting as trap plants for stem borers, and as reservoirs for their natural enemies. Planting Sudan grass around maize field reduced stemborer infestation on maize and also increased efficiency of natural enemies.
- Silverlea (Desmodium uncinatum), a high-value, commercial fodder legume, when intercropped with maize, repelled ovipositing gravid stemborer females, and suppressed striga by more than 40 times
- Molasses grass, when intercropped with maize, not only reduces the infestation of crops by stemborers, but also increased parasitism particularly by the native larval parasitoid, Cotesia sesamae.
3. USE OF RESISTNT VARITIES
- Maize varieties such as Rampur-1, Arun-1, and Rampur composite are moderately resistant to borers (NARC 1999).
- Grow rice varieties such as IR-20, Taichung-176, chinung-142, Palung, Khumal-4, Chandina which are moderately resistant to stem borer complex.
- Rice varieties having tight and extensive leaf sheath are not favored for oviposition by borers.
- Clip tips (2.5-3 cm) of the seedlings just before transplanting to prevent incidence of the borer to the main field since the moths lay eggs near the tip of the leafs (particularly in nursery).
- Synchronize planting time over a large area to prevent congregation of large
4. BIOLOGICAL CONTROL METHOD
- Spray Bacillus thuringiensis (Bt) based formulations such as DIPEL or BIOLEP @ 3 g per litre water during evening hours, repeat application at 10 days interval (NARC, 1999).
- Trichogramma can be very effective and economical control as it can eliminate the pest in the egg stage (Bournier, 1982; Somchoudhury and Dutt, 1988).
- Release Trichodergma chilonis for 6 times @ 50,000 – 100,000 parasitized eggs per week soon after appearance of moths in the crop field (NARC, 1999). Try to create suitable environment for biological control by encouraging natural enemies through appropriate technique. Conserve spiders, water bugs, mirids, damselflies, dragonflies, trichogramma, bracon etc.
5. CHEMICAL CONTROL:
Spray pesticides like Dursban 20% EC (chlorpyrifos) @ 2 ml per liter water three times at 30, 50 and 75 days after transplanting if more than 10% dead hearts appear in the fields or apply Carbofuran or Phorate @ 1 – 2 kg a.i per hectare in the rice field. Before application of chemical insecticides in the soil, repair all the bund or ridges of the field and then drain water from the field while after application pesticides in the soil, do not allow to drain water through the field for 4 – 5 days. While in maize, apply 1 – 2 granules of above mentioned insecticides, in the plant whorl at 4 – 5 leaves stage of plant (Neupane, 2001).