Four moth species
Why is it a problem? There are at least four species of stem-boring moths affecting rice in Bhutan. They kill the central shoot at the vegetative stage and can result in seed loss at later stages. Rice plants can compensate for early damage, and management is rarely needed.
Where and when is it a problem? They occur in all rice growing areas, and can locally reach relatively high densities in some years.
Spotted stem borer (Chilo partellus)
- Adult moths are relatively small (wingspan of 20-25 mm). Forewings are brown-yellowish with darker scale patterns forming longitudinal stripes. In males, hind wings are a pale straw-colour, and in females, they are white.
- Larvae are creamy-white to yellowish-brown in colour, with four purple-brown longitudinal stripes. They usually have very conspicuous dark-brown spots along the back, which give the larvae a spotted appearance. When fully grown they have a prominent reddish-brown head.
- Pupae are up to 15 mm long, slender, shiny and light yellow-brown to dark red-brown in colour.
- Eggs are flat and oval (scale-like), creamy-white, about 0.8mm long. They are laid in overlapping batches of 10-80 eggs on the upper and underside leaf surfaces, mainly near the midribs.
Yellow stem borer (Scirpophaga incertulas)
- Adults differ in colour between the sexes. Males are light brown with numerous small brownish dots, along the sub-terminal area and near the tip of the forewing. The female is yellow, the colour deepening toward the tip, with a very distinct black spot in the centre of each forewing.
- Larvae have a cream body and reddish brown head.
- Eggs are laid near the tip of the leaf blade and are covered in pale orange brown hairs.
White stem borer (Scirpophaga innotata)
- The adult is white and slender and resembles the yellow stem borer, but it does not have a black spot on the forewing. The abdominal tip of the female is white to pinkish white.
- Larvae are similar to yellow borer larvae except that they are white.
- Eggs resemble that of the yellow stem borer and are also laid near the tip of the leaf blade.
Asiatic pink stem borer (Sesamia inferens)
- Adults are light brown-colored with dark brown markings. From a central point in the forewing, a typical radiation of greyish black spreads toward the wing tips, ending in a thin terminal line of dark spots.
- Larvae have an orange-red head. The body is purplish-pink dorsally and white ventrally.
- Eggs are bead-like and laid in rows.
At the vegetative stage of the crop the central shoot dries up and dies off, turning yellow (dead heart). At the ear-bearing stage the ear-head appears completely chaffy and white in colour (white ear-head). Damage to the lower area due to larval feeding leads to drying of the panicle and no grains are formed. Panicles are empty and appear whitish. White heads stands out when looking at a rice field, particularly during grain filling when undamaged panicles are still green. Young rice plants are normally preferred.
Confusion with other pests: There are no other stem-borers in rice.
Lifecycle: Larvae bore into the stems of their host. Multiple generations a year are possible, depending on the location. They overwinter as either larvae or pupae, depending on the species.
Dispersal: Adults fly.
When can damage be expected? Serious outbreaks are rarely observed in Bhutan.
Hosts: Some of the same stem borers have also been reported from barley and maize. However, only the Asiatic pink stem borer (Sesamia inferens) has been reported as potentially causing minor losses in maize. At least some of the species probably also utilise other grass hosts.
Stem-borers rarely occur at damaging levels in rice, and plants can often compensate for early damage. In areas where there may be chronic problems populations should be able to be suppressed through cultural control.
- Look for whiteheads, and rogue out when seen. Confirm that stem-borers are the cause by looking for borer-damage and the presence of larvae. Eggs can also sometimes be seen on the leaves.
- Direct management is only needed if more than 10% of tillers have dead hearts (or there are 20 egg masses/25 plants) at the vegetative stage or 5% white heads at the reproductive stage. Estimates can be made by examining 25 plants.
Effect of variety
Rice varieties with short stature and shorter growth duration appear to suffer less damage. It will also be harder for insects to complete their lifecycle.
- Do not over apply Nitrogen fertilizer as plants will grow too fast and become too soft, making them more easily bored by larvae. Splitting the fertilizer application through the season can help.
- Clip the tip of the leaf blade when seedlings are transplanted, especially for borers that lay eggs on the tip of the leaf. This can be effective when older seedlings are transplanted.
- Harvest at ground level to prevent larvae from completing their lifecycle within the stems.
- Destroy stubble before pupation, for example by ploughing it into the field or flooding. Stubble is a major source of overwintering populations as larvae move from the stems into the soil to pupate.
- Rogue out whiteheads when seen in the field.
- Crop rotation with non-graminaceous crops may reduce borer populations.
- Predators should be encouraged. Three egg parasitoids are used as biological control agents, but they are not available in Bhutan.
The use of pesticides would rarely, if ever, be justified. A systemic pesticide (dimethoate at 2 ml per litre of water) is necessary to reach the stem-boring larvae.
Image acknowledgements: Rice knowledge bank,IRRI