STEM BORERS OF RICE

 

 

 

slide1 

This is a rice production training module produced by the International Rice Research Institute

 

 

 

slide 2

This lesson is on rice stem borers.

 

 

 

 

 

At the end of the lesson, you will be able to:

slide 3

  • describe and identify the damages to rice caused by stem borers;

  • identify and describe the 5 most destructive stem borer species of rice in Asia. Relate these species with geographic distribution.

  • identify and describe the 4 stages in the life cycle of the major stem borers including their appearance, habitat, behavior, mode of feeding, and duration of each life cycle, and

  • describe the most effective methods of controlling the major stem borers of rice.

 

 

 

 

 

slide 4
Rice stem borers are serious pests of rice.   They infest plants from the seedling stage to maturity.  Although worldwide in distribution, rice stem borers are particularly destructive in Asia, the Middle East, and the Mediterranean regions.

 

 

 

slide 5

STEM BORER DAMAGE
Symptoms of stem borer damage are deadhearts and whiteheads.  These visible symptoms on affected plants vary with the growth stage at which plant infestation began.

 

 

 

 

 

 

slide 6
Rice plants from the seedling to maximum tillering stages that have been attacked at the base of the stem have deadhearts.  Deadhearts are the dried up central shoots of tillers on the rice plant.

 

 

 

 

slide 7
Plants that have started bearing panicles when they were attacked usually produce whiteheads.  Whiteheads are discolored panicles with empty or partially filled grains.

 

 

 

slide 8
Although stem borer damage becomes evident only as deadhearts and whiteheads, the larvae can inflict 3 other forms of damage.  These forms of damage are indicated by: transparent or yellowing leaf sheaths, presence of entrance or exit holes on the stem, and disintegrated tissues or broken stems.

 

 

 

 

slide 9
Stem borer larvae may feed within the stem without severing the growing plant parts at the base.  This can result in reduced plant vigor, and many unfilled grains.

 

 

 

 

 

slide 10
Stem borer presence can also be learned by the presence of adult moths and egg masses.

 

 

 

 

 

Let us review by answering the following questions:

slide 11

  • Identify and describe the 2 primary plant symptoms of stem borer damage.

  • What 3 other forms of damage can stem borers inflict on rice plants?

  • Explain the 2 damages inflicted by larval feeding when plant parts are not severed at the base.

 

 

 

slide 12

LIFE CYCLE OF STEM BORERS
The life cycle of stem borers consists of 4 stages:  the egg, the larva, the pupa, and the adult

 

.

 

 

 

slide 13

All rice stem borers lay eggs in masses usually containing 50-250 eggs.  A single female can lay several egg masses per week.

 

 

 

 

 

slide 14
Eggs are laid on the leaf blade or between the leaf sheath and the stem.  Most stem borer species lay eggs or oviposit in the evening.

 

 

 

 

slide 15
Newly hatched larvae enter leaf sheaths and begin feeding on the inner tissues.  Damaged leaf sheaths that have been attacked have transparent patches that later turn yellowish-brown and eventually become dry.

 

 

 

slide 16

After a few days, the larvae leave the sheaths and bore into the stem.

 

 

 

 

slide 17
Larvae feed on the tissues around the node.  As a result, the infested stems are weakened and easily break.  Occasionally, stem borers eat through the node and bore down the stem.  Others leave the stem above the node and enter either another tiller or a neighboring internode of the same stem.

 

 

 

slide 18
As they grow, the larvae of rice stem borer molt (cast off their old exoskeleton) five times before pupating.  The full-grown larvae, before pupating, cut exit holes in the internodes through which the emerging moths later escape.  Usually the opening of these exit holes are plugged with fine white web and cannot be easily detected before the moths have escaped.

 

 

 

slide 19

Rice stem borers usually pupate in the stem straw or stubble.  Sometimes the pink stem borer pupates between the leaf sheath and stem.

 

 

 

slide 20
Once pupation is completed, an adult emerges.  Adult stem borers are referred to as moths. The moths live for about one week, during which they disperse and reproduce.

 

 

 

slide 21
The adults of most stem borers species are nocturnal and are attracted to light, especially to ultraviolet lights.  They also are strong fliers.  Most moths can fly from 5 - 10 miles but can cover longer distances if carried by winds.

 

 

 

slide 22
Stem borers generally produce about 6 generations a year, but the number of generations in a year depends on environmental factors, primarily temperature and crop availability.

 

 

 

slide 23
In temperate areas such as central Japan and Korea, where only one rice crop per year is grown, there are two stem borer generations per year.  The stem borers spend the winter in diapause.

 

 

 

slide 24
In tropical and subtropical regions with single rice cropping regimes, they occur in 3-4 generations per year.  The moths belonging to each generation are often referred to as broods.

 

 

 

slide 25
During periods when there is no rice crop and temperatures are not optimum for larval development, the full-grown larvae enter diapause.

 

 

 

 

slide 26
During diapause, the larvae in the stubble move down in the plant base and may stay 1-2 inches below the ground level.

 

 

 

slide 27
But where two or more rice crops are grown in a year, the stem borers remain active year-round.  They undergo only a temporary period of inactivity in the last larval instar, during brief periods when host plants are not available.  This is apparently true for most of the tropical rice areas where moths are caught in light traps throughout the year.

 

 

 

 

slide 28
Before we continue our discussion on stem borers, let’s pause and review.

  • What are the 4 stages in the life cycle of a stem borer?

  • Describe the sequence of stem borer larval feeding on rice plants.

 

 

slide 29

  • Describe the 3 characteristics of the adult stem borer.

  • Compare the activity of rice stem borers in single-cropped and multi-cropped tropical areas.

 

 

 

slide 30

STEM BORER SPECIES
In Asia, the 5 most destructive stem borers are:  yellow stem borer, Scirpophaga incertulas (Walker); white stem borer, Scirpophaga innotata (Walker); striped stem borer, Chilo suppressalis (Walker); dark-headed stem borer, Chilo polychysus (Meyrick), and pink stem borer, Sesamia inferens (Walker).

 

 

 

 

 

 

slide 31
The African white stem borer, Maliarpha separatella (Ragonot), is the most common stem borer attacking rice in East and West Africa.  The purple-lined stem borer, Chilo agamemnon (Bleszynski), is the most important stem borer in North Africa and the Middle East.

 

 

 

 

 

slide 32
In Central America and South America, the South American white stem borer, Rupela albinella (Cramer) is a serious pest of rice.

 

 

 

 

slide 33

YELLOW STEM BORER
The yellow stem borer Scirpophaga incertulas is common in Southeast and East Asia, China, the Indian continent, and Afghanistan.  It attacks rice throughout its growth period.

 

 

slide 34

The eggs of the yellow stem borers are laid near the tip of the leaf blade.

 

 

 

 

slide 35
The eggs, which hatch in 8 days, are laid in oval masses of 50-200 eggs each.  They are covered with hairs from the anal tufts of the female moth.

 

 

 

 

slide 36

The body of the larva is cream colored and the head  is reddish brown.

 

 

 

 

slide 37
The pupa is yellowish white with a tinge of green.  It turns dark brown just before adult emergence.  The pupal stage takes place inside the stem, and often below the soil surface.

 

 

 

 

slide 38
The adults show sexual dimorphism, or differences in form or color between the sexes.  The males and females are often mistaken as 2 species.  The male is light brown with numerous small brownish dots, along the subterminal area and near the tip of the forewing.  The female is yellow, the color deepening toward the tip, and there is a very distinct black spot in the center of each forewing.  The hindwings are pale and straw-colored.

 

 

 

 

slide 39
The yellow stem borer requires about 46 days to complete development.  Adults can emerge through the water.

 

 

 

 

slide 40

WHITE STEM BORER
The white stem borer Scirpophaga innotata  is found in Indonesia, in the Visayas and Mindanao of the Philippines, Malaysia, and Australia.

 

 

 

 

 

slide 41
The eggs are laid in batches of about 100 and are covered with silky greyish hairs; they resemble the eggs of the yellow stem borer.  The incubation period is about 8 days.  The female white stem borer lays the eggs near the tip of the leaf blade.

 

 

 

 

slide 42
The white stem borer larvae are similar to yellow borer larvae except that they are white.  They reach full growth in about 30 days.

 

 

 

 

slide 43

The pupa is soft-bodied and white.  Pupation is completed in about 8 days.

 

 

 

 

slide 44
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.

 

 

 

 

 

slide 45

The white stem borer requires  about 46 days to complete development.

 

 

 

 

slide 46

STRIPED STEM BORER
Another important stem borer species in Asia is the striped stem borer, Chilo suppressalis.  It is found in the Indian sub-continent, South and East Asia, China, Iran, and Southern Europe.

 

 

 

 

 

slide 47
The adult lays its eggs on the basal half of the leaves.  They resemble those of the dark-headed stem borer but are more yellowish, while those of the dark-headed stem borer are whitish.  The eggs hatch in about 5 days.

 

 

 

 

slide 48
The larva is fully grown in about 25 days. The mature larva has a yellowish brown head and  3 dorsal and 2 lateral brownish abdominal stripes.

 

 

 

 

slide 49

Moths emerge after about 5 days of pupation.

 

 

 

 

slide 50
The moth is straw-colored to light brown with silvery scales and several black dots at the tip of the forewing.  The hindwing is yellowish-white. 

 

 

 

 

slide 51
The life cycle of the striped stem borer requires about 35 days to complete.  In cooler regions, 1 or 2 generations are produced per year; fully grown larvae diapause or lie dormant during the cool season.  In the tropics, 6 or more generations are possible per year.

 

 

 

 

 

slide 52

DARK-HEADED STEM BORER
The dark-headed borer, Chilo polychrysus, is an important pest in Malaysia and Thailand.  It is found throughout Southeast Asia, and in China and India.

 

 

 

 

 

 

slide 53
The eggs of the dark-headed stem borer are laid at the basal half of the leaves or, occasionally, in the leaf sheaths.  The scale-like eggs, arranged in rows, are laid in batches of 20 to 150.  They hatch in about 5 days.

 

 

 

 

slide 54
The newly hatched larva feeds actively and reaches full size in about 25 days.  The mature larva has a black head and thoracic plate.  The dark-headed borer larva has 3 dorsal and 2 lateral brownish stripes distinct on its abdomen.

 

 

 

 

slide 55
The pupa is yellowish-brown with 2 distinct protrusions at the front of the head.  The adults emerge in about 5 days.

 

 

 

 

 

slide 56
The adult moth is light yellow with scattered brownish scales and several tiny black dots near the tip of the forewings.  The hindwings are yellowish-white. 

 

 

 

 

 

slide 57

The life cycle of dark-headed borer requires about 35 days to complete.

 

 

 

 

slide 58

PINK STEM BORER
The pink stem borer, Sesamia inferens is found in the Indian subcontinent, China, and Southeast and East Asia.  It feeds on rice, sugarcane, maize, wheat, barley, and grasses.

 

 

 

slide 59
The adult pink stem borer deposits its eggs between the leaf sheath and the stem.  Of all the species, pink stem borer eggs are probably the most effectively protected.  The beadlike eggs are laid in rows, usually 30-100 per batch.  The incubation period is about 8 days.

 

 

 

 

slide 60
The larva has an orange-red head.  Its body is purplish-pink dorsally and white ventrally.  The larva is fully grown in about 22 days.

 

 

 

slide 61
The pupa is dark brown with a tinge of purple in the head region.  Pupation is completed in about 8 days.

 

 

 

slide 62
The adult is 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.  The hindwings are white.

 

 

 

slide 63

The pink stem borer requires about 38 days to complete development.

 

 

 

 

slide 64
Let us review the lesson we have covered so far.

  • What are the 5 most destructive stem borers in Asia?  Give their common and scientific names.

  • Describe the egg-laying patterns of the 5 important stem borer species in Asia. 

  • Identify and describe their larvae and adults.

  • Which species exhibits sexual dimorphism?

 

 

 

slide 65

METHODS OF CONTROLLING STEM BORERS
Let us now discuss the various methods of controlling stem borers.  The approach to stem borer control should be integrated, where varietal resistance, cultural practices, insecticides, and biological control are used compatibly.

 

 

 

 

 

slide 66
Varieties differ in their resistance to the various stem borer species.  Improved semi-dwarf varieties are generally more resistant to stem borers than are tall traditional varieties.  There are also differences in resistance between improved semi-dwarf varieties, but careful screening methods are needed to reliably detect these differences.  Varietal resistance at IRRI and other research institutes has focused on the striped and yellow stem borers.  Among varieties released by IRRI, IR40 and IR72 are relatively resistant, while IR62 is relatively susceptible.

 

 

 

slide 67
Some morphological features that confer resistance to stem borers are short, narrow stems; more layers of lignified tissue in stems and leaf sheaths, and a large number of silica cells.  Improved semi-dwarf varieties are high tillering and can compensate for stem borer damage at the vegetative stage by producing new tillers.

 

 

 

slide 68
Resistance to stem borers can be greatly increased by genetic engineering with toxin genes from the bacterium Bacillus thuringiensis (Bt).  Transgenic rice plants with Bt genes are under evaluation in several countries but are not yet available to farmers. For more information on Bt rice, consult the IRRI publication on “BtRice: Research and Policy Issues”, IRRI Information series number 5.

 

 

slide 69
Let us proceed to the second method of controlling stem borers: cultural practices.
Four cultural practices can be used to control stem borers:
1.    Harvesting at ground level removes most larvae of all species.

 

 

 

 

slide 70
2.   The combination of plowing and flooding is the most effective way to destroy the stem borers remaining in the stubble.  Most of the stem borer population remains in stubble between crops.

 

 

 

 

 

slide 71
3.   Burning is only partially effective because after harvest the larvae may move below ground level.  Burning stubble uniformly in the field is difficult.

 

 

 

 

slide 72
4.  Stem borer damage can be highly affected by planting date.  In tropical areas, planting later than the neighboring farms can result in higher stem borer damage.  In Japan, delaying seeding or transplanting can evade the first generation moths of the yellow stem borer.

 

 

 

 

slide 73
The use of insecticides is another method that can be used against stem borers.   Insecticides such as fipronil applied as granules to the paddy water or soil can help to protect plants at vegetative stage, but granular insecticides are generally not effective at the reproductive stage.  Foliar sprays are not very effective against stem borers because the larvae spend almost all their development inside the plants.  Government regulations on insecticides and rates of use should be followed.

 

 

 

slide 74

Economic thresholds are sometimes used to determine when insecticide sprays are necessary, but thresholds are difficult to determine for stem borers.  When damage becomes visible, it is usually too late to benefit from applying insecticides.  In addition, it is usually not possible to predict the level of damage at reproductive stage based on the level of damage at vegetative stage.

At vegetative stage, high tillering varieties (with 10 or more tillers per hill) can compensate for deadhearts if there is adequate fertilizer and water. 

 

 

slide 75
Biological control is a very important factor in stem borer management.  There are many natural enemies of stem borers that occur naturally in rice fields.  These include egg predators such as crickets, predators of adults such as bats and spiders, parasitic wasps that attack eggs and larvae, and pathogenic viruses and fungi that attack larvae.  Insecticides use should be kept to a minimum, to protect natural enemies.

 

 

 

 

slide 76
Farmers and pest managers should keep in mind that the percentage of panicles that are whiteheads is often over estimated.  Whiteheads “stand out” when looking at a rice field, particularly during grain filling when undamaged panicles are still green.  It is often a useful exercise to calculate the actual percentage of damaged panicles by sampling hills chosen at random.

 

 

 

slide 77
For the last time, let us pause and review.

  • What  are  the  four  cultural   methods  of   controlling  stem borers?  Explain each.

  • What are some morphological features of varieties resistant to stem borers?

  • What are 2 reasons that limit the effectiveness of insecticides in controlling stem borers? 

 

 

 

 

 
 
GLOSSARY

CREDITS

Adult.   The mature stage of an insect which occurs after the nymphal or pupal stage.  Adult insects have mature sexual organs and usually have wings.

Deadheart.   Dead standing rice tiller caused by stem borers girdling its base.

Diapause.   A period during which the development of the insect is temporarily interrupted.

Economic threshold.   The pest density at which artificial control measures should be applied to prevent an increasing pest population from reaching the economic injury level; a threshold which tells the farmer when he must take action to prevent a pest outbreak.

Egg.   In insects, the reproductive body where the embryo develops and from which the nymph or larva emerges.

Egg mass.   A group of overlapping eggs deposited by the female insect, such as the egg mass of yellow and striped stem borers.

Larva.   The immature stage occurring between the egg and pupal stages in the life cycle of insects having complete metamorphosis.

Exoskeleton.   The outer covering of the insect that provides support.

Nocturnal.    Active at night.

Oviposit.   In insects, to lay or deposit eggs.

Pupa.    A nonfeeding and usually inactive stage which occurs between the larval and adult stages of insect development.

Sexual dimorphism.   A condition wherein the male and female adult insects differ in appearance, as in yellow stem borers.

Thoracic plate.

Whitehead.   White, empty panicles resulting from the attack of stem borers which cut the lower portion of the stem.

CREDITS