Stem Borers

Maize stem borer larva

Photo credit: National Bureau of Agricultural Insect Resource

Source: PlantwisePlus Knowledge Bank

Introduction

In Southeast Asia, maize is attacked by several species of stemborers, which are among the most damaging insect pests in the region. These pests bore into the stem and tunnel through plant tissues, reducing plant vigor, disrupting nutrient flow, and significantly lowering yields.

These pests primarily belong to the order Lepidoptera (moths). In maize, the most damaging species are the Asian corn borer (Ostrinia furnacalis) and the Asian pink borer (Sesamia inferens).

Asian Corn Borer (ACB) Ostrinia furnacalis

The most widespread and destructive maize stemborer in Southeast Asia
Found throughout Southeast Asia.
Causes significant damage: Larvae bore into leaf whorls and stems, causing deadhearts, broken stalks, and poor grain filling
Management involves:
- Biological control (Trichogramma spp.)
- Host plant resistance
- Timely planting and field sanitation

In tropical regions, O. furnacalis can produce two generations each growing season.

Leaf damage caused by Ostrinia furnacalis

Photo credit: James Lutsinger

Emerging larvae of Ostrinia furnacalis

Photo credit: James litsinger

Identification and Symptoms

Egg Stage

Location: Laid on the underside of maize leaves, usually near the midrib.
Appearance:
- Flattened and oval
- White to pale yellow
- Laid in overlapping scale-like masses (like fish scales)
- Each mass contains 10–60 eggs
Hatching: Occurs in 3–5 days, depending on temperature.

Larval (Caterpillar) Stage – Most Damaging

Coloration:
- Newly hatched larvae are creamy white with dark heads.
- As they grow, they become pale pink to light brown, with dark spots on each body segment (one pair per segment).
Size:
- Grow up to 20–25 mm long at final instar.
Head: Dark brown to black.
Behavior:
- First instars feed in the whorl, then bore into the stalk, midrib, or cob.
- Entry points often show frass (sawdust-like excreta).
- Multiple larvae may be found inside a single stalk or cob.

Pupal Stage

Location: Pupation occurs inside the tunnel made by the larva in the stalk or cob.
Appearance:
- Reddish-brown, about 13–15 mm long.
Duration: Lasts about 5–10 days, depending on the climate.

Adult Moth

Size: Wingspan of 20–30 mm.
Color:
- Males: Yellowish-brown with irregular dark markings on forewings.
- Females: Paler, almost whitish, with fewer markings.
Behavior:
- Nocturnal flyers, attracted to light.
- Mating and egg-laying occur mostly during the night.

⚠️ Symptoms of Damage (Field Clues)

Leaf Feeding (early stage)
Shot holes and windowpane patterns on leaves.
Whorl Damage
Feeding inside the whorl causes “deadheart” — drying of central leaves.
Stalk Boring
Larvae tunnel into stems, leading to plant lodging (collapse).
External symptoms: Entry holes and frass at internodes.
Cob Damage
Larvae bore into developing ears, reducing kernel quality and yield.
Cobs may rot due to secondary infections.

⚠️ Confusion with Other Pests

May be confused with:
Fall Armyworm (Spodoptera frugiperda): Attacks whorl but does not bore stems.
Pink borer (Sesamia inferens): Bores at the base; larvae lack dark spots.
Use larval pattern (spots and pinkish color) and location of damage for accurate diagnosis.

Research: Biological Control of Asian Corn Borer in Myanmar Using Indigenous Trichogramma

Myint, Y. Y., Huang, X., Bai, S., Zhang, T., Babendreier, D., He, K., & Wang, Z. (2023). Field evaluation of Trichogramma strains collected from Myanmar for biological control of Asian corn borer, Ostrinia furnacalis (Guenée)(Lepidoptera: Crambidae) and sustainable maize production. Crop Protection, 171, 106284. Link

Maize is Myanmar’s second most important crop after rice and is cultivated year-round, often in intercropping or crop rotation systems with rice, pulses, vegetables, and other crops. In recent years, the use of chemical pesticides in maize production has increased significantly (Myanmar Agriculture in Brief Yearbook, 2019). At the same time, a notable decline in natural populations of Trichogramma wasps—key egg parasitoids used in biological control—has been observed in major maize-growing areas.

Recognising this, Myint et al. (2021) conducted a study to identify effective indigenous Trichogramma strains for the control of Asian Corn Borer (ACB) (Ostrinia furnacalis), a major pest of maize in Myanmar. The research identified three promising strains:

Two strains of Trichogramma ostriniae from Taunggyi and Yatsawk
One strain of T. dendrolimi from Yatsawk, all in Shan State

These strains showed strong host-searching behaviour and high egg parasitism rates in both laboratory and semi-field trials. Field evaluations confirmed their potential as effective biological control agents for ACB. The T. ostriniae strain from Yatsawk produced the most promising results:

Achieved the highest egg parasitism rate at 89%
Followed by T. dendrolimi (Yatsawk) and T. ostriniae (Taunggyi), both at 67%

In maize plots treated with the T. ostriniae (Yatsawk) strain:

Kernel weights were 1.2 to 1.4 times higher than in plots treated with other strains
Grain yields increased by 60% compared to untreated control plots

Further field trials showed that two to three inundative releases of Trichogramma significantly:

Reduced plant damage by 60–80%
Minimised yield loss by up to 50%

The best economic return was observed with two releases at a medium release density, achieving a cost-benefit ratio of 1:31. These findings support the scaling up of a sustainable biological control program using locally adapted Trichogramma strains, particularly T. ostriniae from Yatsawk, to enhance maize productivity and reduce reliance on chemical pesticides in Myanmar.

Trichogramma sp. Photo credit: Dr Robert Copeland

Sesamia inferens (Asian Pink Stemborer)

Sesamia inferens (Noctuidae, Lepidoptera), commonly known as the Asian pink stem borer or gramineous pink stem borer, is a highly polyphagous pest affecting maize, sugarcane and rice at different growth stages. This pest has been reported in several Southeast Asian countries, including Malaysia, the Philippines, Indonesia, Singapore, Thailand, and Vietnam (Baladhiya et al., 2018). The symptoms of S. inferens infestation are similar to those caused by other stem borers. The larvae tunnel into the centre of young maize plants and damage the growing point, which can cause the plant to dry up and die, a condition known as a "dead heart." Sometimes, ring-like cuts on the lower part of the stem can be observed. If the damage is bad, the stem might break, and the plant will stop growing properly. In serious cases, the plant stays short, and the cob and tassel end up growing close together (Kafle, 2013).

Adult of Sesamia inferens Walker

Photo credit: Young Chan (Publisher: iNaturalist)

Life cycle and Identification

Eggs: Adult moths lay between 120 and 348 eggs under the leaf sheath. The eggs are always clustered in several rows, covered by the leaf sheath. They are creamy white in colour and semi-globular in shape, being flattened on the top. As they develop, the eggs change from creamy white to brown, ash grey, and finally pinkish before hatching (Baladhiya et al., 2018)

Egg cluster of Sesamia inferens on maize (×6)

Photo credit: Viswajyothi

Larvae: The neonate larvae are pink to purplish-pink on the dorsal side, lighter on the ventral side, and have a smooth, cylindrical shape with a reddish-brown head. The larval stage consists of six instars. Pink colouration develops on the larval body during the second instar and becomes prominent in the fourth instar. The total duration of the larval period is approximately 23 to 39 days (Baladhiya et al., 2018)

Larva of Sesamia inferens

Photo credit: PlantwisePlus Knowledge Bank

Pupae: Pupation occurs either inside the stem or between the stem and leaves. The pupae are robust, light brown, and have a purplish tinge on the head region. One type of pupa is stout and dark brown. The pupal stage lasts from 9 to 12 days. During this stage, the sex of the borer can be easily distinguished. Males are smaller, have tapering abdomens, and possess two small bumps in front of their genitalia, whereas females are larger, with broad abdomens and no markings (Baladhiya et al., 2018)

Pupa of Sesamia inferens (a) Male (b) Female

Photo credit: Viswajyothi

Adult: Moths are straw-colored and have a broad, dark brown triangular streak running down the centre of their bodies. The male moths are slightly smaller than the females. Males have pectinate antennae, while females possess filiform antennae. Adult females typically live for about 6 to 8 days, whereas males have a shorter lifespan of around 4 to 6 days (Baladhiya et al., 2018)

Adult of Sesamia inferens

Photo credit: PlantwisePlus Knowledge Bank

Sugarcane Moth Borer (Scirpophaga excerptalis)

Occasional pest of maize, especially when grown near sugarcane
Distribution: Found in Indonesia, Malaysia, and parts of Thailand
Damage: Similar boring behavior; less significant than primary stemborers

Sugarcane Moth Borer Research Project in Indonesia

Kristini, A., Samson, I. D. P., BSES, M. D. N. S., & BSES, M. (2023). Integrated pest management of stem borers and insect vectors of viral diseases of sugarcane in Indonesia. ACIAR. Link here

Moth borers are significantly contributing to the decline of sugarcane production in Java, Indonesia. Pre-project estimates indicated that stem borer infestations in commercial sugarcane fields in Java exceeded 10%, resulting in millions of dollars in losses. This project conducted extensive monitoring across over 930 sugarcane fields in Java during 2008–2009 and followed up with monthly surveys in hotspot areas.

Repeated surveys confirmed three key stem borer pests:
- Chilo sacchariphagus (stalk borer)
- Chilo auricilius (stalk borer)
- Scirpophaga excerptalis (top borer)

Borer Ecology & Damage Patterns

Adult and larval populations peak during the rainy season (January–May), aligning with stem emergence when plants are most vulnerable.
The three species were found co-occurring within individual plants, with no competitive displacement, meaning a field can suffer simultaneous infestation by multiple borer types.

Biological Control Trials

Trials comparing standard vs. increased release rates of Trichogramma egg parasitoids highlighted that previous release guidelines were insufficient, necessitating updated recommendations for effective control.

Low Natural Parasitism Rates

Natural parasitism of stem borer eggs and larvae in Java was very low, highlighting the need for augmented biological control strategies (egg, larval, or pupal parasitoids).

Why This Matters?

Multi-species threat: The presence of multiple stem borer species requires integrated responses rather than targeting a single pest.
Seasonal targeting: Knowing rainfall-linked population peaks helps optimise timing for interventions.
Improved biocontrol: Enhanced Trichogramma release rates provide a stronger foundation for biological pest management.