Spotted stem borer

Spotted stem borer (Chilo partellus) is an exotic plant pest not present in Australia. This insect is a serious threat to Australia’s grains industry.

Spotted stem borer (Chilo partellus) is a stem boring insect which is a serious pest of maize, millet and sorghum. The spotted stem borer caterpillars damage these crops by boring or tunnelling inside their plant stems (Figure 1).

Notifiable status

Spotted stem borer (Chilo partellus) is not a notifiable plant pest/disease in NSW.

However, if you suspect Spotted stem borer (Chilo partellus):

• Call the Exotic Plant Pest Hotline 1800 084 881
• Email biosecurity@dpi.nsw.gov.au with a clear photo and your contact details

A full list of notifiable plant pests and diseases can be found in Schedule 2 of the NSW Biosecurity Act 2015.

Description

Spotted stem borer moths have small yellow-brown bodies with 20-25 mm wingspans. They have 2-3 dark spots on their light yellow-brown forewings which also have darker scale patterns. The hindwings are dirty white-grey (Figure 2).

Pupae have shiny, light yellow-brown to dark red-brown bodies up to 15 mm long.

Caterpillars have dark red-brown heads, four purple-brown longitudinal stripes and dark-brown dorsal spots along their creamy white to yellow-brown bodies (Figure 3). Fully grown caterpillars are up to 25 mm long.

The flat, oval, creamy white eggs are about 1 mm long. They are laid in clusters of 10-80 eggs.

Damage

Spotted stem borer caterpillars can cause plant stunting, “dead hearts” (the youngest still unfolded leaves wilt and die), broken stalks and delayed crop maturity in infested crops. Infested plant stalks are also easily broken by wind.

Damage to the flower heads may interfere with grain formation, causing “chaffy heads” (grain heads with empty or light weight grains) in infested sorghum crops.

Spotted stem borer caterpillars cause 50% yield losses in infested maize and sorghum crops in South Africa.

Lifecycle

Spotted stem borers have four lifecycle stages: eggs, caterpillars, pupae and moths.

Female moths lay their egg clusters near the leaf mid-rib on the undersides of the host plant’s leaves. Eggs hatch into caterpillars after 4-10 days incubation.

Caterpillars feed on the young leaves for 5-6 days before tunnelling into the plant stems. They can over-winter in plant stems and crop stubble for up to six months, before completing their lifecycle early in the following growing season. Inside the plant stems the caterpillars become pupae before emerging as moths.

The female moths live for 2-5 nights after emergence, when they mate and lay their eggs on nearby plant hosts. Spotted stem borers can have five or more lifecycles in a year, with each lifecycle lasting 4-6 weeks.

Host range

The main host plants of the spotted stem borer are maize (Zea mays), sorghum (Sorghum bicolor) and pearl millet (Pennisetum glaucum).

Other hosts include rice (Oryza sativa), foxtail millet (Setaria italic), finger millet (Eleusine coracana), sugarcane (Saccharum officinarum) and grasses (Poaceae spp.).

Spread

Spotted stem borers are spread from farm-to-farm by infested planting material, machinery and vehicles. The caterpillars move about and within a host plant, while the moths fly between the host plants at night.

Distribution

Spotted stem borer is a native insect of Asia, which has since become established in Africa. It is present in Afghanistan, Bangladesh, Benin, Botswana, Cambodia, Cameroon, the Comoros, Congo, Eritrea, Ethiopia, India, Indonesia, Iran, Israel, Japan,  Kenya, Laos, Lesotho, Madagascar, Malawi, Mozambique, Nepal, Pakistan, Reunion, Rwanda, Somalia, South Africa, Sri Lanka, Sudan, Swaziland, Taiwan, Tanzania, Thailand, Togo, Turkey, Uganda, Vietnam, Yemen, Zambia and Zimbabwe.

Actions to minimise risks

Put in place biosecurity best practice actions to prevent entry, establishment and spread of pests and diseases:

• practice “Come clean, Go clean”
• ensure all staff and visitors are instructed in and adhere to your business management hygiene requirements
• source propagation material of a known high health status from reputable suppliers
• monitor your crop regularly
• keep records