Pests of Field Crops in Southern Africa

PINK STEM BORER

(Lepidoptera: Noctuidae)

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The pink stem borer, Sesamia calamistis, is a pest of sporadic importance in graminaceous crops.  The larvae have a brown head and pinkish-brown body, with dark spiracles down the sides.  They are much lighter in colour than the similar maize stem borer (Busseola fusca).  They can reach about 30 mm in length, although this depends on the size of the stems in which they bore.

The pupae are roughly 15 mm long and a pale reddish-brown, and the adult moths, with a wingspan of 25-30 mm, have yellowish-brown forewings and white hindwings.

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Text Box: The pink stem borer has been recorded on maize, sorghum and other millets, wheat, sugar cane, rice and other grasses.  It is not a major pest of maize, but on occasions it has been reported causing quite extensive damage to very young, early-planted maize plants, and to sweetcorn crops.

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Text Box: In maize, the damage takes the form of withering of the central leaves of young plants.  The attack comes much earlier than that of the maize stalk borer (Busseola fusca) which is a more widespread and serious pest of maize.  The significance of this is that while the normal maize stalk borer funnel treatments would probably give some control of pink stem borer, especially if they were insecticidal sprays rather than granule applications, it arrives before these treatments have been applied, often in maize plants less than 15 cm tall.  Another difference between the two pests is that the eggs tend to be laid low down on the plant stem, and the young larvae tunnel straight into the stems, avoiding the funnel.  Thus there are none of the characteristic feeding marks found on the unfurling leaves of the funnel left by the maize stalk borer, which can be used for scouting. Several pink stem borers will penetrate the stem successfully, while maize stalk borer infestations tend to be only one or two successful individuals per plant.  (Many actually die from drowning or natural enemies before they penetrate the stem.)  In sweetcorn crops that are early-planted, the most serious damage is to the cobs themselves where rows of seeds are destroyed by the caterpillars’ feeding activities.

In wheat, which is grown in Zimbabwe through the winter months, relatively low numbers of “white ears” caused by S. calamistis are noticed in most seasons. When an affected tiller is noticed, a sharp tug will easily detach it and the hollowed stalk will contain piles of frass and sometimes the pinkish caterpillars.  The base of the stalk remains green.  Rodents or termites can also cause white ears, in which case the stem is eaten away entirely.  Even when there is an affected ear for every square metre, the damage to wheat will be negligible, considering the dense wheat stands generally grown in Zimbabwe. Their presence, however, may be an indication that large numbers of this pest are present to start off the coming summer season, when maize is planted.

In rice the damage by pink stem borer is very similar in appearance to that in wheat.  In sorghum and millets, damage by pink stem borer may lead to stunted growth or poorly developed heads, and occasionally damage to sugar cane can occur in a very similar fashion to that described for maize, with attack at ground level giving rise to “dead hearts”.

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Text Box: The adult moths lay their eggs in elongated batches of 25 to 40 at night under the leaf sheaths near the base of the plant.  (In the case of sweetcorn damage, the eggs tend to be laid under the cob bracts or in the cob axils.)  The eggs hatch in about a week and the young larvae soon penetrate the stem or the developing cobs.  During the summer the larval period is 3 to 6 weeks, and the caterpillars pass through six or seven larval instars.  They pupate in the plant stalks or in the tunnels caused by feeding on the cobs, and after 10 to 14 days, new moths emerge.  In the winter months, development takes place at a much slower rate.

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Text Box: It is fortunate that this is not normally a serious pest of maize, as its habit of entering the stem low and almost immediately after hatching, makes it very difficult to control.  If dead hearts are seen in maize in numbers that are significant, it is possible that sprays rather than granules applied on the very young plants so that the chemical runs down the stem, will do a reasonable job of control.  However, the damage is usually noted long after it is possible to prevent it. Of course, pre-planting systemic granules applied to the furrows, while expensive, will probably give good control of this pest, (as well as the common maize stalk borer, maize streak vectors, some soil pests and nematodes) if the maize crop is a valuable one. Taking note of the amount of infestation in preceding wheat crops should give warning of the possibility of problems when early maize or sweetcorn is to be planted. 

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Identification

Identification

Host Plants

Damage

Life Cycle

Control

Host Plants

Damage

Life Cycle

Control

Pink stem borer in a wheat stem.

‘White ear’ of wheat caused by pink stem borer.

Pink stem borer.