Note: The “stem borers” or “stalk borers” include a number of moth species that attack the stems of cereals. For practical purposes, stem borers will here be treated as one pest until more information is available on the occurrence and damage caused by them in Namibia. COMMON NAMESfimbi weemweenge (Oshikwanyama); ombolo (Oshikwaluudhi, Oshingandjera); sorghumstaamboorder (Afrikaans)PEST STATUSStem borers are considered as minor pests in fields used for subsistence agriculture, where cattle are led in after harvest to eat the crop residues. The borers are more important, however, on irrigated grain crops and may be the main pest problem in large commercial fields.All the main grain crops such as maize, sorghum (Sorghum bicolor) and pearl millet (Pennisetum glaucum) are attacked. Stem borers are not really a problem in pearl millet but maize and Sorghum may be heavily attacked. Stem borers appear to be widespread in Namibia but information is not available yet on the distribution of each species. Farmers have reported two types of symptoms on maize plants. One species attacks young plants from the very bottom of the plant and bores up through the stem. This leads to “dead heart” where the shoot wilts and dies. Another species attacks from the funnel of the leaf and moves downward inside the stem. This does not lead to dead heart leaves. It is still uncertain which species actually produce each of the reported symptoms. DESCRIPTIONThe mature caterpillar of Chilo is 25 mm long, pale brown with dark spots along the body. The head is dark brown. The adult moth is white or straw-coloured with a few darker spots, wingspan is 25 mm.CONTROLUnder normal rain-fed conditions there will usually be no need for control of stem borers. The pest survives as a caterpillar or pupa inside dry stems of cereals during the dry season.For partial control of stem borers it is best to remove all crop residues from the field after harvest. This method is, however, against the ideas of soil conservation where the residues should be left in the field to increase soil fertility and reduce soil erosion. Chemical control is mainly relevant to large fields of irrigated maize. Stem borers are generally difficult to control by spraying as they are hidden deep inside the plants. If chemical control is found necessary, please contact the FSRE pest officers for precise advice on chemical control. The contact numbers and addresses are listed on the cover. Many institutions and projects are involved in stem borer control research as it is a major maize pest in Africa. The ARC - Grain Crops Institute in South Africa is working on an interesting use of trap crops for stem borers. They tested Napier grass (Elephant grass) and Blue thatching grass (Hyparrhenia tamba) as trap crops around the maize fields. Two of the stem borers (Busseola fusca and Chilo partellus) prefer to lay eggs on these plants instead of maize, but strangely enough the larvae do not survive well on these preferred plants. It needs to be tested out under Namibian conditions but it seems that Maize plots would benefit from guard rows of Napier grass. PREDATORSA few large wasps have hatched from stems of maize and sorghum that have been infested with stem borers. These wasps have not yet been identified to species.OTHER NOTESStem borer species cannot be identified from the caterpillars alone, so it is necessary to rear adult moths from stems that have been attacked by caterpillars. Old stems are collected and kept in cages, until the adult moths eventually emerge.Note that Chilo stemborers really like to attack Sorghum plants. Most fields in Namibia have some left-over sorghum plants after harvest and these are usually heavily infested with Chilo caterpillars. I have not seen the other stem borer (Busseola fusca) for many years, but am quite sure I reared a few specimens from maize plants just west of Oshakati town during the mid-nineties. We were overworked so the specimens were lost. I am sure fusca is present in our maize fields, but our collection methods may favour Chilo all the time and we seldom visit a pure maize stand. fusca prefers maize instead of sorghum.
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