Pests of Field Crops in Southern Africa

MAIZE STALK BORER

(Lepidoptera: Noctuidae)

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This caterpillar is usually regarded as a major pest of maize in the sub-tropics.  There are three different stalk borers found infesting maize, but the most important species is usually Busseola fusca, which is by far the most predominant species at altitudes over 1200m.  At lower altitudes another species can predominate, this being Chilo partellus (family Pyralidae), which is more commonly found in sorghum and will be dealt with separately. A third species, the pink stem borer, Sesamia calamistis, is more unusual, but is occasionally found attacking maize also (see general section).

The moths of B. fusca themselves are seldom noticed, but they are stout moths with reddish-brown forewings, pale creamy hindwings and a wingspan of 25 –35 mm.  The larvae start life a dark colour, but once larger, they are usually dirty pinkish-brown, and can reach a length of nearly 40 mm.  They form reddish-brown pupae within the stem.

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Text Box: The main hosts of Busseola fusca are maize and grain sorghum. Indigenous grass hosts could play a role in the survival of this pest, but there are few types which are of sufficient bulk to allow the larvae to mature within the stems.

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Text Box: Maize stalk borer usually attacks maize that has reached a height of between 30 cm and one metre tall.  Characteristic feeding marks found on the leaves unfurling from the funnel indicate that the young larvae have been there for several days.  These take the form of fine holes, or “windows” in the leaves (one epidermis left intact).  If several larvae were present, there can be quite a lot of tattering damage to the growing leaves.  Later, damage is caused to the actual stems, as the larvae hollow out their feeding tunnels, and fill them with frass (caterpillar droppings).  They may retard growth completely on young maize plants, and cause the development of suckers, or severely weaken the stems of larger plants so that they will not stand up to wind.  A second generation of the caterpillars, or a late attack, results in damage to the growing cobs, as the caterpillars feed directly on the grain and allow fungal infections to become established.

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Text Box: Round white eggs, slightly depressed at top and bottom, are laid in clusters of up to 80 beneath the leaf sheath.  The moths definitely prefer younger plants and the main attack by this pest seems to be within about a month of the first rains of the season.  After about a week, minute larvae hatch and crawl up the stem towards the funnel.  At this stage some migration may occur to neighbouring plants, and during this time they are very susceptible to weather and predators.  They tunnel down into the funnel, feeding on young furled leaves, and when these later open, they display the feeding marks described above.  The larvae then make their way down to the growing point. This may be destroyed, but usually one or more larvae manage to penetrate the stem below.  They will feed here, filling the feeding tunnels with frass, and after 30 to 40 days, having passed through six instars, they reach maturity.  Before pupating, they bore an “escape hatch” in the side of the stem with only a thin outer layer remaining.  As this dries, it forms a conspicuous window on the stem that indicates the presence of the pest.  The pupal stage lasts two to three weeks, depending on prevailing temperatures, and the emerging moths fly off to initiate a second generation of stalk borers.

The second generation appears from the beginning of January on older maize or younger late-planted maize.  Eggs are again laid beneath the leaf sheaths, or occasionally beneath ear bracts.  These larvae may feed on tassels or on the developing cobs before penetrating the stem.  Not all second-generation larvae pupate immediately, upon reaching maturity.  Larvae hatching from eggs laid after mid-February invariably enter a diapause state.  Having tunnelled down into the lower end of the stem (even below ground level), they hibernate in the dry stubble over the next months, and only become active again in the spring to complete the life cycle.  They then pupate, first tunnelling an escape hatch, and after about three weeks, moths emerge from the pupae.  This takes place from early October, mainly giving rise to the first generation moths of the next season.

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Text Box: Stalk borer moths do not fly very long distances, and infestation levels are definitely affected by tillage practices within an area.  Deep ploughing and burial of crop residues are effective since, unlike cutworm moths, these moths are not adapted to crawl through the soil upon emergence from the pupal cases.  Similarly, destruction of stubble lying on the surface to expose the overwintering larvae to desiccation and predators can result in the death of the majority of them.  Therefore, while conservation tillage is known to increase the incidence of stalk borer attack, any technique that knocks over standing stalks and cracks them will reduce the carryover of larvae to the next maize crop.

Apart from cultural methods of control, there have been attempts to breed maize with low susceptibility to maize stalk borer, and in some countries to use maize which has the Bacillus thuringiensis (“Bt”) gene incorporated.  There is no doubt that both of these methods will assist in keeping populations of the pest at acceptable levels.  In addition, synthetic pheromones have been developed and used successfully to determine pest levels so that chemicals can be applied timeously.  In the case of this pest, the pheromone is derived from the female moths and used as a “sex bait” to lure male moths into traps.  Increased trap catches indicate a moth emergence flight.

Chemical control of first generation larvae can be achieved by the use of granular treatments applied at planting.  These are obviously preventative treatments and do not take into account the size or presence of an infestation.  They are also extremely costly.  They may, however, be justified with valuable crops (e.g. seed crops) and in cases where other pests are also a problem, such as maize streak virus vectors and soil pests.  Granular soil treatments of this type are mainly carbamate insecticides such as carbofuran, benfuracarb or furathiocarb.  To date, none of the seed dressing treatments that have been registered for other sap-sucking pests (mainly leaf hoppers) have been effective for maize stalk borer control.  It is important to note also that drought conditions have an adverse effect on soil systemics and adequate moisture must be available if they are to work properly. Another caution that must be given is that soil systemics are not suitable if the maize crop is intended for consumption as “baby corn”.  The reason for this is that baby corn is harvested much earlier than grain maize, and residues of the chemical may still be higher than desired within the plant.

If preplanting treatments are not used, the crop should be scouted regularly (e.g. weekly) from about two weeks after planting, and funnel treatments applied over the entire crop when about 10% of the plants show the feeding damage described earlier.  A number of granular and spray treatments are registered for this purpose. These include low strength (e.g. 1%)  endosulfan and trichlorfon granules, which are applied by hand or machine to the actual funnels of the growing plants, or sprays of various organophosphate insecticides.

Control measures may not always be warranted in early plantings that are attacked by first generation stalk borers.  However, in later plantings (after mid-November), infestations can be severe and control is frequently necessary in these crops.  Very late plantings are attacked by second-generation stalk borers, and here the damage can be very severe. It is essential to inspect these plantings before the appearance of the tassels, as the larvae can not be effectively controlled by spraying the plant after the tassels have appeared.  In these cases, sometimes a second insecticide application is necessary about two weeks after the first.  Of course, this may be uneconomical if the yield potential of the crop is low.

Full control of the first generation of stalk borers will not automatically exempt one from a second-generation attack since moths will fly in from adjacent maize crops.  Use of soil systemics should give about eight weeks’ control, and thereafter a scout should be employed to look for signs of a second-generation attack.  Appearance of emergence holes in the stems is a useful indicator of these second-generation moths.

There are a number of natural enemies of maize stalk borer and some interesting work has been carried out to assess their levels of effectiveness.  In the Zimbabwe highveld (over 1200 m) the most abundant species is a parasitic wasp, Cotesia sesamiae (Hymenoptera: Braconidae), which actually enters the stem tunnels to lay eggs on the stalk borers.  This parasitoid tends to reach a peak late in the season. The second most common parasitoid is a fly, Sturmiopsis parasitica (Diptera: Tachinidae), which deposits first instar larvae at the tunnel entrances.  Although it does not reach the same levels of parasitism as the former, it does tend to peak in numbers earlier in the season.

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Identification

Identification

Host Plants

Damage

Life Cycle

Control

Host Plants

Damage

Life Cycle

Control

Early instar of stalkborer in Maize funnel. Yellow frass can be seen from feeding.

Stalk borer in a maize stem among the frass.

Maize stalk borer in the cob.

Scout for feeding damage from stalkborer caterpillars. 

Moth emergence holes from last season’s stubble.