Wheat and Barley

Pests of Field Crops in Southern Africa

WHEAT AND BARLEY

(Triticum aestivum and Hordeum vulgare, Poaceae)

Text Box: See general section. It is not uncommon, when the wheat crop is nearing maturity, to find varying numbers of Helicoverpa armigera caterpillars on the ears. As is normal with this pest, the colouring may vary considerably, but in drying wheat it tends to be shades of brown to almost black, rather than green. The bollworms can and do consume the drying grains, and occasionally may reach relatively high numbers (one or more per square metre), but even these are unlikely to do much damage to the yield of the very densely planted wheat crop. It is therefore not usually necessary to spray for this pest in wheat, although some farmers have felt the need. Since synthetic pyrethroids should be avoided in the winter season, other suitable pesticides must be found. It is, however, not very practical to spray the drying wheat and unless numerous caterpillars are detected per square metre (the threshold level has never been calculated), it is probably better to leave the pest to its natural enemies. Significant populations of the pest at the end of the winter crop may serve as a warning of problems to come in susceptible summer crops such as cotton, tobacco and horticultural crops. TOP

African Bollworm

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African Bollworm

Text Box: This pest has been described in general terms in the general section under “dusty surface beetle” (Gonocephalum simplex). It is a localised pest, occasionally necessitating some replanting of the crop. The larval stage of this beetle, known as false wireworm, can be found in the soil following certain summer crops, and can cause large patches of poor or absent crop stand. The same areas tend to be infested year after year, as the beetles do not move very far from suitable places. The preference seems to be for loamy soils, rather than sandy soils. Control of false wireworm is difficult in the wheat crop, since affected areas are often only noticed about three weeks after planting. For those who have history of this problem in certain lands, the cheapest and most effective control is by baiting of the adults in the preceding crops, before eggs are laid in about April. Although some soil-applied chemicals are effective in controlling the pest, application is difficult, particularly if the chemical needs to be incorporated (such as is the case with the organophosphate, chlorpyrifos). Farmers often just replant the affected area, and the second planting is usually not affected by the false wireworms, although this solution has its own problems, since these patches of the crop will be later to reach maturity than the rest of the crop. TOP

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Vertebrate Pests

Text Box: On occasion, numbers of darkish green caterpillars may be found consuming the leaves in patches. This is the lesser armyworm, Spodoptera exigua, but it is seldom a significant problem. Spot sprays of a contact insecticide may be used for control if necessary. In addition, migratory locusts (Locusta migratoria) may be seen to be building up towards the end of the wheat season on the crop. It is unusual for the grasshoppers to cause much damage to wheat, but if any large numbers of hoppers are detected, the authorities should be alerted and correct identification made. TOP

Text Box: Amongst the other pests that are sometimes seen in wheat or small cereal crops is one that may be regarded as a major pest – the Quelea bird, Quelea quelea. This small finch flocks in massive numbers, descending on wheat and barley crops when the ears are beginning to mature to feed on the seed. The birds waste as many seeds on the ground as they consume and can do extensive damage to the crop, leaving the ears stripped and bare. It is not recommended that farmers undertake control of the birds themselves – this should be the job of experts, (in Zimbabwe, the Problem Bird Control Unit of the Dept of National Parks). However, it has been the farmers’ task to locate the roosting areas so that control measures can be taken in these areas. These are usually in dense, homogeneous vegetation, and may consist of thickets of thorn scrub, young ‘gum’ plantations, reeds or Hyparrhenia (thatching) grass. The birds begin moving from the lands in small groups in the evenings, flying along definite paths to water before going to their roosts. They start arriving at their roosts (which may contain millions of birds) for about 30 minutes before sunset and continue for about 15 minutes afterwards. Tremendous noise can be heard at this time for up to half a kilometre away. The roosts are totally vacated shortly after sunrise, when the birds split up into many small feeding flocks. ‘Bait’ or ‘trap’ roosts have been grown with some success. These consist of napier fodder or even sugar cane, and a hectare seems to be the best size. This arrangement makes for easier and more successful control operations, and may limit the number of non-target bird species that are killed in control operations. Rodents are another vertebrate pest in winter cereal crops and also a cause of the appearance of ‘white heads’ in wheat. In southern Africa, multi-mammate mice (Mastomys natalensis) undergo irregular but dramatic population explosions. This species has a very high reproductive rate, breeding year-round, but more prolifically in the wet season. Outbreaks seem to follow years when good rains occur late in the wet season and particularly after an intense drought, resulting in abnormal vegetative growth and a good food source during the dry season. These outbreaks are often more serious in the ‘Lowveld’ areas ( below 500 m altitude). In wheat, small groups of white heads, or patches of abnormally high numbers of dead heads, may indicate damage by mice. The stems will be seen to be completely severed, with the stem characteristically being cut at a 45-degree angle. The collected ears can be found on the ground between the rows, sometimes near a hole which is the entrance to an underground burrow. For short-term control, many rodenticides are commercially available, and some of these are conveniently formulated in small wax blocks or pellets, which makes them waterproof and easy to drop into holes. They are, however, expensive and may attract non-target species. In the long-term, habitat modification is the best method of keeping rodent numbers acceptable. This includes the establishment of broad, low-cover strips around lands with erection of T-perches or use of trees to encourage predatory birds, efficient harvesting which prevents food spillage, clean areas between cultivated lands, and clean farmyards and secure food stores, to avoid build-up of rodents. TOP

Vertebrate Pests

Text Box: Leaf hoppers of the genus Cicadulina are well-known as Maize Streak Virus vectors. At the end of the rainy season, in about April, large numbers of the hoppers can be found in the grass verges of crops, and from here they may invade irrigated wheat crops. These small and very mobile sap-suckers can transmit the virus to wheat, but unlike in maize, the effect is seldom serious, appearing only on later tillers at field edges. The effect, appearing as stunting and yellow striping on the leaves, is sometimes mistaken for herbicide damage. In areas where MSV is endemic in maize, it may be advisable to apply a systemic insecticide to the wheat (possibly as a mixture with the early herbicide) in order to knock back the overwintering generations that will attack the following summer maize crops. TOP

Text Box: See pink stem borer in general section. Every season, very low numbers of individual premature ‘white heads’ are noticed in the wheat crop. When an affected tiller is noticed, a sharp tug will easily detach it, and the hollowed stalk will be seen to contain wads of frass and sometimes the pinkish-brown caterpillar, Sesamia calamistis. The base of the stem remains green and unaffected. Damage by this pest is usually completely negligible in wheat, although abnormally high numbers may signal a problem for following maize crops. Other pests may also cause prematurely white heads. Amongst these are rodents (see below) and termites. The latter may, in hot and dry areas of production, cause some damage to wheat at the field’s edges. The soil workings around stem bases and up the stems make identification simple. A disease of wheat also causes ‘white heads’. This is ‘take-all’ (Gaeumannomyces graminis), and it appears in larger patches, not individual tillers, and the stems are not severed in any way, but have a blackish stain at their bases. TOP

African Bollworm on Wheat ear.

Wireworm. The larva of the Dusty Surface Beetle.

Dried wheat stalk having been killed by pink stem borer.

Quelea damage to barley. White heads show where the grain has been taken.

Rodents typically leave short lengths of stem on the ground.

Pink stem borer in wheat stalk.