Pests of Field Crops in Southern Africa

APHIDS (Wheat, Barley)

(Hemiptera/Homoptera: Aphididae)

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Text Box: There are a number of aphid species that attack wheat and barley. Since these crops are often grown with full irrigation through the dry winter months, they tend to be densely planted and grow very lush, an ideal environment for aphids. The earliest of the aphid species to appear in wheat crops is usually Rhopalosiphum padi, which is a rounded aphid, dark green to blackish, with a reddish-brown area at the bases of the siphunculi. It is a small aphid (1.5-2.0 mm long) with short antennae reaching about halfway along the body and short siphunculi. It is most often found on stems and leaves in the lower canopy, but may occur on the ears later on. At about flag leaf appearance in wheat, another species (Metopolophium dirhodum) can be found in the wheat crop, usually confining itself to the underside of leaves and sometimes the stems in mid-canopy. This is a larger aphid (2.0-3.0 mm long) and is pale yellow or yellow-green with a barely discernible darker green line running the length of the body, and long antennae reaching beyond the bases of the siphunculi, which are themselves long and colourless. These aphids are easily disturbed and drop from the plant readily. A third species that tends to appear when the wheat crop is in ear is the brown ear aphid, Sitobion avenae. This is a similar size to the above species, is pale pinkish-brown (although a green form exists), oval in shape and has fairly long, dark antennae, long dark siphunculi and long legs. It can infest the upper leaves, but is most often found on the ears between the developing grains. A fourth species of aphid that attacks wheat is the common wheat aphid, Schizaphis graminum, but it has seldom been seen in Zimbabwe, presumably because it has a preference for dryland wheat. It is a small green aphid, with a pronounced dark green stripe down the body and fairly short antennae. The “Russian wheat aphid” (Diuraphis noxia), which has become established in South Africa, has not been recorded in Zimbabwe, and this may be due to the standard application of overhead irrigation which presumably prevents establishment of the pest in the rolled-up youngest leaves. Barley may be infested by Rhopalosiphum padi, M. dirhodum and S. graminum as described above, but a more serious aphid pest of this crop is the related maize aphid, Rhopalosiphum maidis. This is very similar to R. padi in size, slightly more elongated in shape, but is dark blue-green with smaller dark areas at the bases of the siphunculi. TOP Text Box: In addition to the details given above, maize and oats are host to all but M. dirhodum, and sorghum is host to all but M. dirhodum and R. padi, but many indigenous grasses are included as hosts for some or most of the species listed. TOP Text Box: The degree to which yield damage is caused by aphids varies depending on the species, the population levels they reach and the stage of growth at which the crop is attacked. In wheat the earliest species to attack, R. padi, is seldom considered to be serious when feeding on lower canopy leaves. On occasion it may reach very high levels however, particularly in a dense crop during a mild winter, and infested spots may be noticed with yellowing foliage and lots of sticky honeydew. The next species, M. dirhodum, has been shown in trial work to be capable of causing yield reduction by virtue of its feeding on the flag leaves if populations become quite dense. Infested leaves usually become yellow (particularly at the bends) and senesce early. This species seems to be more prevalent following a drought. The later species to appear, S. avenae, often appears quite late in the wheat crop (after flowering), but heavy populations feeding on developing seed heads can cause substantial yield loss. In general, damage is considered to be negligible before flowering, but during ear development, continual sap-sucking can cause reduced yields, presumably through reduction in kernel size and in shortening the functioning period of the important flag leaves. Copious honeydew production may also make combine-harvesting difficult. In barley crops, attack by R. maidis is usually during the vegetative stages, and heavy populations will cause yellowing foliage in depressed patches in the crop. There is also the danger of barley yellow dwarf virus being transmitted. This has not been a problem in the region as yet but remains a potential problem. TOP Text Box: Aphids initially appear in the crop as winged adults which have flown in or been carried in by air currents. They arise from summer grasses or grass crops in the vicinity. Reproduction of live young by asexual means is very rapid and if conditions are favourable (e.g. no frost), heavy populations may soon be reached. Populations often go unnoticed until this point. If temperatures drop sharply at night (which is the norm in the winter months), the populations may be kept at an acceptably low level. Later in the season, however, during the months of August and September, temperatures increase quite sharply, and late populations may explode. TOP Text Box: It has been a reasonably common practice to include a systemic aphicide spray with the herbicide application that is regularly made at about three weeks after planting. This is very early for aphids and has been regarded as a preventative measure. At the time that this practice was first introduced, it was very inexpensive, since there was no extra cost or time involved in application (which is by tractor-drawn equipment). It would certainly keep early aphid populations down (although they are regarded as having relatively little effect on yield) and may help to control maize streak vectors (leaf hoppers) surviving from the previous summer. However, insecticides have become much more costly as time has gone on, and it must be stated that this early application would not prevent a later application from being necessary if a late aphid population explosion occurs. The systemic is incapable of lasting the several intervening weeks. Since it is the grain-filling stage that is important, it is advisable that crops be scouted soon after flowering, if only for presence/absence of the pest. Much has been written about threshold levels, but realistically, aphid populations are difficult to count or assess in small cereal crops. Scouting will give a good indication of the population level, and how widespread it is within the crop. (Many farmers regard the appearance of large numbers of flying swallows above the crop as a good indication of the build-up of aphids within the crop!) Taking into account the weather prognosis (is it expected to warm up sharply, or are late cold spells to be expected?), a decision can then be made as to whether control measures are necessary or not. In addition, aphids are very susceptible to the large numbers of predators (particularly ladybirds and Syrphid larvae) and parasites which follow their appearance every season, and these may exert a good deal of control, but usually not rapidly enough. The cold weather at the beginning of the season prevents their early establishment. In terms of insecticides, the commonly applied ones are systemic organophosphates such as dimethoate or demeton-s-methyl. Softer insecticides, which will have far less effect on the natural enemies, such as pirimicarb, are available but unfortunately these are usually much more expensive. Application methods have included hand-spraying (laborious, time-consuming and quite hazardous), use of tractor-drawn equipment (research has shown that 3-5% yield loss can be incurred from tractor damage in the established crop) and aerial spraying. The latter, while quick and effective, is obviously very costly and its advantages would have to be carefully weighed against cost. In addition, the very dry and windy conditions prevailing in August and September would need to be taken into account to limit evaporation as a serious application problem. A fourth method which has occasionally been used by enterprising farmers, and which needs much more critical assessment, is the application of insecticide through the irrigation pipes. This, if it proves to be efficient and safe, would have the advantages of causing no damage to the crop and being of no extra cost. As usual, harvest intervals should be observed. TOP

Identification

Identification

Host Plants

Damage

Life Cycle

Control

Host Plants

Damage

Life Cycle

Control

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Rhopalosiphum Padi on wheat. (Above and below)

Sitobion avenae on wheat ear.

Sitobion avenae on wheat ear.

Ladybird larva, predator of aphids.

Left:      Rhopalosiphum padi. 1.5– 2mm long, body  rounded and dark green with rusty patches at the rear.

 

Middle:  Metopolophium dirhodum. 2—3 mm long, yellow body, long antennae.

 

Right:    Sitobion avenae. 2—3 mm long, body elongated, pinkish brown, long antennae.