DistributionOccurs in both the summer and winter rainfall regions, but is much more problematic in the summer rainfall areas where it is the most important pest of wheat. Different biotypes of RWA occur.
IdentificationRussian wheat aphid is a small (<2.0 mm), spindle shaped, pale yellow-green to grey-green aphid with extremely short antennae and a “double tail". It is difficult to see the cornicles with the naked eye.
Feeding position and damage symptomsYoung susceptible wheat plants infested with RWA
become prostrate and the leaves roll tightly closed. Symptoms on older leaves
include longitudinal, white or pale yellow stripes, which may turn
purple when cold conditions prevail, and tightly rolled leaves trapping
emerging ears (i.e., bent ears). Yield losses of more than 60% can occur if the
RWA is not controlled chemically.
ControlThe best control option for RWA is the use of resistant cultivars. In contrast to susceptible plants only small white or yellow blotches and spots occur on the leaves of plants with resistance and the leaves do not roll tightly closed. The presence of new RWA biotypes can result in the RWA resistant cultivars being less effective but, although damage occurs, cultivars with resistance are often not as severely damaged by the new biotype(s) as susceptible cultivars. Therefore resistant cultivars are still recommended. Producers should scout their fields regularly and be aware that it may be necessary to apply insecticides if aphid populations increase.
Wheat is most prone to damage by RWA during the period between the emergence of the flag leaf [known as growth stage 14 (GS 14)] and the awn (GS 18). Chemical treatment at GS 12 (Joubert scale - when first node is visible above soil level) will ensure that the upper two leaves are protected from aphid infestation and this will limit yield loss. Spraying before GS 12 is recommended only in cases of severe infestation, i.e., > 30%, a scenario which may occur during spring in the Eastern Free State or under very dry conditions in the Western Free State. Re-infestation of this wheat may occur during the susceptible period necessitating an additional spray, while some damage may already have occurred when spraying takes place after GS 12. Infestation levels at various yield potentials, which necessitate spraying, are presented in Table 1. Seed treatments and soil systemic insecticides are available for control of aphid populations and control is possible for up to 100 days after planting.
What is a RWA biotype?Essentially a biotype is a strain of RWA that has the ability to overcome resistance genes deployed in commercial wheat cultivars rendering them susceptible. The different biotypes all look the same and can only be differentiated by the susceptible or resistant reaction of the wheat plants they feed on.
To date four RWA biotypes have been identified in South Africa; RWASA1 in 1978, RWASA2 in 2005, RWASA3 in 2009 and most recently RWASA4 in 2011. This new RWA biotype, is distinguished form RWASA1, RWASA2 and RWASA3 on the basis of its virulence to Dn5-based resistance in wheat.
It is critically important to monitoring the diversity and distribution of RWA biotypes to be able to manage this insect pest successfully. This information is used by pre-breeders and breeders to identify resistance genes which can be used to control the new biotypes.
Samples to determine biotypic diversity in an area, are collected not only from cultivated wheat but also from alternate host plants used by RWA. The collected aphids are reared as clone colonies and each colony is tested using a set of differential lines. This "differential set" contains wheat lines that each have a different, known resistance gene present in them. The plants reaction to the clone colonies tells reseachers which biotype the clone is.
RWA are always present in wheat producing areas, but often in low numbers. When damaging RWA biotypes are present in an area it may not always result in economic damage, but there is always the potential of the populations reaching economic pest status when conditions for RWA growth and reproduction are favourable. Because of the ever changing nature of ecosystems and with it the population structure and damage potential of pest insects, it is very important to continue screening for new biotypes on an ongoing basis. The information gained through this research will keep us up to date with changes that occur in the field and enable us to react to these changes in time.
Clone colonies of RWA biotypesScreening for RWA biotypes
Follow these links to see the distribution of RWA biotypes in South Africa.
DistributionOccurs sporadically in both the summer and winter rainfall regions, but is more problematic in the summer rainfall areas.
IdentificationAdults reach a size of 1,8 to 2 mm and are pear shaped. Colour is usually light yellowish green with a dark green stripe on the back. Cornicles are pale green with a distinct black tip.
Feeding position and damage symptomsFeed on the upper side of leaves. Leaves turn yellow and then later form brown chlorotic marks after aphid feeding.
ControlInfestations during hot, dry conditions seem more injurious. Chemical interventions can be considered when 30-40% of the tillers are infested with 10 or more aphids per tiller.
DistributionThis aphid occurs annually in the Western Cape and irrigation areas, but sporadically in the summer rainfall region.
IdentificationBody is rounded and adults vary in length from 1,5 to 2 mm. Antenna are short, approximately two thirds of the body length. Characteristically the body is dark green in colour with a red brown area visible between the cornicles. Cornicles are short but clearly visible.
Feeding position and damage symptomsAphids generally feed on the leaves and stems of the wheat plant. During the latter part of the season aphids may migrate into the ears where they extract sap directly from the developing kernels. Damage symptoms usually become visible only during high aphid population pressure resulting in yellowing of plants.
ControlThe Oat Aphid is less harmful than RWA and generally occurs simultaneously with the Brown Ear Aphid and Rose Grain Aphid (see below). For this reason, the damage potential of this particular species can not be determined easily. Population increase generally occurs after the flag leaf stage and chemical control can be considered when 50% of the tillers are infested with 10 or more aphids per tiller.
Distribution An annual pest in the Western Cape Province and on wheat produced under irrigation throughout the country but occurs sporadically in the summer rainfall areas.
IdentificationA relatively large aphid averaging 2 to 3 mm in length. These aphids may be brown or green in colour. The brown form is more common than the green form. Antennae reach the basis of the cornicles and are brown to black in colour. Siphuncili are long and characteristically black in colour.
Feeding position and damage symptomsThese aphids feed on leaves of plants but after ear emergence, they move up to the ears. Damage caused by these aphids is not defined. When present in high numbers leaves may turn yellow.
ControlThe Brown Ear Aphid is less harmful than Russian Wheat Aphid and generally occurs simultaneously with the Oat Aphid and Rose Grain Aphid (see below). For this reason, the damage potential of this particular species can not be determined easily. Population increase generally occurs after the flag leaf stage and chemical control can be considered when 50% of the tillers are infested with 10 or more aphids per tiller.
DistributionThis aphid occurs mainly on wheat produced under irrigation throughout the country as well as on dry land wheat in the winter rainfall region of the Western Cape.
Identification A relatively large cereal aphid measuring 1,5 to 3 mm in length. Light- to yellowish green in colour with a dark green longitudinal stripe on the back. Cornicles are long without any conspicuous colour. Antennae long with darkened apices, reaching beyond the base of the cornicles but not exceeding the total body length.
Feeding position and damage symptomsThis aphid is often found in small colonies on the underside of the leaves, making it less conspicuous than some of the other cereal aphids. Flag leaves may become infested while, at high population densities, aphids may also be observed feeding on the stems. Leaves may turn yellow when high populations are feeding on the plants.
ControlThe Rose Grain Aphid is probably the least injurious of the aphid species associated with wheat. Although population increase generally occurs prior to the flag leaf stage, chemical control should only be considered when 70% of the tillers are infested with 10 or more aphids per tiller.
DistributionA sporadic pest in both the small grain production areas of the summer and winter rainfall regions and on wheat produced unde irrigation throughout the country.Identification Small to medium in size, 1,5 to 2,5 mm in length. Blue green in colour, with dark green markings around the base of the cornicles. Antennae short, more or less one third of the body length, and black in colour. Winged aphids are dark green in colour.
Feeding position and damage symptomsGenerally feeding in the axil of the leaf but also sometimes found together with the RWA inside curled-up leaves of susceptible cultivars. Damage symptoms and thresholds for this particular species in South Africa are unknown.
ControlMixed populations of Maize Aphid, Brown Ear Aphid and Oat Aphid do occur and should be controlled when 50% of the tillers are infested with 10 or more aphids per tiller.
DistributionThe grain chinch bug is a sporadic pest of wheat and restricted to the small grain production areas of the Western Cape.
IdentificationNarrow elongated bugs with wings folded over the abdomen. The bugs are blackish in colour and measure 4 to 5 mm in length. Lighter markings are generally present on the wings. When crushed, a vile odour is emitted. Eggs are laid between the leaf sheaths and are at first white in colour progressively darkening to orange. The small wingless immature stages are yellow to orange in colour, often found in large numbers during spring on various grasses and small grains, including wheat. During November – December adult bugs migrate to trees and shrubs over summering under the bark and other suitable sites of refuge. Here the bugs remain without feeding until mid-June to mid-July. Form here bugs move back to the grasses and wheat crop where mating and egg lying occurs.
Feeding position and damage symptomsChinch bugs are sap sucking insects and high infestations may lead to slightly yellow withered plants. These symptoms may be aggravated by warm, dry conditions. Where the wheat crop is in the ear, sap may also be sucked from the seed.
ControlTwo systemic insecticides are registered in South Africa against chinch bug on wheat.
DistributionOccurs sporadically in the small grain production areas of the summer rainfall region.
IdentificationThe adult stinkbug is shield-shaped measuring 4 to 7 mm in length. The bugs are generally blackish in colour with orange and yellow markings. Pale yellowish-white eggs are laid on the foliage or soil surface and darken shortly before they hatch. The immature stages are generally reddish in colour and undergo five moults.
Feeding position and damage symptomsThese sap-sucking insects feed on the leaves and stems inflicting damage only when high populations occur. Typical feeding symptoms is characterised by white rosette-shaped markings occurring on the leaf surface. A variety of vegetable crops such as cabbage and cauliflower are attacked and also the weed Raphanus sp. These host plants can act as sources of infestation should they occur near wheat crops.
ControlNo chemicals are currently registered in South Africa against Bagrada Bug on wheat.
DistributionOccurs sporadically in the North Western parts of the Free State and in parts of Mpumalanga where wheat is produced under irrigation.
IdentificationA soft-bodied wedge-shaped insect about 2,5 mm in length. The head is yellow with two black spots on either side. The rest of the body is brown although the underside is generally of yellow appearance. The wings are soft, transparent with a brown longitudinal stripe. When disturbed, insects often jump to safety.
Feeding position and damage symptoms
The pest status of this insect is attributed to the fact that it can transmit maize streak virus to wheat, after feeding on the sap of infected maize or some grass species. Immature virus-infected plants may have a rosette or stunted appearance while longitudinal yellow stripes may be observed on the leaves, a condition in wheat called streak of wheat stunt. Bent or curled leaves may appear when plants are infected at a late stage. Direct feeding damage may be inflicted to young plants when high leafhopper populations are present.
ControlNo chemicals are currently registered in South Africa against leafhopper on wheat. However, infestation can be prevented by later planting dates and/or in areas away from maize fields to prevent direct spreading of the insect to emerging wheat.
DistributionA sporadic pest in the summer rainfall regions and annual occurrences can be expected specifically in the western parts where drought conditions are frequently encountered.
IdentificationThe mites are very small measuring about 0.5 mm in length, dark brown with pale yellow legs. Eggs are generally laid beneath clods and are either active i.e. red in colour and not visible to the naked eye because it is too small and the same colour as the soil, or dormant i.e. white eggs clearly visible on the underside of clods. Under favourable environmental conditions eggs hatch within 9 to 11 days. Dormant eggs may remain in the soil for long periods and hatch during July/August following light rainfalls. Dry conditions favour larval development and adulthood can be attained within 8 to 11 days. Only females occur and eggs are laid within 2 days after reaching adulthood. Subsequently, mite populations often reach pest status under dry conditions.
Feeding position and damage symptomsThe brown wheat mite feeds on sap from leaves by inserting two needle-like stylets into the leaf. This phenomenon causes a typical dapple appearance on the leaf surface. During high mite populations the leaves may have a bronze appearance with some leaves even die off as a result of intense feeding. Plants may also become severely stunted, head poorly, and turn white. Due to mite damage often occurring under drought conditions, mite and drought damage should be clearly distinguished. Individual mites are extremely small and can scarcely be seen with the unaided eye. If an infested plant is held over a piece of white paper (folded to form a trough) and tapped sharply, several mites will fall onto the paper and can be seen moving about.
ControlIn South Africa, two systemic insecticides are registered against the Brown Wheat Mite on wheat. Rainfall of more than 12 mm will destroy mite populations.
DistributionA pest of small grains and pastures in the Western Cape.
IdentificationAs the name implies, these mites are black in colour. Mites are small, pear-shaped and about 1 mm in length with reddish legs. The eggs hatch after the first winter rainfalls. Only females occur and periodically lay eggs which hatch within 8 days. During the onset of warm dry spring conditions the female mite produces eggs with a thicker egg shell which she retains within her body until after her death. These eggs remain on the soil surface and hatch following good autumn rains.
Feeding position and damage symptomsFeeding results in silvery white scars visible adjacent to the main nerve on older leaves. During high population densities, leaves may discolour and plants may have a wrinkled appearance with small plants possibly dying off. These mites have a wide range of food plants, including pigweed or ‘gousblom’, lucerne, oats and many vegetable crops.
ControlOne chemical insecticide is currently registered in South Africa.
DistributionThis pest occurs throughout the small grain production areas of the Western Cape, although economical damage is caused only in isolated areas.
IdentificationThis “slug” is the immature stage of a metallic green beetle often observed in wheat fields during June. The beetles have an orange breast and measure about 5 mm in length. Light green eggs are laid adjacent to the main vein on the upper side of the leaf. After hatching the larvae are white but soon become a blackish colour when they become covered by a slimy substance giving them a slug-like appearance.
Feeding position and damage symptomsThe larvae feed on the upper side of the leaf between the veins resulting in white longitudinal stripes visible on the leaves.
ControlNo chemicals are currently registered against the Grain Slug on wheat.