Types of Aphids on Cereals

1. The cereal aphid (CA) (Rhopalosiphum padi).
This aphid is small, dark green to yellow green with a brown patch on its back. It is oval in shape and 1.2-2.4 mm long. It is found lower down on the stems and leaves (except when the populations are large) on all types of cereals, and is also common in pastures from which it emigrates and infests newly sown autumn and spring cereal crops. Its distribution in cereal paddocks is patchy.
As aphid numbers increase with time the aphids slowly spread from the initial focus and infest more plants in the vicinity. This gives rise to stunted circles in the cereal crop. Some of the strains of BYDV that CA transmit also severely stunt cereals. 
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2. The rose grain aphid (RGA) (Metopolophium dirhodum).
RGA is a pale green-yellow aphid with an elongated shape, 1.6-3.3 mm long. It first appeared in New Zealand in 1982 when populations of 50-150 aphids per tiller in December were common and yield losses of 15-30% were recorded. RGA is found on all parts of the plant and is more regularly distributed through the cereal crop than CA. RGA transmits BYDV more evenly throughout the crop than CA.
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3. The corn leaf aphid (CLA) (Rhopalosiphum maidis).
This bluish-green aphid is elongate, 0.9-2.4 mm long and found inside pre-emerged rolled leaves. It is also associated with honey dew. CLA was associated with major yield losses in some cereal crops in mid Canterbury in 1985. It causes severe stunting and turns some crops completely yellow due to BYDV infection. CLA is more common on maize crops.


4. The blackberry cereal aphid (BCA) (Sitobion species).
This apple-green aphid is elongate and 1.6-3 mm long. It is found more on the upper leaves and heads of cereals and grasses. It is known to transmit BYDV but its main effect is direct feeding on the flag leaf and ripening heads of cereals. No major outbreaks have been recorded in New Zealand, but more than five aphids per head have been found in some Canterbury paddocks. Overseas this population density is considered to be the threshold above which small grains are produced.
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5. Apple grain aphid (AGA) (Rhopalosiphum insertum).
AGA looks similar to CA but is found on the host plants closer to ground level. Very little is known about the aphid in New Zealand. It was first recorded in Canterbury in 1987.

 

Life Cycle
Large flights of aphids occur during April-May. They fly from pasture, rogue cereals and cereal forage crops and land on the newly emerged autumn-sown cereals. During winter these aphids increase slowly in numbers until the weather becomes warmer and they start to multiply rapidly. During spring, aphids also colonise emerging spring sown cereals and multiply rapidly. By mid December aphid populations reach their peak and begin to emigrate to late-sown cereals and pastures.

Detection
Counting the number of aphids on 20 or more tillers throughout the crop will give an indication of aphid numbers. Patches of stunted cereals should be checked to see if CA aphids are present. Yellowing in the crop caused by BYDV is also a sign that aphids are or were present.

Damage 
Direct aphid feeding on the leaves causes yellowing and premature death of the leaf, stunting of the tillers, reduced grain size and set. Large populations of CA aphid have caused severe stunting in crops. Small, shrivelled grains are consequently produced. Direct feeding on the heads at grain fill by BCA causes shrivelled grain, smaller grain weight and decreased seed set. Feeding on the flag leaf by RGA also causes poor grain fill and set.

The spread of BYDV by aphids has resulted in yield losses of up to 30%. The symptoms range from severe yellow patches in crops to a general scattering of yellow plants throughout the crop. Infections of BYDV in the early growth stages of a crop cause greater yield losses (due to secondary spread), death of secondary tillers and poor grain fill (due to reduced photosynthesis). Symptoms range from yellowing of the leaf in barley crops to a red and yellow discolouration in wheat and oats.

Yield Losses 
Yield losses vary depending on which aphids are present in the crop, the type of cereals, the number of aphids per tiller and the percentage of BYDV in the crop. Losses of up to 30% have been recorded in research trials and farm trials (40 aphids per tiller at stem elongation gave a 30% yield loss).

 

Control 

Cultural Control 
Sowing cereals after the main autumn aphid flights, i.e., in mid to late May onwards, reduces BYDV infection and the size of resident aphid populations during winter. Early spring sowing ensures that plants are well established before the spring aphid flights. Some late-sown spring barley crops host damaging levels of aphids in the early, critical growth stages.

 

Biological Control 
A parasitoid (Aphidius rhopalosiphi) was introduced to New Zealand in 1985 to control RGA. The parasitoid spread rapidly with rates of parasitism recorded at over 90%. RGA is now largely only a problem in spring-sown cereals. The same parasitoid also parasitises the other cereal aphids, but to a lesser extent.

Large populations of ladybirds (Coccinella spp.), lacewings (Micromus tasmaniae), syrphids (Melanostoma spp.), and entomophagous (insect-eating) fungi have been recorded in cereals in November - December and have been associated with sudden declines in aphid numbers.

Chemical Control 
Early autumn-sown crops, which emerge during the autumn aphid flights, should be checked and if aphids are present an aphicide applied. This will help prevent secondary spread of BYDV.

A BYDV forecast for Canterbury winter-sown cereals emerging from late May onwards has been developed by Crop & Food Research. It is based on 17 years data of aphid flights and BYDV infection.  

Control of high levels of aphids (>50 per tiller) at booting (Growth Stage 39-41) has given significant yield increases (up to 16%). An insecticide specific for aphids, i.e., one that does not harm aphid predators and parasites, should be used 

If more than five BCA per head are found at flowering spraying is recommended