Pest biology

Aphids

Summary

Bird cherry-oat aphid, Grain aphid and Rose-grain aphid are three aphid species often found in cereal crops. Yield losses of up 30% have been seen in Danish trials. The aphids are 2-3 mm long, have cornicles and are usually greenish brown. Depending on the crop it is necessary to scout for the aphids as early as the beginning of elongation. Because the different species are found in different places in the crop, it is necessary to study the crop closely. Use the control guidelines to calculate the need for control as well as the dosage.

Symptoms

The aphids secrete honeydew, which gives the plants a bright, greasy look. Secondarily, black mould fungi can appear on the honeydew. Attacks often begin in sheltered areas of the field (near hedges, etc.). The bird cherry-oat aphids often sit on the straws quite close to the soil surface. Here they can propagate for a long time before moving further up the plant. The bird cherry-oat aphid prefers spring barley but is also common in oats, spring wheat and winter wheat. Especially in spring barley it is important to look at the bottom part of the crop. The grain aphids sit on the underside of the leaves before the earing of the cereals. After earing they are mainly found in the ears. All cereals are attacked, but after earing the aphids prefer wheat and oats where they can be found in the ears in large colonies. Spring barley with many grain aphids before earing can be a forewarning of a later attack on wheat. The rose-grain aphids always sit on the underside of the leaves where they can easily be missed. Their yellowish green colour melts into that of the leaf, and they also easily fall to the ground. Rose-grain aphids can live on all cereals but are especially common in spring barley, oats and wheat.

Time of attack

The aphids arrive in barley and wheat from late May to late June (from stage 31). The most extensive propagation takes place in June-July. The size of the population often peaks near the milk ripeness stage and drops when the nutritional conditions becomes poor (beginning ripeness or too large aphid density). Natural enemies also reduce the population.

Biology

In the spring the winged females leave their winter residences and seek out the corn fields where they give birth to wingless young. These propagate by giving birth to live young without any fertilisation taking place. Later in the summer winged aphids are born who can seek out new crops and winter hosts. When and how many winged aphids appear depends on food and aphid density. Aphids can be spread by the wind over large areas. Bird cherry-oat aphids overwinter as eggs on bird cherry (winter host). In the spring several generations are developed here. During May winged aphids are developed who fly into the corn fields (summer hosts). After harvest the bird cherry-oat aphids stay on cultivated and wild grasses until they seek out bird cherry trees in the autumn. Overwintering as an adult in open areas only takes place in regions with a milder climate than Denmark. The grain aphid stays on grass and cereals all the year, that is to say it does not alternate between hosts like the bird cherry-oat aphid and the rose-grain aphid. Overwintering eggs are laid in the autumn. The rose-grain aphid overwinters as egg on roses. In the spring 2-3 generations are developed here. Winged aphids fly into the corn fields from mid-May to early June. After harvest they stay on alternative hosts and in October-November fly back to the winter host where they lay their eggs. The aphids are highly procreative; one female can give birth to approx. 50 or more young. After approx. 2 weeks these aphids will be procreative themselves. The virgin birth has special importance for the rate of propagation as all individuals are exploited as dams. Climatic requirements: The aphids thrive best in hot and dry years. A generation develops in 10 days at 15-20°C. Low temperatures slow down the development. Strong rain can kill many aphids. Strong winds and very high temperatures inhibit aphid development.

Control measures

Natural enemies: Ladybirds are the best known. Among other enemies are flower flies, ground beetles, green lacewings, bugs, parasitic wasps and parasitic fungi. The natural enemies can reduce the aphid population significantly but often do not appear in high numbers until late in the growing season. Conditions influencing propagation: Late cultivars are more exposed to attack than early cultivars. High levels of nitrogen promote aphid development. Fungus control is favourable to the aphid as this keeps the plant material fresh and green for a longer period. Forecast service: Aarhus University, Faculty of Agricultural Sciences and the Danish Agricultural Advisory Service issue forecasts. Keep informed on the Internet under Varsling/Registreringsnet (= Warning/registration network developed). Chemical control: Use the control guidelines in Crop Protection Online. The specific aphid insecticides should be used because they spare the beneficial animals. Aphids should not be controlled preventively as this involves a risk of renewed propagation. Very severe attacks can be difficult to control through pyrethroids. In case of treatment with pyrethroids application must take place outside the bees’ flying hours (that is, spray from 9 p.m. to 3 a.m. Danish standard time). When you choose treatment technique the position of the aphids should be considered. In barley it is necessary to drive slowly and use a large amount of water as the aphids sit in the bottom of the crop. In wheat the aphids are easy to control as they sit in the ears.

Names and distribution

English: Different species: Grain aphid/Bird cherry-oat aphid/Rose-grain aphid in cereals. Other names: Aphids in cereals. Systematics: Order Hemiptera (true bugs), Family Aphididae (aphids). Distribution: Aphids are found in most parts of the world. In the tropics and subtropics, however, Bird cherry-oat aphid and Rose-grain aphid are only found in high-lying areas with a cooler climate. In Denmark the Bird cherry-oat aphid is most common on barley and the Grain aphid on wheat.

Hosts and importance

Up to 30% yield loss in cereals due to aphid attacks have been reported. The aphids cause damage by sucking just as secretion of honeydew makes the leaves sticky and reduces the yield. Bird cherry-oat aphid can cause large yield losses in spring barley and Grain aphid in winter wheat. In non-Danish trials a reduction in protein content, sedimentation value and germination capacity has been observed. A reduction in grain size, germination capacity and extract yield has been measured in spring crops. The aphids can transmit the virus disease barley yellow dwarf virus, which is not normally found in Denmark. Hosts: All cereals as well as cultivated and wild grasses act as hosts. Maize and dicotyledonous species such as shepherd’s purse, iris and spurrey can also be attacked. Bird cherry-oat aphid has bird cherry as its alternative host, and rose-grain aphid overwinters on wild roses; however, not on dog roses.

Charactaristics

Both winged and wingless aphids are found. Aphids have 3 pairs of legs, clearly visible antennae and cornicles that stick out from the abdomen. The bird cherry-oat aphid is approx. 2 mm long and a little broader or more drop-shaped than the grain aphid and the rose-grain aphid. Its colour is green or brownish green. A reddish brown area is visible on its abdomen; from this area the relatively short cornicles stick out. The length of the antennae is a little more than half the length of the body. The grain aphid is approx. 2-3 mm long and varies very much in colour: yellowish green, green, red or reddish brown, brown or brownish black. The top side of the abdomen is dark. The completely black cornicles are a characteristic feature. The antennae have approximately the same length as the body. The rose-grain aphid is 2-3 mm long and usually yellowish green. A characteristic, darker green dorsal stripe is visible on its back. The antennae have approximately the same length as the body.

Names in different languages

Danish :	Bladlus
German :	Getreide-/Hafer-/Bleiche Getreideblattlaus
English :	Aphids
Latin :	Sitobion avenae
Norwegian :	Kornbladlus/Havrebladlus
Swedish :	Sädes-/Havre-/Grønstrimmig gräsbladlus

Bird cherry-oat aphid

Grain aphid

Rose-grain aphid

Grain aphid in a wheat ear

Parasitised grain aphid

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