Aphid control and BYDV prevention

A presentation at the ITLUS conference last December from Dr Louise McNamara from Teagasc, Oak Park, stated findings and recommendations from past and recent research on barley yellow dwarf virus (BYDV) and aphid control.

Control of aphids is important, especially in terms of the prevention of BYDV in cereals. BYDV can be spread by winged and unwinged aphids which become virus carriers through feeding on infected plants. These infected winged aphids then carry the virus to the next landing site, which they may have no control over. Flight patterns are influenced by temperature and wind direction and field orientation can then affect where that aphid (which may be more blown around than flying) actually lands.

Grain aphids of different sizes and colours and a mummified carcase towards the bottom right of the leaf.

Aphid control has become a less certain process since the announcement a few years ago that a proportion of the grain aphid population, the main species which carries the MAV strain of the virus, carried partial resistance to the commonly used pyrethroid insecticides. This is known as knock-down resistance (kdr) but, while the term carries the word “resistance”, it is widely believed that a proportion of aphids which carry kdr can still be controlled by full rates of a pyrethroid spray.

Kdr cereal aphids in Ireland carry a single recessive gene for resistance. Aphids carrying a double recessive gene, which would confer full resistance, have not yet been detected. That said, there have been instances where aphid populations have not been adequately controlled in the field using pyrethroid sprays and further Teagasc work has since shown that a second resistance mechanism is also present in Irish grain aphid populations.

This was confirmed at the Tillage conference earlier this year following work presented by Lael Walsh, Walsh Fellow at Ashtown (EPIC project). She tested a number of populations of grain aphids known to carry the kdr trait. The testing confirmed that the populations tested have a range of sensitivity to pyrethroids.

She concluded that these resistant populations were also exhibiting metabolic resistance whereby the aphids had developed an improved capability to detoxify the insecticide in the gut before it was taken into their circulation system.

So we now have two resistance mechanisms to cope with. Kdr is still a huge worry due to the lack of alternative chemistry to help protect the pyrethroids and this is further exacerbated by the probable loss of Redigo Deter as an insecticidal seed dressing from next year onwards.

Understanding the aphid

Control of aphids is an increasing challenge. Louise emphasised that we must utilise all possible husbandry and cultural methods to help minimise pressure from the virus by doing all we can to favour the crop while disfavouring the aphids.

To help do this, a greater understanding of the aphids and their life cycle is important. At a recent ITLUS workshop on crop pests, Louise explained the complexity of aphid biology and lifecycle and this helped outline the challenge involved in controlling them. Suffice it to say that reproduction is complex in aphids.

Aphids can alternate sexual and asexual production. The latter is very much about rapidly producing a live copy or clone of itself. These are exact copies so if one is resistant to anything, all the offspring are too. These clones or nymphs can be born with babies inside of them so the process can get into exponential growth very quickly when mild weather and especially temperature permits.

The sexual phase is about two different aphids crossing like any other animal so the offspring is different to the parents. They can produce either live young or eggs which can be useful for overwintering. Most aphids are born wingless but at certain times a proportion is born with wings to help spread the colony. It is thought that the trigger for this may be related to the density of the specific colony and the quality of the host plant.

Aphids feed by virtue of highly specialised mouth parts called stylets. These can penetrate deep into the leaf and tap into the phloem from which they extract plant fluids. They feed on the amino acids and excrete the sugars which form the sticky honey dew substances associated with aphid feeding. But as well as feeding they can also inject viruses into the plant if that aphid is infected from elsewhere.

Virus strains

There are a number of strains of the virus that cause trouble and the spread of these tends to be specific to individual aphids. For example Louise said that the common mild strain of the virus – the MAV strain – is spread by both the grain aphid and the rose grain aphid. The bird cherry aphid is the carrier of the very severe RPV strain (also known as cereal yellow dwarf virus). And both the grain aphid and the bird-cherry aphid are carriers of the PAV strain

These are the three main cereal aphid species that we worry about in Ireland from a virus perspective. A small number of winged aphids can carry the initial infection into a crop to individual plants and then local propagation and spread generate the infected patches.

Table 1: Aphid recognition

Know the important aphids

The individual species are named based on where they have been found to overwinter. The rose grain aphid is identifiable by its rosy bum and short siphunculus and short cauda. It can overwinter on roses and also as adults on grass.

The bird-cherry aphid mainly overwinters on bird-cherry trees either as eggs or adults or both.

The grain aphid always overwinters on grasses (including cereals) and mainly as adults but some eggs can also be present.

Grain Aphid

Aphid flight from BYDV infected sources is the main vehicle for virus spread over bigger distances. Flight, or the production of winged aphids, is influenced by a number of factors including day length, temperature, colony crowding and host plant crowding. When these conditions occur, winged aphids are produced and these can then fly considerable distances with the help of air currents.

Once an infected aphid lands and begins to feed the virus is then spread into the host plant. Subsequent infection or secondary spread then happens from that initial point of infection. As over-crowing takes place on a leaf, individuals fall off and climb up on to an adjoining plant or tiller. Hence the patch.

Because environmental conditions are so important in the movement and feeding of aphids, we can know that early planting in autumn coupled with mild conditions increase the infection risk from BYDV. Early infection is most severe and there is no threshold for treatment because very small numbers can bring about the initial infection points. These can then grow into patches of infection if the aphids are not controlled in some way.

In general aphid activity tends to be lower from November until the end of March when it is generally colder and so crops emerging in that time window tend to have lower infection risk. But planting prior to October in the autumn or after April in spring tends to be associated with much higher risk of BYDV infection. Infection risk is predominantly a temperature issue.

Monitoring movement

Aphid numbers are counted weekly on a continuous winter barley crop (untreated with insecticides) in Oak Park to give an indication of the build-up in field conditions. It is interesting to see the way numbers can surge at unexpected times during winter in particular.

This can be seen in Figure 1 which shows the number of aphids found during counting from October to February inclusive over the past two winters. These numbers can show occasional weekly surges which may or may not have any impact on the infection or spread of BYDV in cereal fields. Still, it is interesting and perhaps worrying to see the many surges in Figure 1 that occurred from December to March this year.

A quick glimpse at Figure 1 shows a significant surge in numbers during December last year and also the peaks in January this year and last. It seems likely that the build-up in January 2017 may be the reason why there was such good yield responses in 2017 from the application of an insecticide later in that month.

The traditional monitoring work at Oak Park involves using a suction sampler to catch the aphids on the crop. While this is relevant, the major risk from BYDV comes from aphids that fly into the crop at different times in the season because this is how virus is spread initially. Once a winged aphid arrives, it will then begin reproduction to build a colony which will spread to adjoining plants as numbers build.

So monitoring of winged aphids in flight is now being developed at Oak Park. A series of such trapping devices around the country could greatly improve our ability to detect BYDV risk more accurately. Louise said it is possible to build a suction sampler, like those in the UK, which will both catch aphids flying at a height and also conduct an ELISA test to detect the presence of a virus.

It would appear that the ability to have this technology in Ireland would greatly improve our ability to monitor and react to any real threat of virus transmission and potential for infection.

A typical BYDV infected barley leaf.

Using IPM

The use of integrated pest management (IPM) should always be employed and maximised for aphid control. This will involve using species which parasitise or eat aphids to help decrease their numbers. However, given that the natural predators of aphids are also temperature dependent for their multiplication, Louise suggested that natural predation of aphids by things such as ladybirds, hover flies, parasitic wasps etc may be more effective in summer when high aphid numbers can be found actively feeding on plants compared with catching a small number of aphids closer to winter.

While we occasionally see large number of aphids feeding on our crops, we can only imagine what these numbers would be if nature did not provide us with their natural enemies. At the ITLUS workshop in February, Robin Earl, Teagasc Walsh Fellow, gave us a little insight into some of these natural enemies and the feeding activity they can engage in.

The parasitoid wasp is a bit more like a flying ant than a wasp but it is a cunning predator. There are more than 6,000 species in the UK. Parasitoids are important in the control of most pests. The wasp appears to sting the aphid but what it actually does is lay an egg inside a live aphid where it hatches and feeds on the living aphid until it eventually grows to kill its host, climb out and leave a mummified carcase of its host behind.

The hoverfly is much more wasp-like in appearance and there are more than 260 different species in the UK, Robin reported. Aphids are the predominant prey of species with predatory larvae and each can consume up to 1,200 aphids.

There are 53 different species of ladybird in the UK and five are important predators of aphids. Their most common food is aphids and scale insects, although a few species feed on fungi and the 24-spot feeds on plants.

The lacewing is another ferocious aphid consumer. It is estimated that a single lacewing can consume about 1,500 aphids. Their effectiveness in controlling aphid outbreaks is, however, uncertain.