By Andy Doyle

For many years, we have controlled aphids in field crops with little need for caution on efficacy. But a presentation at this year’s Teagasc Tillage Conference indicated that caution is now needed.

Steve Foster from Rothamsted reported the presence of resistance in the grain aphid to pyrethroid insecticides and he stated that this has also been confirmed in Irish aphid populations.

The good news is that the reported resistance is only partial – full rates of individual pyrethroid products will still achieve full control, that is unless full-blooded resistance develops.

But the risk is real and states the need to sharpen up on application procedures to ascertain where aphicide is really necessary, as distinct to where application is mainly precautionary.

Pyrethroid resistance

First, the origin of resistance and how it is manifested. At the conference, Steve Foster gave a brief and simple summary of what is known about this resistance.

Basically, a change or mutation has taken place in the grain aphid population which makes these specific aphids less susceptible to pyrethroid insecticides.

This change has resulted in a reduced ability of these actives to knock down a population that is already present on the plant and this has become known as knock-down resistance (KDR).

This then confers a level of resistance in the aphids which have the mutation but this is not full-blown resistance. This means that the aphids which carry this KDR mechanism are still largely controlled by pyrethroid insecticides, providing full spray rates are used.

At the conference, Steve explained the genetic basis behind this resistance. In simple genetic terms, a genetic trait, such as this resistance, is severe when the aphid carries the trait on both of its pair of chromosomes, i.e. they have two copies of the resistant mutation (generally described as RR).

However, the KDR resistance is only present on one of the two chromosomes in all of the resistant aphids tested to date, so it is only partial resistance.

A normal susceptible aphid is designated as being SS, that is the susceptible gene is present on both chromosomes and it will be easily killed by an insecticide.

However, high-tech analysis has shown that in the aphids that are less well controlled, the resistance is only present on one of the two chromosomes, i.e. it is designated as SR (part-resistant but mainly susceptible).

While most of Steve’s work tested aphids from Britain, a number of Irish aphid samples were tested for the presence of KDR last year. Almost half of these samples, which were mainly from south Leinster and Munster, tested positive for the presence of the KDR mutation (SR types). So we are not immune from this problem and its potential consequences.

The great fear now is that a further change in the aphid population could result in the production of RR types, which would most probably be fully resistant to pyrethroids (as seen in other insect pests). However, the KDR resistance has been present in Britain for many years now and Steve has not yet managed to find samples of aphids carrying this double resistance (RR).

So, for the time being, the important thing is to ensure a full kill of the aphid population when spraying takes place. The use of full rates of pyrethroid is important and the advice is to continue to use these actives for now.

Altered behaviour

While reduced control is one of the consequences of the presence of KDR, Steve reported one other consequence also. It is not uncommon that mutations that carry resistance or reduced susceptibility to a product may be altered or compromised in some other way to reduce some of the new-found advantage.

In the case of KDR resistance, it may carry a behavioural handicap. Other work at Rothamsted showed that another aphid species carrying the KDR mutation appears to respond differently to threats of danger.

Aphids normally respond to a threat of danger by producing a substance which sends out a signal to warn all neighbouring aphids of a danger and they immediately begin to scatter away from the danger area.

However, Steve showed that peach-potato aphids which carry the KDR mutation do not heed this warning and this makes them more vulnerable to attack by natural predators, such as parasitoid wasps.

The fact that they do not move away makes it easier for these natural predators to more quickly knock down a greater proportion of this element of the population.

This could also be the case in grain aphids and is a potentially important finding. If there is increased susceptibility to natural predators, this makes it increasingly important to not damage these beneficial insect populations.

Presumably this characteristic would also make these aphids more vulnerable to attack by other predators, such as ladybirds, hover fly larvae, etc.

The use of blunderbuss insecticides that are less selective will also kill many of these predators. And while they would give good aphid control, the absence of predators may result in a faster return to high aphid numbers. And if RR types do emerge, they could more quickly become dominant in the total population in the absence of these natural predators. But for the time being, the presence of these predators in our crops helps lessen the need for chemical control and this could yet prove to be our closest ally for continued aphid control.

Population balance

The importance of minimising insecticide use can only be reinforced by the confirmation of resistance. And we must be increasingly aware of the important role that beneficials play.

At the conference, former Teagasc researcher Tom Kennedy presented information which showed the way aphid numbers evolve in the presence or absence of control. In this instance, plots were treated with chlorpyrifos, which is a much broader based insecticide and will kill predators as well as aphids.

The results are shown in Figure 1. The population trend is shown in the untreated plot where aphid numbers began to build in late May into early June and then fell again naturally, probably due to the intervention of natural predators. A further increase occurred in mid- to late-June.

In the chlorpyrifos-treated plot, numbers were knocked down to zero until early June following a 4 May treatment, but then they began to increase dramatically in the absence of natural predators which were also killed by the insecticide. The net result was that by 23 June the numbers were much higher in the treated plot than in the untreated plot.

The message from this piece of research is that we should intervene as little as possible and try to choose products that are less severe on natural predators which are vital to the subsequent suppression of any re-bounce in the aphid population.

And with a level of resistance now in the population, we need to give these natural predators every chance to pick up on any surviving aphids in the hope that we can continue to prevent the appearance of full-blown resistance.

However, this advice is likely to be more applicable where aphid feeding is the threat but will be less applicable for BYDV prevention.

BYDV prevention

In most years, the main target of aphicide application is the prevention of BYDV transmission by grain aphids.

At the conference, Tom Kennedy summarised many years of Teagasc research on BYDV prevention and this indicated that infection risk is most closely associated with the time of year. And he believes that the general findings over many years are still equally valid today.

A basic understanding of the biology of the aphid is important in this regard. The virus can be transmitted by infected aphids that have already fed on other plants that contain the BYDV virus.

This could be from infected crops, infected volunteer plants on stubbles or waste areas, other infected plants or from infected plants within the same crop. But if the aphid is not infected with the virus, it cannot transmit the disease. So overall infection risk is a factor of the presence of winged aphids and the presence of the virus.

Tom emphasised that it requires very specific conditions before aphids take flight. These include temperatures above 10°C to 12°C, wind speed less than 8km/h, brighter days from cloudy to sunshine (not dull) and with relative humidity below 70%.

So these are quite specific risk criteria and generally most likely in September / early October in autumn and from mid-May onwards in summer. This risk translates to early-sown winter crops and late-sown spring crops, Tom said.

This is important because it explains the basic recommendations for BYDV prevention that have been in use for many years, and may continue to apply in the future.

While our main risk is from the mild MAV strain of BYDV transmitted by the grain aphid (Sitobion avenae), and to a lesser extent from the rose grain aphid (Metopolophium dirhodum), this risk is much lower with early planting in spring and normal to late planting in autumn.

Applying aphicides

Planting prior to the middle of October and from the second week in April onwards is where the infection risk is consistently real, Tom said.

So winter cereal crops planted in September should receive an insecticide in mid-October with a follow-up insecticide in early November. An insecticide seed treatment can be used to replace the first spray in this situation. An aphicide is unlikely to be necessary where planting takes place after mid-October.

On spring cereals, Tom is of the view that crops planted prior to early April are unlikely to benefit from the application of an aphicide spray. However, crops planted from early April onwards are at much greater risk from BYDV infection and are therefore more likely to benefit from an aphicide spray. This should be applied at the four- to five-leaf stage of the crop.

A summary of aphicide recommendations are shown in Table 1. These are general recommendations and growers need to be aware that there may be exceptions. Risk from aphids may be greater at the shoulders of these times in warmer and coastal regions of the country. Aphid and BYDV risk is always greater when crops are sown into grassy stubbles or following grass. In these instances, greater care is necessary and earlier or alternative approaches might be considered.

Pyrethroid insecticides like cypermethrin, etc., can still be used for BYDV prevention at the appropriate timings, but full rates need to be used to ensure aphid kill.

Feeding damage

Occasionally, as happened in 2011, aphids can cause damage to yield resulting from the sheer weight of numbers feeding on the crop. This is normally an ear feeding issue on wheat but, as 2011 proved, it can be an issue with all crops.

The traditional threshold used here is an average of five aphids per ear on the wheat or if half the tillers have aphids present. This feeding can be significant and treatment is likely to be worthwhile.

Tom emphasised that pyrethroids should never be used as ear sprays and, as such, the options on wheat are dimethoate and pirimicarb (Aphox) but only pirimicarb is cleared for late use on spring barley.