The biology and control of cereal aphids

Background.
This review has been prepared by authors from the main organisations which have been involved in research into cereal aphids in Great Britain. Recent research findings are summarised and recommendations made for improvements in current aphid control practice and for research in areas where further opportunities are identified. The review is restricted to problems of direct aphid damage to cereals in the summer and does not consider the aphids role as vectors of barley yellow dwarf virus.

Surveys of aphid incidence have confirmed that aphid problems are mainly confined to 'outbreak years' with few, if any, fields requiring treatment in the intervening years. Aphids can increase in number very quickly in suitable conditions and in these outbreak years sprays are often applied too late to prevent yield loss, which mainly occurs between flowering and the milky-ripe stage. ADAS advice has been to check crops during early flowering and spray if 5 or more aphids per ear are found; further checks are advised later if conditions are suitable for aphid increase. Counting aphids has proved too unpopular with farmers and, following a detailed analysis of aphid survey results, a threshold of two-thirds of ears infested was substituted in 1988. The University of Southampton has made a simulation model available through PRESTEL which allows more refined decision making based on simple aphid assessments and other factors.

Either of these systems would improve decision making greatly if more widely adopted; at present neither is used in most cases and sprays are still applied too late to be effective. This has led many independent consultants and farmers to the view that it is preferable to apply a cheap aphicide in tank-mix with a fungicide routinely during ear emergence as a precaution. In non-outbreak years, whilst cheap, this practice is unlikely to be profitable and due to the broad spectrum of activity of the insecticides used, many beneficial insects are killed leading in some cases to additional pest outbreaks. The present situation is, therefore, considered unsatisfactory in both cases, usage of aphicides being too late in outbreak years, and too great in between.

Biology.
The life cycles of the two principal pest species differ, the grain aphid (sitobion avenae) spending the winter actively feeding on cereals and grasses whilst the rose-grain aphid (Metopolophium dirhodum) overwinters as eggs on roses. The forecasting and monitoring of the two species, therefore, requires different approaches and farmers should make a clear distinction between the problems posed by them.

Deficiencies in the knowledge of the underlying mechanisms of varietal resistance to aphids are identified and recommendations made for further research. Such resistance has been identified in wheat, but clarification of the mechanism is required to enable exploitation by plant breeders. The genetics of aphids are also poorly understood and fundamental studies are required to determine clonal stability and the potential within populations to overcome varietal resistance and develop resistance to insecticides.

Natural enemies.
The role of natural enemies in restricting aphid outbreaks has received much attention. In addition to specific aphid predators, parasites and diseases, the greater effect of general predators, such as ground beetles and spiders, in preventing outbreaks has been recognised. However, no simple method of quantifying predation in the field has been developed and an evaluation of the importance of natural enemies has not been possible. Wide variations in levels of predation occur from field to field and the underlying causes for this have been investigated. This has provided the opportunity for influencing the degree of predation and experiments are in progress on the enhancement of natural enemy levels in fields by the creation on non-crop habitats. This work is considered timely and worthy of continuation.

Recent research has demonstrated the feasibility of using low rates of aphicides to limit aphid numbers without harming natural enemies and thus enhancing the relative rate of predation. Further testing of this approach is considered high priority and a proposal for further work is made.

Monitoring and forecasting.
At present aphid flights are monitored by the Rothamsted Insect Survey (RIS) network of suction traps and populations in fields are regularly monitored by staff of the IACR and ADAS. Warnings are issued in the RIS Aphid Commentary and in ADAS Crop Intelligence reports. Possible improvements in the aphid monitoring and forecasting systems currently in use are identified. These include simplification of field assessment methods, development of a greater understanding of the reasons for, and sources of, aphid outbreaks and the assimilation of the data sets required for this purpose. Much of this work is in hand as part of existing research but will need sustained funding to achieve the desired objectives.

Damage assessment.
Aphid feeding has been shown to reduce the flow of assimilates to the developing grain, and the principal cause of yield loss is from reduction in grain size. The bread-making quality of the grain is also reduced. However, since significant reduction of yield occurs at lower aphid densities than would affect quality, decisions on spray application can be based on potential yield reduction alone.

Conclusions.
Simulation modelling of aphid population build-up and the damage caused has shown that the critical factor in aphid control is timeliness, both in decision making and in spray application. This requires regular field monitoring to detect changes in aphid numbers, an exercise which can be expensive in time or cost of field walking. To overcome this difficulty, a system of "directed field scouting" is proposed whereby the available forecasting methods are used to tell farmers when and where to asses aphid numbers providing rapid decision making guides in the process. Payment for this system would involve a direct charge for access while a greater uptake of the information should ensure its continued provision and development.

The review team proposes that this system forms the best base for the future adding the use of resistant, or partially resistant, varieties and the substitution of low-dose selective aphicides for broad-spectrum insecticides to enhance natural controls. Natural enemy levels could be further enhanced by the provision of suitable habitats. None of these measures involves a significant increase in cost to the farmer; overall the total cost of controlling aphid damage should be substantially reduced. The current cost of poor control practice to the industry fluctuates widely according to aphid numbers. However, taking all factors into account, the cost, including yield losses where control is not achieved, is estimated to vary between £2 million and £24 million at present day values with a median of about £5 million.

The following recommendations are made for further research:

The value of reduced-rate, selective pesticides requires testing with a view to replacing out-dated, broad-spectrum insecticides. The following components should be investigated:
I. The effectiveness of control.
2. The implications on development of insecticide resistance.
3. The economics and effects on yield and quality.
4. The effects on natural enemies and any enhancement of their role.

The initial evaluation should be on winter wheat, and an extension to spring wheat and other cereals could be considered later.