Impact of changes in arable agriculture on the biology and control of wheat bulb fly

Abstract

Wheat bulb fly is one of the most serious insect pests of winter wheat in Britain. The pest is currently restricted to the eastern counties of England and Scotland. Eggs are laid in July and August in bare or partially covered soil. The eggs hatch in January, February and March. Winter wheat is often attacked when sown after crops such as potatoes, sugar beet and vining peas. Wheat after fallow and fallowed set-aside is also at high risk of attack. Each larva enters a host plant below the soil surface at the basal node to bore upwards and feed within a leaf shoot. Larvae migrate between shoots and plants while completing three larval instars (stages). Infested shoots turn yellow, wither and die to produce deadhearts. Pupation occurs in the soil between mid-April and mid-May and adult flies emerge in June. The severity of attack varies considerably between years and is mainly dependent on the number of eggs laid in the summer and the interaction of larval attack with crop growth and development during the winter.

A number of factors influence yield loss by wheat bulb fly. Generally, crops sown before the end of October tiller well and tolerate pest attack better than those drilled in November or December. However, damage tends to be worse in cold winters, owing to slower crop development and a reduction in the capacity of the crop to compensate for loss of plants or tillers during the winter. Previous cropping, the availability of exposed soil and weather conditions during the egg-laying period also influence the risk of attack. Highest egg numbers occur in fallow or in the large areas of soil exposed in open-canopy or low-standing crops such as potatoes or sugar beet. A rough and freshly cultivated soil surface appears to be preferred for egg laying. Hot, dry conditions during the egg-laying period of July and August reduce the number of eggs laid.

Egg numbers and the associated risk of attack vary considerably between fields and between districts. The potential risk of wheat bulb fly attack can be estimated reliably at field level only by taking soil samples in the autumn to extract and count the number of eggs. Water and soil traps have also been used to provide an early warning of potential egg numbers. As an alternative to sampling or trapping, a regional forecast of attack has been developed using a multiple regression model to predict egg numbers.

There are now fewer insecticides available for control of wheat bulb fly than at any time over the last decade and all are broad-spectrum organophosphorus products. Concern over the general environmental safety and toxicity of organophosphorus insecticides may, in future, threaten the continued approval and use of these chemicals. The current range of insecticides should be safeguarded by fostering a rational and precise approach for their use.

Seed treatments (fonofos and chlorfenvinphos) are widely used as a low-cost preventive measure when crops are sown after mid-October. Efficacy is reduced by earlier sowing. Insecticide sprays may be applied at egg hatch to kill larvae before they enter plants (egg hatch spray) or after larval invasion when the symptoms of attack become visible (deadheart spray). Individual insecticide applications provide only partial control of the pest. When damaging attacks are expected, various control strategies, combining several types of insecticide, have been shown to be cost-effective, although none is fully effective in completely eliminating damage. Research and development of integrated control strategies, utilising cultural and biological techniques in addition to insecticides, may improve the relatively poor standard of control often obtained solely from chemical control.

There are various natural enemies of all life stages of the pest, many of which may be exploited in future as biological control agents. Adult wheat bulb flies are infected by a fungal parasite (Entomopthora spp.) and may also be preyed upon by certain flies, spiders and birds. Eggs and pupae are destroyed by several species of predatory beetle. Pupae are also attacked by parasitic staphylinid beetles (Aleochara spp.). Levels of natural mortality can be high. Many larvae die between hatching and plant invasion. A greater understanding of the factors affecting larval survival could, therefore, contribute to new or improved methods of control.

Cultural control of wheat bulb fly is often overlooked but there are a number of options. Sowing date, depth of drilling, seed rates, cultivations, trap fallows and crop rotation can all be used to reduce egg numbers or the crop's ability to tolerate larval attack.

Various other methods of control have also been studied. Attempts have been made to control populations over a large area, using either insecticides or cropping restrictions. Control of adult flies with insecticides at their emergence sites before they lay eggs is another strategy but is environmentally undesirable. Further research of environmentally benign insecticides or biocontrol agents could prove useful for this purpose. Research into larval host location may enable the disruption of this process to prevent plant invasion. There are also a number of beneficial invertebrate species which could be investigated as candidate biocontrol agents. There is potential to develop an integrated pest management (IPM) system for wheat bulb fly by unifying existing knowledge of cultural, biological and chemical control measures.

In the current economic conditions, it is prudent to examine the cost-effectiveness of wheat bulb fly control. An analysis of insecticide experiments has shown that chemical control is cost-effective above a treatment threshold of 2.5 million eggs/ha. There is scope to increase the efficiency of egg hatch sprays and deadheart sprays by improving the accuracy of spray decision making and timing of application. Research into enhancing the efficacy of dimethoate, the only remaining deadheart spray, may limit the number of applications needed to achieve acceptable levels of control.

The resurgence of summer fallowing in the management of set-aside has caused widespread concern about the risk of wheat bulb fly attack and the potential of an overall increase in the frequency of the pest. Results of monitoring suggest to date that set-aside fallows may cause localised increases in wheat bulb fly populations in districts currently considered to be at marginal risk. Experiments are also underway to investigate cultural control measures against the pest in set-aside fallow. The possible increase in wheat bulb fly populations resulting from set-aside fallowing will heighten the need for further research into cost-effective and environmentally acceptable forms of control.

An improved understanding of crop growth and development in relation to the severity of wheat bulb fly attack could reduce expenditure on insecticides. Plant growth models are likely to become increasingly important in determining the ways in which pests affect plant growth, development and yield. Precision farming provides the opportunity to adjust inputs according to the spatial distribution of factors within a field and so potentially reduce pesticide use. Decision support systems and geographic information systems should also contribute in the development of precision farming. A prototype expert system for the management of wheat bulb fly control has been developed with HGCA funding. This system could be refined by inclusion of GIS technology and predictive models to estimate the distribution and severity of the pest.

There is considerable concern over misuse of pesticides. In particular, the potential for residues in food, effects on non-target organisms and the development of insecticide resistance have stimulated a move towards a rational approach to pesticide use. Pressure for the adoption of environmentally friendly integrated control strategies will increase and their development should be a major research priority.

There is a dual need to reduce reliance on insecticides and improve the cost-effectiveness of wheat bulb fly control. Suggested priority areas for further research include alternative control strategies such as biological control, control of adult flies, population control incorporating trap fallowing, varietal tolerance and interference with host location. Decision support systems and predictive models also have a further role in forecasting damage and technology transfer to the farmer. Most importantly, improvements gained in individual methods of chemical, cultural or biological control require unification in a practical and fully researched integrated pest control strategy which may be readily adopted by the agricultural industry.