Primary symptoms

Healthy crops are more tolerant to pests and remain more pest-free.

Insects cause physical damage (above) and some help spread plant diseases.

Summary

Worldwide hundreds of insect species feed on wheat. Some are adapted specifically to wheat and the conditions in which it has been grown historically. They are generally in balance with the system. But with the spread of wheat into new regions in recent years, particularly regions with hotter, wetter and more humid conditions than traditional areas, populations of many new and old pests have erupted into significant outbreaks. It will take time for predators to build up to control pest levels in these regions, and to develop varieties resistant to all pests. Until then, crop rotations, careful selection of planting time to put the crop out of step with the pest, the use of resistant varieties when they are available and chemical approaches to control are required. These break the cycles of the pests. This discussion will focus on a few of the problem insects in the new wheat zones. The pests described do not occur in all wheat areas.

Are above ground insects a problem?

• Are stems of plants neatly severed with consequent death of the spike? This could have been caused by the innocuous-looking sawfly (Cephus pymaeus)
• Are there sections of crop with extensive brown dried up patches. Look closely at individual plants. Are leaves and stems covered with aphids (greenbugs) and their sticky secretions?
• Look for signs of longitudinal yellowed streaks on the stem and leaves, severe rolling of the leaves and curling of the rachis. Unroll the leaves. Are aphids feeding there? These are likely to be Russian wheat aphid (Diuraphis noxia). Heavy infestations in dry seasons can cause 80% crop losses.
• Check the crop for areas of low vigour. Look more closely for dead areas on leaves and stems. This could be due to one of the suni or wheat shield bugs (Eurygaster integriceps) that extract fluids from leaves and stems, and from grains in the milk stage. Look for the tiny feeding nymphs.
• Are some spikes dead and white? Look for signs of stem borers, but be aware that crown rot disease and take all can cause white heads also.
• Are there areas of plants that are stunted and dull green, such as occurs after water stress? Pull back leaf sheaths and check there and at the base of stems for pale red or white larvae about 1.5 mm long. Check also for dark coloured 'flax-seed' puparia that can easily be pulled away from the plant. These are the stages of the hessian fly (Mayetiola destructor) that can cause large crop losses particularly in North Africa. The fly resembles a mosquito in size and shape.

Causes of insect problems

• Continuous wheat cropping or rotating wheat with alternative host crops allows insect populations to build up in the locality. What rotations are used in the area? Is it feasible to alter them to include a non-host crop?
• Are fallows used in the area? Were isolated self-sown wheat and barley plants rogued from these fallows? If not, these could be infested with Russian wheat aphid and be the source of the pest re-infecting cropped areas.
• Insects have been blown in or migrated from outside the region such as might occur with locusts. When were the symptoms first noticed and were they a problem last year?

What to do about insect problems

• Management: Healthy crops are more tolerant to pests and remain more pest-free. Follow good crop production practices to minimize problems. Chemical control, while effective, is often not economically feasible and if used imprudently may cause more problems than it solves. It should always be the last resort.
• Tillage and residues: Minimize sawfly and hessian fly problems by using recommended best practices for crop production for your area. Use clean tillage and ensure crop residues are fully turned in after harvest. For hessian fly, use resistant varieties when available.
• Timing: Check whether it is feasible to delay or advance planting to encompass a fly-free period.
• Monitoring: Aphids in Egypt and Sudan (probably oat bird-cherry aphid Rhopalosiphum padi and greenbug Schizaphis graminum respectively) can cause 20-30% yield losses if infestation is heavy. Monitor populations regularly. Once more than 35% of plants are infested, spray the recommended chemical. Control of lower infestations is not worthwhile.
• Variety: Russian wheat aphid does not proliferate in humid areas or where there is heavy rain. Wheat varieties that are resistant to Russian wheat aphid are available. These should be used in areas known to have the pest to reduce or eliminate the need to spray.
• Institutional and governmental support: Count feeding nymphs and adult suni bugs. When nymph numbers average 6-12/m2 or adults 2-3/m2 throughout the field alert the local authorities. Control programs by aerial spraying are often in the hands of government.
• Rotation: Use crop rotation to control stem borer. Generally infestations of stem borer cost more to control than the yield benefits from extermination.

NOTE. Litsinger & Barrion 1988 and Miller & Pike 2000 have more detail on the insects discussed and extensive information on other insects.