Wheat Bulb Fly

Key Points

• Wheat Bulb fly can be a devastating problem
• Attacked crops are unlikely to yield their full potential
• Badly attacked crops could need re-drilling
• Dursban WG eliminates the pest before it damages plants

Introduction

Wheat Bulb fly — Delia coarctata — is a small fly which lays its eggs in exposed soil in July and August. These eggs hatch from January to March the following year, depending on soil temperatures. Once hatched the larvae move through the soil and bore into the base of cereal plants feeding on the central shoot. This causes the characteristic dead-heart symptoms to appear. Although the outer leaves remain green during early stages of attack plants become dull in appearance and, unless examined carefully, the attack may go unnoticed until dead-hearts become visible in February or March.

Winter wheat, winter barley, rye and early sown spring wheat can all be attacked. Crops that have not started to tiller before they are attacked can be destroyed completely. Spring wheat and spring barley drilled December to March can be killed before emergence. Traditionally the main risk area is eastern England where Wheat Bulb fly is a major pest of winter wheat. Yield losses depend on pest numbers, plant populations and other factors, but a guide is a potential loss of 0.7 tonnes per hectare if 20% of plants are attacked.

Biology

Adult Wheat Bulb fly are small, weakly flying insects similar to small houseflies. They are normally present from mid-June to September and lay eggs on bare soil between mid-July and early September. Eggs need cold weather to break diapause and hatch in late December and January.

Larvae are white and legless with obvious black mouth hooks at the pointed head end. The rear end is blunt and carries six lobes. They burrow into the base of cereal plants and move upwards. Wheat Bulb fly larvae grow very quickly in March and April and can reach 12mm in length. These larger larvae then move to adjacent shoots causing further damage. Feeding ends in early May and larvae then pupate in the soil. Approximately 40% of eggs laid result in larvae attacking shoots. There is only one generation of Wheat Bulb fly per year.

Attacks are worst where there is bare soil, especially if freshly cultivated, for egg laying during July and August. Wheat crops following fallow or ploughed set-aside, potatoes, vining peas, sugar beet and onions are most at risk. Wheat Bulb fly do not move very far and tend to predominate in the eastern counties of England where crop rotations particularly suit their life cycle. However, attacks do occur in eastern Scotland, Lancashire and the Cotswolds.

To minimise the risk of attack from Wheat Bulb fly avoid leaving bare soil in July and August. Rotational choices could include drilling winter oilseed rape after set-aside or switching to spring sown cereals. If winter cereals are drilled late consider the use of an appropriate seed treatment and avoid drilling too deeply.

Product Choice

The first action must always be to carry out a Risk Assessment. This can be done using the Dursban* WG Wheat Bulb Fly Risk Assessment Chart (627KB PDF). Depending on the outcome of the Risk Assessment, Dursban WG should then be applied at the rate of 1.0kg/ha in 200 – 1000 litres of water per hectare.

There are two main types of insecticide used to control Wheat Bulb fly:

• Egg hatch sprays are applied as soon as eggs hatch, but before plant invasion has occurred. Dursban WG is an egg hatch spray.
• Dead-heart sprays are applied after the larvae have invaded plants. Dimethoate is an example of a dead-heart spray.

In some cases a dual approach must be adopted. This is especially useful in years where pest populations are particularly high or in situations where egg hatch is protracted. A repeat application of Dursban WG may be required in high risk situations and on soils with high organic matter content. Later applications of Dursban WG, once an attack is established and dead-hearts are visible, will be less effective. Under these conditions an approved systemic insecticide such as an approved formulation of dimethoate should be used.

Dursban WG provides good control of Wheat Bulb fly and protects crops from attack. Applications at egg hatch, before larvae have entered the plant, means damage is reduced by not having to wait for dead-hearts before deciding to spray. Dursban WG has a residual life in the soil which means that most larvae will be controlled as they hatch over a period of time.

Always check the Cereals Pestwatch pages for the most recent bulletins.

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