A guide to corn earworm from Simon Preece of DuPont Pioneer.

One of our maize trials under film in Devon showed signs of corn earworm activity. The larvae were found on about 1% of plants feeding on the silks and to a lesser extent on the leaves.

Historically, corn earworm has not been a significant pest in the UK. These notes are provided for information and not as a suggestion that treatment is required.

South West Farmer:

Corn Earworm larvae feeding on maize plants in Devon this year

Helicoverpa zea is commonly known as the corn earworm. The larva of the moth is a major agricultural pest in warmer areas of the world. The caterpillar feeds on a range of plants during the larval stages and the species has been given many different common names including the cotton bollworm and the tomato fruitworm. The species migrates seasonally, at night, and can be carried downwind up to 400 km.


The eggs are laid individually on leaf hairs and corn silks. The eggs are initially pale green in colour but over time they turn yellowish and then grey.


Following hatching, larvae feed on the leaves and the reproductive structures of the plant and usually develop through four to six instars (stages between two moults).

Holes can be found on the leaves and the excretions of the larvae can be found on the leaves and whorls. The larvae can be quite varied in their colouring ranging from light green to more brown versions.

The caterpillar can be distinguished from cutworms by the pale coloured head. Where the larvae feed on the silks then fertilisation of the kernels will be impaired and the ears will show irregular kernel placement and areas of the ear with no kernels.

Corn earworm is a particular problem in sweetcorn where insect damage makes the product unsalable. Corn earworm caterpillars are very cannibalistic and it is therefore rare to find more than one caterpillar on a single corn ear.

South West Farmer:

Showing the larvae excretions in the leaf axle of the maize plant

South West Farmer:

Later in the season the larvae feed on the kernels; the damage can lead to some late season disease on the damaged ear.

Photo: Pioneer Web Site


Mature larvae migrate to the soil. At 5 to 10 cm below the soil surface they pupate for 12 to 16 days. Pupae are vulnerable to freezing temperatures hence the lower populations further north and at altitude. Pupae can make use of diapause to wait out adverse environmental conditions, especially at high latitudes and in drought conditions.


Adults have forewings that are yellowish brown in colour and have a dark spot located in the centre of their body. The moths have a wingspan between 32 to 45mm and live over thirty days in ideal conditions. They are nocturnal and hide in vegetation during the day. Females can lay up to 2,500 eggs in their lifetime. In areas where the insect is particularly damaging pheromone traps are used to capture male moths in order to monitor levels of infestation.


In the USA insecticides may be used where damage is severe or where pheromone traps indicate high adult moth numbers. Insecticides are usually applied to foliage in a liquid formulation with particular attention to the ear zone, because it is important to apply insecticide to the silk. Corn earworm has become resistant to some insecticide active ingredients. Susceptibility to Bacillus thuringiensis also varies. In high infestation areas deep ploughing is conducted in order to disrupt the overwintered pupae.

Natural predators

The parasitic wasp and green lacewings are two beneficial insects that help to control corn earworm. The parasitic wasp lays its eggs inside the earworm's egg while green lacewings eat the larvae of the earworm.


The presence of corn earworm larvae observed so far is not at a level that will have significant impact on the crop yield. If the warm weather continues then there is the possibility that numbers could increase with further in-migration of the adult moths or the emergence of a second generation later in the season when the larvae feed on the kernels and will be hidden by the wrapper. It is unlikely that infestation will reach levels where treatment is economically justified.