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Corn earworm

Helicoverpa armigera

Other common names: Heliothis, Tomato grub

Caterpillars of the corn earworm (or cotton bollworm) are the major pest of cotton, but can also be problematic in pulses, canola and occasionally cereals. They are more common in the northern or coastal regions of eastern Australia, particularly in warmer regions. This species has developed high levels of resistance to insecticides and management needs to draw on biological and cultural options and the use of appropriate insecticides. 

The corn earworm is a common and widely distributed pest that has been recorded from all states and territories within Australia. They are more common in eastern Australia, particularly in warmer regions such as northern New South Wales and southern Queensland. In cooler regions such as Victoria, South Australia and Tasmania they are generally only problematic in summer, and are most commonly associated with irrigated horticultural crops such as tomatoes and sweet corn. They are occasionally found in dryland broadacre crops such as winter canola and wheat or summer forage brassicas.  

Corn earworm is a warm season pest with a wide host range particularly cotton.

Adult moths have a wing-span of 30-45 mm and are light brown to red-brown in colour with numerous dark spots and blotches. The hind wings are pale with a dark band along the lower edge. The most obvious characteristic is a small pale patch within this dark band.  

Corn earworm adult; note the pale patch in dark band of the hind wing (Source: cesar)

Larvae reach 40 mm in length and there can be substantial colour variation that includes shades of brown, green and orange. All colours usually have darkish strips along the body and bumpy skin with sparse stiff stout hairs. They have a ‘saddle’ on the 4th body segment and have a sharp downward angling at the rear of the body. Larvae have white hairs around the head and black hairs along the body.

Corn earworm larva (Source: cesar)

Distinguishing characteristics/description of corn earworm (note the light patch in the dark band of the hind wing of the corn earworm (Source: Bellati et al. 2012)

Corn earworm generally has 4 to 5 generations per season. Larval activity increases in warmer conditions, and larval activity and feeding cease when temperatures fall below 12°C. Corn earworm populations mostly arise from agricultural and horticultural industries, not from native non-crop plants.

Lifecycle, critical monitoring and management periods for the corn earworm (Source: cesar)

Corn earworm larvae are rather aggressive, occasionally carnivorous and may even cannibalise each other. If disturbed, they fall from the plant and curl up on the ground.

Caterpillars and moths are similar to the native budworm and lesser budworm. They can only be distinguished using a microscope. They may also be confused with armyworms and cutworms, although these have smoother bodies with fewer distinct hairs. 

Corn earworm has a wide host range and can attack all field crops, particularly cotton, sorghum, maize, sunflowers, chickpeas, lupins and lucerne. The species is also occasionally found grazing on wheat and barley heads. They can cause damage to foliage, flowers and pods on canola.

Larvae attack all stages of plant growth. Feeding on stems and leaves is rarely extensive. Larvae can damage seedlings by eating terminal buds but in more advanced growth, larvae will attack all above ground plant parts, including pods and seeds. Most damage occurs in late spring and summer. Corn earworms are at their most damaging when they feed on the fruiting parts and seeds of plants. Holes or chewing damage may be seen on pods and/or seed heads. In cotton, the main damage is to squares, flowers and bolls.

Losses attributed to corn earworms come from direct weight loss through seeds being wholly or partly eaten. Grain quality may also be downgraded through unacceptable levels of chewed grain. Caterpillars eat increasing quantities of seed and plant material as they grow. The last two growth stages (5th & 6th instar) account for over 90% of their total grain consumption. 

Because corn earworm moths are less inclined than native budworm moths to undertake long-distance migratory flights, problems in spring and summer generally arise from local populations that survive over winter as diapausing pupae in winter crops.

Sweep netting is a quick and easy method to sample most crops. Monitor crops for activity by taking a minimum of 5 sets of 10 sweeps and calculating the average number of larvae per 10 sweeps. The use of pheromone traps (which attract male moths) can provide an early warning of moth arrival to an area or their emergence from local winter diapause. These should be set up in early spring. Visual sampling for eggs and larvae is the main technique used for routine sampling in cotton.

Canola:

• Flowering – podding: 5-10 larvae per m2 (Berlandier & Baker 2007), or >5 larvae (10 mm or longer) per m2 (McCaffery 2009)

Lupins (NSW):

• Flowering- podfilling:

o Albus lupins for human consumption: 1- 2 larvae per m2 (<5 mm long)

o Stockfeed: 1-2 larvae per 10 sweeps

Field peas (NSW):

• Flowering – podding:

o Stockfeed: >4 larvae (4- 9 mm) per 10 sweeps

o Human consumption: 1-2 larvae (4- 9 mm) per 10 sweeps

Chickpeas (NSW):

• 1- 4 per m2 (using a beat sheet) or 5 per m2 (using a sweep net). Use QDAFF formula to fine tune threshold.

Faba beans (NSW):

• 2- 4 larvae per m2 (less than 10 mm)

• Human consumption: 1 per m2

Cotton

• up to flowering, > 2 larvae per metre row.

• After flowering, a standard egg threshold is > 5 brown eggs per metre row. Thresholds for Bt cotton are higher (Bailey, 2007). 

Management Options:

There are many natural enemies that attack corn earworm. These include shield bugs, damsel bugs, assassin bugs, native earwigs, tachinid flies, orange caterpillar parasite, two-toned caterpillar parasite, the egg parasitoid Trichogramma wasps, lacewings and spiders. Naturally occurring fungal diseases and viruses also play an important role in some seasons.

In southern crops, windrowing canola or desiccating pulse crops such as field peas may be an option to advance the drying of crops when small-medium size larvae are present. Cultivating and slashing can reduce pupal and larvae survival.

In northern areas, chickpeas can be used as a trap crop to capture overwintering populations merging from diapause. Avoid successive plantings of summer legumes. Weed management in and around crops can prevent the build-up of corn earworm and other insect pests.

There are numerous insecticides registered against corn earworm. Control is complicated because field populations are resistant to endosulfan, synthetic pyrethroids and carbamates, as well as lower levels of resistance to organophosphates, spinosad and indoxacarb (there is a substantial literature on this, see References). In cotton, a comprehensive resistance management framework exists for growers to follow. 

Timing of chemical applications and coverage are critical. Target small larvae up to 7 mm in length and apply insecticides before larvae move into flowering pods. Soft options include the use of Bt (Bacillus thuringiensis) and nucleopolyhedrosis virus (NPV) based biological insecticides.

Corn earworm populations are resistant to several insecticides and control can be complicated.

This article was compiled by Paul Umina (cesar), Garry McDonald (cesar) and Sandra Hangartner (cesar).

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Baker GH, Tann CR and Fitt GP. 2011. A tale of two trapping methods: Helicoverpa spp. methods: (Lepidoptera, Noctuidae) in pheromone and light traps in Australian cotton production systems. Bulletin of Entomological Research 101: 9-23.

Baker GH and Tann CR. 2012. Mating of Helicoverpa armigera (Lepidoptera: Noctuidae) moths and their host plant origins as larvae within Australian cotton farming systems. Bulletin of Entomological Research 103: 171-181.

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Date

Version

Author

Reviewed By

Feb-2015

1.0

Paul Umina (cesar), Sandra Hangartner (cesar) and Garry McDonald (cesar)

Bill Kimber (SARDI)

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