Corn earworm is a significant threat to sweet corn production. It can damage the market product by feeding directly on it. It isn’t easy to control and is commonly found in high numbers at the end of the season. Pyrethroids are the primary insecticides used for corn earworm control. Still, there is growing concern that some populations have developed resistance.
Effective control programs are necessary to minimize damage, especially on late-season corn. Corn earworms are variable in color and hairy larvae ranging from yellow to green to red to brownish black. They may be found feeding in the ear tips after silking and are cannibalistic, with rarely more than one per ear or whorl.
Corn Earworm management in Maize
The life cycle of Corn Earworm
The corn earworm is a species of moth (Helicoverpa zea) that lays its eggs singly on leaf hairs and corn silk. The eggs are pale green at first and hatch in 3-4 days. The larvae are variable in color, with a rough texture due to micro spines, and feed on the plant’s reproductive structure.
They grow through 6 instars and develop in around 31-13 days, depending on temperature. When mature, they drop to the ground, pupate in a chamber 5-10 cm below the surface, and emerge as yellowish-brown moths with a small dark spot on their forewings. The hind wings are creamy white, and black. The moths are active at night and can live for 5-15 days, with females able to lay 500-3000 eggs in that time.
Identification of Corn Earworm in Maize field
- Corn earworm moths are buff-colored and about the same size as other moths in the cutworm family.
- Female moths lay eggs on green silks of sweet corn, which can hatch in a few days. The larvae then move into the ear and feed on green silk, later tunneling into maturing kernels.
- Corn earworm caterpillars are cannibalistic, and only one caterpillar is usually found in a single corn ear.
- After hatching, they take two to three weeks to become full-grown, causing increasing damage as they grow.
- After feeding, they drop into the soil, dig a cell, and pupate.
Damage symptoms of Corn Earworm in Maize field
- The maize earworm feeds on the fruiting stage and the host plant’s foliage.
- The larvae feed on the silk hairs and then bore into the ear to eat the kernels. This results in a track of damaged kernels and a long line of brown frass.
- Due to their cannibalistic behavior, only one earworm is usually present per cob. The damage to the cob and developing leaf blades creates numerous ragged holes, affecting pollination and grain filling and potentially reducing yield.
- Additionally, the damage can create a favorable environment for the growth of other diseases.
Cultural control of Corn Earworm
- Culturally controlled corn earworms can also be achieved by plowing fields, which kills many overwintering pupae. However, due to its highly mobile and migratory nature, reinvasion from distant sources can be expected.
- Some sweet corn varieties with tight husks around the ear tip can offer slight resistance to corn earworms. However, the pest can still cause damage to these varieties.
- Transgenic varieties that express the Bt-toxin in the silk have been shown to suppress corn earworms. Still, these are currently only available in limited quantities.
- In addition to these cultural control methods, biological control effectively reduces corn earworm populations. Natural enemies such as parasitic wasps, birds, and predator insects can help keep earworm populations in check.
- In some cases, the release of the parasitic wasp Trichogramma can also provide control. However, more than these methods may be required. They must be combined with other management strategies, su