Sweet Corn XXXVI

Corn Earworm

Whitney S. Cranshaw

Corn Earworm

Identification (and life cycle/seasonal history)

Corn earworm is, by far, the most common species of caterpillar found in sweet corn ears and the only species found west of the Rockies.  In the High Plains, European corn borer, western bean cutworm and, in the south, southwestern corn borer are sometimes also found in corn ears.  However  corn earworm is the key insect pest of the crop in most areas.  Corn earworm is also an important pest of many other crops including tomatoes, peppers, and tomatillos.  In these crops it is commonly referred to as tomato fruitworm.

Corn earworm winters as a pupa about 3 to 5 inches deep in soil.  Successful wintering in the region regularly occurs in western Colorado and much of eastern Colorado, south of approximately I-70.  However, mild winters can allow survival further north.  Overwintered populations are supplemented by migrants from the southern US, which annually spread throughout the US. Late spring frosts can cause significant mortality of early-emerging moths.

Corn earworm moths are buff-colored and similar in size to other moths of the cutworm family.  Females lay eggs singly (not in masses) on green silks of sweet corn.  Eggs can hatch in a few days and the larvae move into the ear, feeding originally on green silk.  Later the larvae tunnel into maturing kernels.  Corn earworm caterpillars are very cannibalistic and rarely is more than one caterpillar found in a single corn ear.  Following egg hatch the caterpillars develop for a period of two to three weeks before becoming full grown.  They cause increasing damage as they get older and larger.  When feeding is done, they drop to the soil and dig a cell where they pupate.

Plant Response to Damage

Early season corn earworm feed in the whorl of sweet corn plants causing damage that sometimes resembles that of early season European corn borer. The amount of earworm pressure on sweet corn ears may sometimes be predicted from the number of whorl feeding earworm in the field prior to ear formation.

Ear feeding corn earworm caterpillars tend to limit feeding damage to the ear tip, and this area can be extensively damaged.  Entry by this insect into the side of the ear occurs occasionally, but it is more common with other caterpillars found in sweet corn.

Wounding by corn earworm provides an entry court for some kernel-rotting fungi.  Some insects, notably sap beetles, readily invade sweet corn ears through corn earworm wounds.

Management Approaches

Biological Control

Corn earworms are natural affected by several biological controls.  Probably most important are general predators common to corn - minute pirate bugs, lady beetles, and green lacewing larvae. Fungal diseases can occasionally impact corn earworm populations.  However, these can not consistently suppress corn earworm below damaging levels.  Furthermore, natural enemy populations in sweet corn are greatly reduced by insecticide applications made to the crop.

Cultural Control

Plowing of fields should kill many of the overwintering pupae.  However, this insect is highly mobile and  migratory reinvasion from distant sources can be expected. Some sweet corn varieties with tight husks around the ear tip are slightly resistant to corn earworm, but injury can also occur to these varieties.  Transgenic varieties that express the Bt-toxin in the silk can suppress corn earworm, but these are currently available in limited quantities.

Sampling

The size of adult flights of corn earworm can be good predictors of the intensity of later larval infestation.  Corn earworms are readily attracted to blacklight monitoring traps.  There are also very effective pheromone lures for males.  However, since corn earworm is a strong flier it is not often captured efficiently in standard sticky-bottomed traps.  Instead cone-shaped 'Heliothis' traps are used for this species.  The pheromone of the corn earworm is also fairly unstable and needs replacement every few weeks. 

Corn earworm pheromone trapping is a useful tool in determining spray schedules.  If good records are kept and monitoring methods remain constant then changes in captures at traps become good predictors of infestation levels.

Spray programs in western Colorado are based on pheromone captures. Experience has showed that if corn earworm captures exceed a certain level, moving from a three to a two day spray interval is necessary to maintain control. If a higher threshold is reached, an insecticide with ovicidal properties is added to the spray. Treatment thresholds were developed with trial and error over several years, and have been lowered as problems appeared. Treatment thresholds will vary for different areas and production systems.

Chemical Control

Sweet corn becomes attractive to corn earworm egg laying when green silks first emerge. These silks are where corn earworm lay their eggs.  Sweet corn produces silk continuously until harvest, so there are always attractive oviposition sites on the ear. The target for corn earworm applications are eggs and/or newly hatched larvae before they enter the ear tip. Regularly scheduled insecticide applications are necessary to provide residual on the newly grown silk near the tip of the ear. When many corn earworm moths are laying eggs, a two or three day spray schedule may be necessary to provide good control. Regular sprays may be needed until within two or three days of harvest to eliminate larval damage.

Problems with control of corn earworm have been observed in many locations.  This can have many possible causes: 1) Beginning the spray schedule too late. The initial sprays should be applied as the first silks appear within a field. 2) poor coverage of the ear tips, 3) extremely heavy egg laying during periods of high populations, 4) improper spray schedules, 5) resistance to the insecticide used.  Corn earworm is beginning to become resistant to pyrethroid insecticides over much of its range.  Rotation with insecticides in  different pesticide classes  with different mode of action is recommended.

Honeybee hazard caution:  Honey bees will forage for corn pollen and applications made during pollen shed have risk to area honey bee hives.  Avoid using products of highest hazard to honey bees (e.g., Penncap-M, certain carbaryl formulations) and work with area beekeepers to limits pesticide application hazards to sweet corn.

Product List for Corn Earworm on Sweet Corn:

Insecticide	Lbs Active Ingredient Per Acre (Fl oz or oz. product)	Preharvest Interval, Remarks
Baythroid 2	0.025-0.044 lb (0.8-1.6 fl oz.)	(0 day, 12 hour reentry)  Pyrethroid insecticide. Maximum amount allowed in 7-day interval 2.8 fl. oz/A
Warrior	0.02-0.03 lb (2.56-3.84 fl. oz.)	(1 day, 24 hour reentry)  Pyrethroid insecticide.  Maximum application of 0.25 lb AI/acre per season of 3.84 pt/acre of product per season
Asana XL	0.03-0.05 lb (5.8-9.6 fl. oz.)	(1 day, 12 hr reentry)  Pyrethroid insecticide.  Maximum application of 0.25 lb AI/acre per season.
Avaunt	0.045-0.065 lbs ai (2.5-3.5 oz)	(3 days. 12 hrs reentry machine harvest, 14 days for hand harvest) Carbamate insecticide.  Do not use more than 14 oz product per acre in growing season.  Minimum 3 day interval between applications.
Mustang, Fury	0.035-0.05 lb (3.0-4.3 fl. oz.)	(3 days PHI, 12 hr reentry) Pyrethroid insecticide.  Maximum 0.3 lbs ai/acre per season.
Larvin 3.2	0.5-0.75 (20-30 fl. oz.)	(0 day, 48 hour reentry)  Carbamate insecticide.  Maximum application of 300 fl. oz./acre per season.
Lannate 2.4LV	(1-1 1/2 pt)	(1 day PHI, 48 hours reentry).
Lannate 90SP	(1/3-1/2 lb)	Carbamate insecticide. Maximum application of 6.3 lbs AI/acre per season.  Has some activity against eggs.
Success, SpinTor	(3-6 fl. oz.)	(1 day)  Naturalyte insecticide.  Maximum application of 16 fl. oz/acre per season.
Entrust	(1-2 oz.)	(1 day)  Naturalyte insecticide.  Formulation suitable for Certified Organic production. Maximum application of 9 oz/acre per season.

The information herein is supplied with the understanding that no discrimination is intended and that listing of commercial products, necessary to this guide, implies no endorsement by the authors or the Extension Services of Nebraska, Colorado, Wyoming or Montana. Criticism of products or equipment not listed is neither implied nor intended. Due to constantly changing labels, laws and regulations, the Extension Services can assume no liability for the suggested use of chemicals contained herein. Pesticides must be applied legally complying with all label directions and precautions on the pesticide container and any supplemental labeling and rules of state and federal pesticide regulatory agencies. State rules and regulations and special pesticide use allowances may vary from state to state: contact your State Department of Agriculture for the rules, regulations and allowances applicable in your state and locality.

Categories: Sweet Corn, Insects, Corn Earworm

Date: 01/26/2004