Field Corn XI-5

European Corn Borer

Frank B. Peairs

 

 

European Corn Borer

Identification (and life cycle/seasonal history)

European corn borer (ECB) larvae are cream colored to pinkish caterpillars, which are marked with small, round brown spots.  The head capsule is dark or reddish brown.  Fully developed larvae are 25 millimeters (one inch) in length.  Male moths are distinctly darker and slightly smaller than the pale yellow female moth.  The average adult wing-span is about 25 millimeters (one inch).  The forewings are buff colored with darker bands running in wavy lines across the wings.

First generation.  In early spring, overwintered larvae pupate and emerge as moths during late May or early June.  On warm, calm, humid evenings in June, female moths fly from weedy or grassy margins into cornfields and lay eggs.  Eggs are laid in masses of 15 to 30 on the underside of corn leaves, usually near the midrib.  The egg masses have a scaly, glossy white appearance.  As the egg masses mature, the black head capsules of the developing larvae become visible, and the egg mass is described as being in the "blackhead stage." Once an egg mass reaches the blackhead stage, hatching will generally occur within 36 hours.  Larvae require from four to six weeks to complete development.  Mature larvae, fifth instar first generation, change into pupae within the corn stalk.  Adult moths emerge in mid-July to early August to form the second generation.  Adults fly at night but do not cause damage.  ECB usually go through two generations each year.

Second generation.  The female summer moth prefers to lay eggs in corn that is tasseling and in the green silk stage.  Later-maturing fields are more attractive to egg-laying moths than fields approaching maturity.  Second generation larvae bore into the tassels, ear shanks, ears, and stalks.  These larvae usually overwinter and do not pupate until the following spring.

Plant Response and Damage

ECB usually go through two generations each year.  The young larvae feed first on the leaf near where they hatched.  As the larvae grow, they move to the whorl or leaf sheath area, and feed.  When leaves emerge, the "shot hole" feeding signs in the leaves can be seen.  Most of the mature larvae will bore into the stalks, feed, and finish development there.  Second generation larvae cause ear damage, tunneling in the shank and feeding on silks, kernels and cobs.  Signs of infestation include: dropped ears, broken shanks, stalk breakage, sawdust-like castings on leaves, and holes in the stalks.

Management Approaches

Resistant Varieties

Some hybrids have useful resistance to the first brood of European corn borer, which feeds in the whorls and later enters the stalk.  Control can be expected with Bt corn hybrids, except those containing just the YieldGuard Rootworm event.   See ?Managing Corn Pests with Bt Corn?, http://www.ext.colostate.edu/pubs/crops/00708.html, for details on the use of Bt corn hybrids to control corn pests. 

Biological Control

Beauveria bassaina is a fungus that attacks and eventually kills ECB larvae.  Nosema pyraustae is a protozoan organism that infects eggs, larvae, pupae, and adults.  Ladybird beetle larvae and adults and lacewing larvae feed on eggs and newly hatched larvae.  Two parasitic wasps, Eriborus terebrans and Sympiesis viridula, are known to parasitize ECB larvae.

Cultural Control

Planting date may affect infestations since egg-laying females are attracted to taller corn.      

Chemical Control

Granules applied by ground and air, as well as some center-pivot applied liquids, have given the best results in university tests.  The second brood feeds in leaf axils and the ear tip, and later enters the stalk or the ear.  Second brood damage increases the possibility of lodging and ear drop losses, so heavily infested fields should be harvested early.  Aerially applied and center-pivot applied liquids have performed better than granules for control of second-generation larvae in university tests.  Monitor treated fields for spider mites as applications for control of second generation are often associated with mite outbreaks.

The need to treat European corn borer can be determined by a simple method based on average Colorado conditions, or by a more complex method which takes into account treatment costs, individual field yields and current market conditions (Table XI 2).  Incorrect treatment decisions, by either method, are much more likely with second-generation infestations.  This is partly due to the second-generation egg-laying period, which can last up to four weeks making proper treatment timing very difficult.

By the simple method chemical control of the first generation is economical when 25 percent of the plants have feeding damage and larvae are present in the whorls.  Once larvae have entered the stalk, control is impossible.  Survival of second-generation larvae is highest during pollination, so treatments should be considered when weekly scouting has an accumulated total of 25 percent of plants with egg masses.  This percentage should be raised to at least 50 percent after pollination.  If egg laying continues after the treatment, a second application may be justified under some circumstances. 

A more complex method allows you to take into account control costs and yield expectations.

First Generation Procedures

Visit fields weekly after moth flight activity is reported.  Earlier planted, larger corn will be more likely to have eggs. 

1. For each 25 acres per field, sample one location.

3. Dissect two of the damaged plants and record the total number of ECB larvae present.

7. If not then resample if (1) the moth flight is still underway; (2) mostly small larvae were present; and (3) little stalk tunneling had been initiated.  If most of the larvae have already entered the stalk, a treatment will not be effective.

  Second Generation Procedures

1. Visit fields weekly after moth flight activity is reported.  Later corn is more likely to have eggs.

3. At each location dissect 10 plants, paying close attention to silks and leaf axils.  Record the number of larvae present.  If scouting for eggs, strip leaves from each of the 10 plants, examining each for eggs.  Record the number of egg masses present.

5. Compare your observed borers per plant figure to the figure in Table XI - 4 corresponding to your expected control costs and crop value or use the values in the footnotes to convert the borers per plant to the action threshold in egg masses per 100 plants.  The ECB counts in the table are break-even figures.  If you want more return per control dollar, multiply the borers per plant or egg masses per 100 plants figure by the desired return before comparing it to your observed value.  For example, multiply by 1.5 to get a 150% return for each dollar of control costs.

7. If not then resample if (1) the moth flight is still underway; (2) mostly small larvae were present; and (3) little tunneling had been initiated.

Table XI - 2.  European corn borer field scouting form.

Date:		Field ID:
Acres:		Location:
Sample #	First Generation		Second Generation
	Damaged plants per 20 plants	Borers per 2 damaged plants	Borers in 10 dissected plants	Egg masses in 10 plants
1
2
3
4
5
6
7
8
9
10
Total
Average
Estimated larvae per plant (Damaged plants in 20) x (borers per plant) x             =			(Ave.  ECB/10 plants)/10 = Or
Action threshold (see Table XII-3) Table value x desired economic return (see Step 6) x             =			(Ave. egg masses/10 plants) x 100 = Action threshold (see Table XII-4) Table value x desired economic return (see Step 5) x             =

Decision:

TREAT

DON?T TREAT

RESAMPLE

 

Table XI - 3.  Action threshold values for first generation European corn borer larvae expressed as larvae per plant.

Control Costs per  Acre ($)		Expected crop value (expected bushels per acre x expected market price)
		250	275	300	325	350	375	400	425	450	475	500
		Action threshold (European corn borer larvae per plant)1
	6	0.60	0.55	0.46	0.43	0.40	0.38	0.35	0.33	0.32	0.32	0.30
	7	0.70	0.64	0.54	0.5	0.47	0.44	0.41	0.39	0.37	0.37	0.35
	8	0.80	0.73	0.62	0.57	0.54	0.50	0.47	0.45	0.42	0.42	0.40
	9	0.90	0.82	0.70	0.64	0.61	0.56	0.53	0.51	0.47	0.47	0.45
	10	1.00	0.91	0.78	0.71	0.68	0.62	0.59	0.57	0.52	0.52	0.50
	11	1.10	1.00	0.86	0.78	0.75	0.68	0.65	0.63	0.57	0.57	0.55
	12	1.20	1.09	0.94	0.85	0.82	0.74	0.71	0.69	0.62	0.62	0.60
	13	1.30	1.18	1.02	0.92	0.89	0.80	0.77	0.75	0.67	0.67	0.65
	14	1.40	1.27	1.10	0.99	0.96	0.86	0.83	0.81	0.72	0.72	0.70
	15	1.50	1.36	1.18	1.06	1.03	0.92	0.89	0.87	0.77	0.77	0.75
	16	1.60	1.45	1.26	1.13	1.10	0.98	0.95	0.93	0.82	0.82	0.80
	17	1.70	1.54	1.34	1.20	1.17	1.04	1.01	0.99	0.87	0.87	0.85
	18	1.80	1.63	1.42	1.27	1.24	1.10	1.07	1.05	0.92	0.92	0.90

1Assumes (1) 100% efficient scouting; (2) 5% yield loss per plant; and (3) 80% control.

Table XI-4.  Second generation European corn borer action threshold values expressed as larvae per plant2.

Control Costs per  Acre ($)		Expected crop value (expected bushels per acre x expected market price)
		250	275	300	325	350	375	400	425	450	475	500
		Action threshold (European corn borer larvae per plant)1
	6	1.23	1.12	1.03	0.95	0.88	0.82	0.77	0.72	0.68	0.65	0.62
	7	1.44	1.31	1.20	1.10	1.03