European Corn Borer Management in 2005

• Late-May and early June rainfall patterns in 2004 were especially detrimental to European corn borer (ECB) reproduction, and populations were low as a result.
• This year's populations have much less to do with next year's infestation levels than do conditions next year during moth flights, mating, egg laying and hatch.
• Field surveys of ECB infestation levels have shown that in years following low infestations, ECB levels are just as likely to be high as low.
• Research indicates that even at very low infestation levels such as 0.5 borers/plant, hybrids with HX11 or YGCB2 insect protection provide a positive economic return.
• In addition to controlling ECB, the HX1 trait helps protect against five other major corn pests including southwestern corn borer, black cutworm, western bean cutworm, fall armyworm and corn earworm.
• The YGCB trait helps protect against southwestern corn borer and corn earworm, in addition to ECB.

Levels of European corn borer (ECB) populations were very low in most major corn-producing areas of the U.S. in 2004. High rainfall during egg laying and hatching was detrimental to survival of young larvae. But growers should not become complacent about ECB control. Field surveys of ECB infestation levels have shown that in years following low infestations, ECB levels are just as likely to be high as low. This Crop Insights will discuss weather conditions that affected ECB populations in 2004, and management of this destructive insect pest in 2005.

Corn borers overwinter in corn stalk residue as full-grown larvae in suspended development (diapause). When temperatures reach 50° F. in the spring, development resumes. Larvae pupate and emerge as adult moths, usually in late May or early June in the central Corn Belt. Adults fly to grassy areas to mate and then to selected corn fields to lay their eggs. These first-generation moths target the tallest corn fields in a locality for egg deposition.

European corn borer moth and egg mass on corn leaves.

Late-May and early June rainfall patterns in 2004 were especially detrimental to corn borer reproduction (Figure 1). As the map shows, rainfall during this period was 2 to 6 inches above normal in much of the Corn Belt. High rainfall limits moth movement for mating and egg laying, physically washes small larvae from plants, and promotes diseases that attack egg masses and larvae.

Total Precipitation Departure from Mean in Inches May 15, 2004 to June 15, 2004

Figure 1. Precipitation departure from normal, May 15 - June 15, 2004. Source: Midwest Climate System (MICIS).

One constant in ECB history is the difficulty of predicting outbreaks. This is because this year's infestation levels have much less to do with next year's numbers than do conditions next year during moth flights, mating, egg laying and hatch. When inclement weather accompanies these ECB activities, larval survival may be reduced to a fraction. But under optimal conditions, each female moth can produce over 400 eggs and spread them over many plants and fields, allowing ECB populations to swell rapidly.

Second generation ECB levels also depend greatly on weather patterns and their effects on moths, larvae and natural enemies. This makes predicting ECB levels just as complex as predicting long-term weather patterns.

Each year, a fall survey of ECB levels across the state of Illinois is conducted by the University of Illinois. An analysis of that survey shows that corn borer infestations were very low (0.5 or less borers/plant) during 5 of the 20 years from 1983 to 2002. In four out of five of those years, infestations the following year exceeded 0.5 borers/plant, which would have made use of corn-borer resistant hybrids a profitable decision. The average infestation the year following a very low infestation was 1.3 borers/plant. At that infestation level, corn borer resistant hybrids would be expected to deliver nearly 10% higher yields than non-resistant hybrids.

Table 2. ECB infestation levels in Illinois the year following a very low infestation year, 1983 -2002. Source: University of Illinois ECB Fall Survey.

* 0.5 or less borers per plant.

Understanding yield loss due to ECB is important when considering various management options for this pest. The major damage caused by ECB is due to tunneling in stalks, ear shanks and ears. Tunneling disrupts water and nutrient transport in the plant and increases risk of stalk lodging and ear drop. In addition, damage may allow higher levels of stalk rots and ear molds.

To help determine yield loss levels due to ECB, Pioneer researchers tested hybrids with the Herculex1 I or YieldGard2 Corn Borer insect protection traits vs. their normal non-Bt counterparts in replicated research trials. These studies were conducted in 119 locations over six years.

Plots were evaluated for corn borer damage by examining 10 consecutive plants in each plot. Stalks were split on these 10 plants and the number and total length of tunnels were recorded. Plots were harvested for yield, moisture and test weight measurements. The results are shown in Figure 2.

Yield Advantage of Pioneer Hybrids with Corn Borer Protection Compared to Their Normal Counterparts

Figure 2. Relationship of ECB tunnels/plant to the yield advantage of Pioneer hybrids w/ the HX1 or YGCB gene over similar hybrids without the gene, 119 environments, 6 years.

This graph demonstrates that Pioneer brand hybrids with the HX1 or YGCB gene gave an average yield advantage of approximately 7% for every corn borer cavity per plant. This relationship was demonstrated at low as well as high ECB infestation levels.

Figure 3 shows the additional income expected from resistant Pioneer hybrids at various levels of corn borer pressure. This figure shows that even at very low infestation levels such as 0.5 borers/plant, hybrids with HX1 or YGCB insect protection provide a positive economic return. Should a moderate infestation of 1.0 to 2.0 borers/plant occur, these resistant hybrids may provide corn growers with $25 to $50 of additional gross revenue/acre. At high infestations of 2.0 to 3.0 borers/plant, the advantage for resistant hybrids is $50 to $75 gross revenue/acre.

Figure 3. Economics of using Pioneer resistant hybrids.Benefit is based on $2.25 corn, regression formula for % yield loss/borer (Y = -0.70 + 7.85X), and 150 bu/acre. Cost is the additional cost of hybrids with corn borer resistance above that of non-Bt hybrids.

The value of corn borer resistant hybrids may be even greater in fields most susceptible to damage, including:

• Early planted fields
• Late planted fields
• Fields with a history of high ECB damage
• Fields that are harvested late
• Fields that are scouted less (e.g., a field far from home)
• Fields that are difficult to spray (terraces, point rows, oddly shaped fields, etc.)

Choosing the right Bt hybrid is critical for maximizing economic returns. In fact, the differences in performance among Bt hybrids are just as great as differences among non-resistant hybrids. Remember that the same rules of hybrid selection apply to all hybrids and technologies. Growers should review fall harvest data, then select a total value package that includes excellent yield for maturity, standability, disease resistance and other important traits.

In addition to controlling European corn borer, the Herculex I trait helps protect against five other major corn pests including southwestern corn borer, black cutworm, western bean cutworm, fall armyworm and corn earworm. The YieldGard Corn Borer trait helps protect against southwestern corn borer and corn earworm. Additional information about these corn pests follows.

Southwestern corn borer is a major pest of corn in the Southern Plains states, including Texas, Oklahoma and Kansas. It may also be problematic in the Midsouth states of Arkansas, Mississippi, Kentucky and Tennessee, as well as southeast Missouri and southern Illinois. Losses from this pest are from death of the growing point of young plants, tunneling in the stalk, and girdling the plant, which often results in severe lodging. Management of southwestern corn borer by insecticide application is difficult, because timing is critical to achieve effective control.

Black cutworms generally do not overwinter in the primary corn producing states, but migrate in from southern states on early spring weather systems. Thus, black cutworm infestations are sporadic, but damage can be severe when outbreaks occur. Each larva cuts an average of 3 to 4 plants during its lifetime, usually at night while remaining hidden during the day. They tend not to move on the soil surface when the ground is dry or crusted, but remain in burrows well below the surface, where they are difficult to control with sprays.

Western bean cutworm is now considered a serious threat to corn in at least 10 states. Historically, the primary corn-growing regions affected by this pest have included Colorado, South Dakota, Nebraska, western Kansas and the panhandles of Texas and Oklahoma. But in the last five years, WBC has migrated gradually eastward into Iowa and Minnesota as well. WBC damages corn primarily by larval feeding on developing kernels in the ear tip and then the ear. This causes direct yield losses and also allows secondary insects and diseases to invade the ear. Once larvae enter the ear, they are almost impossible to reach with insecticides.

Fall armyworm is a very serious pest of corn in the southern, eastern and southeastern Corn Belt. The threat of damage is even greater for late planted corn, or corn following winter wheat. Larvae of the fall armyworm feed extensively on leaf tissue before moving deep in the whorl. Larvae also feed directly on ear tissue, which may be even more damaging to yield than severe leaf feeding . Damage is most common on late pre-tassel corn. Scouting and management of fall armyworm must take place in a timely fashion before larvae burrow into the whorl or enter ears.

Corn earworm can be found throughout the major corn-growing areas of North America. This insect does not overwinter in the northern U.S., but reestablishes itself each spring when moths migrate from southern areas. The major damage to corn is from ear feeding. Larvae feed for 2 to 4 weeks, causing direct yield losses by devouring kernels and indirect losses by providing an entry for ear rot organisms. Insecticide applications for corn earworm are not considered practical in field corn because a single application will not provide adequate control.