(Lepidoptera: Pyralidae)

1. BIOLOGY AND ECOLOGY

The European corn borer, Ostrinia nubilalis , is becoming a serious threat to corn producers across the prairies. Outbreaks have occurred in Manitoba in 1955, 1963, and 1982-1984 (Anon., 1985). Invariably, corn grown in the Carman, Morden, and Altona areas is involved.

Corn borer moths are buff-colored, have brown markings on the wings, and have a wing-span of about 25-30 mm (Anon., 1985). The female moths are paler, with fewer markings, than the male moths (Wressell and Hudon, 1966; Mutuura and Monroe, 1970).

Eggs are flat and overlap each other, much like fish scales. Newly-hatched larvae are 3 mm in length, white to tannish-grey in color, and covered with rows of small brown spots. They shad their skins (moult) 4 times. Thus, there are 5 larval instars. Full-grown larvae are similar in appearance and about 2.5-3.0 cm in length. Some larvae have a pinkish tinge.

Pupae are reddish-brown, about 2.5 cm long, and pointed on one end.

Introduced to North America in 1917, the European corn borer (Mutuura and Monroe, 1970) occurs from

Alberta, east through Manitoba to Newfoundland. It reached and began to spread through Manitoba in 1948.

European corn borers have the potential to be present wherever corn is grown in Manitoba.

Sweet corn is the preferred host (Wressel and Hudon, 1966). Grain and silage com is also damaged (Anon., 1985).

Over 200 other plants including cotton, sweet and hot peppers, beans, potatoes, tomatoes, oats, sugar-beets and large-stemmed flowers and weeds have been racorded as hosts (Dively and McCully, 1979; Sanborn et al., 1985; Webb et al., 1987).

Like all moths and butterflies, these insects have 4 life stages i.e., the egg, larva, pupa, and adult. European corn borers overwinter as full-grown larvae in corn stalks, cobs and plant debris on the soil surface. Only one generation per year has been observed on the prairies (Palaniswamy et al., 1990).

Sometimes, a second generation is attempted but usually fails.

During the late-spring, each overwintered, full-grown larva chews an exit hole and returns to its tunnel to spin a cocoon in which to pupate. Female moths begin to emerge from corn debris in mid-June. They quickly mate and begin developing their eggs. The resident moth population may be swelled by moths flying in from the south in some years. They are strong fliers.

Egg-laying begins in late-May. Peak egg-laying and flight occurs in mid-July, during warm nights. Female moths will lay up to 500-600 white eggs, in batches of 15-25 eggs, on the undersides of corn leaves, near the midrib. The egg masses are about 6 mm in diameter (Hudon et al., 1984; Anon., 1985).

The eggs hatch within a week (mid-July through early August). The young, white, black-headed larvae disperse and feed on leaves and work their way to the whorl of the plant (Ross and Ostlie, 1990). Pinholes or shotholes are signs of borers already moving in the plant.

During later stages of borer development, most larvae enter the tassel, stalk, and ear shank, but they may also feed on the silks, kernels and cobs.

Nearly-mature larvae bore into the stalk by late summer to overwinter. They complete their development by pupating the following spring. The new adults emerge in the early summer to reproduce.

1. Seasonal Abundance:

Corn borers are most abundant in mid-summer, during their egg stage, before natural mortality factors, including adverse weather conditions and natural enemies, begin to affect their survival.

2. Responses to Environmental Factors:

Weather conditions are a key factor in corn borer development and survival (Showers et al., 1978; Lee and Spence, 1988; Lee, 1988a; Godfrey and Holtzer, 1991). Low moisture, heavy rainfall, or high temperatures, immediately after hatching, will affect the survival of the young larvae. Damage to the corn plant is least during periods of warm, moist weather.

Cool nights may delay moth flights and mating, leading to later than normal egg hatching (Kelker et al., 1990).

Some workers suggest that mild winters and increased no-till corn acreage may also be factors contributing to corn borer problems (e.g., Musick and Petty 1973; Musick and Beasley, 1978).

However, others suggest that there is no difference (e.g., Umeozor et al., 1985; Andow and Ostlie, 1990).

3. Importance:

All above-ground portions of the corn plant may be attacked. The young larvae bore into growing leaf whorls and feed on the developing leaves. This gives the loaves an etched or shot-hole appearance. Young larvae boring in the leaf midrib will cause leaf breakage. Feeding damage to developing tassel stalks weakens the tassels. Stalks, damaged by tunnelling larvae, fail to translocate nutrient to the ears and are often broken off by wind.

Older larvae bore into the stalks and ear shanks, disrupting the normal movement of nutrients and water. Such damage results in reduced yield. One corn borer larva can cause a 3% yield loss (Anon., 1985).

Stem breakage and ear drop are common damage symptoms. The tunnelling and boring permits secondary infection and damage by stalk- and ear-rotting fungi.

Feeding damage to the ears of sweet corn makes them unremarkable (Hudon et al., 1984).

4. Natural Enemies:

Birds, lady beetles (several species of coccinellid beetles), and other predacious insects may feed on some of the young, exposed larvae or the emerging adult moths but these predators seem to have little impact on corn borer population levels (Sparks et al., 1966; Hudon et al., 1984). Some birds may dig out full-grown borers from corn stalks as part of their winter diet.

There are also a few predators of the eggs of the European corn borer (e.g., mites; lacewings, Obrycki et al., 1989, anthocorid bugs, Reid, 1991).

Although there has been some research carried out on introduced parasites of the corn borer (e.g., parasitic tachinid flies and ichneumonid and braconid wasps) and on the use of bacteria (e.g., Bacillus spp.) and fungi (e.g., Beauveria and Nosema spp.) for their control (Lewis, 1975; Lewis and Bing, 1991). It may be some time before these organisms are commercially available for use by Manitoba corn farmers.

1. MANAGEMENT

Beginning in early-July (or when corn is about 0.5m high), corn growers should check their fields on a weekly basis. Move well into the field before you begin sampling (Lee, 1988b). Check 20 plants in each of 5 different areas of the field to get a reliable idea of the infestation level.

Egg masses can be seen on the lower surface of the leaves, usually near the mid-rib. The eggs are white when laid. Just before hatching, the egg mass darken to the black-head stage because of the darkening head capsules of the larvae. This signal of imminent hatching can be used to time sub-sequent treatments (Hawkins and Devitt, 1952).

Shot-holes in the young leaf whorls will often signal the presence of young larvae. These larvae are about 3 mm in length.

Later, as the larvae mature, boring dust (also called frass) may be seen on leaves, on stems and on kernels within the ear.

Large numbers of moths can signal an outbreak. Driving through corn country in the evening, windshields can quickly become coated with the remains of European corn borer moths.

Entomologists often use blacklight traps or pheromone traps to monitor adult corn borer activity (Legg and Chiang, 1984; Palaniswamy et al., 1990). The pheromone traps can be as simple as milk cartons, coated on the inside with a sticky material, and bafted with tthe sex pheromone lure, suspended within the carton. Commercially-available traps usually stand up to weather longer.

1. Threshold/Action Population Level:

During a severe outbreak, 50-100% of the cam plants in a field will have one or more cam borers present (Anon, 1985).

Economic loss in grain and silage corn may occur when 50% of the plants have shot-holes in the leaves, caused by the young cam borers (Anon., 1985). Lower infestation levels (e.g., 15% of the plants shot-holed) may reduce the quality and sales of sweet corn.

Remember that application timing is critical with the European corn borer. It control is necessary, you want to get to the larvae before they bore into the stalk and the cryptic larvae become untreatable. Quick action is especially important during periods of warm weather when corn borer development is more rapid.

3. Management Alternatives - Non-Chemical:

Ploughing down corn stubble and residues will reduce the number of overwintering larvae by up to 99%. Thus, for farmers using conventional tillage, it can be an effective control method (but only if strong-flying moths, flying in from the south or from neighbouring corn fields or weedy areas, do not reinfest subsequent crops). Deep ploughing in the fall or spring (by May 1) prevents the emerging moths from reaching the soil surface.

For zero till farmers, infested fields should be harvested as soon as possible to minimize loss caused by ear-drop or stalk breakage. Mowing after harvest may be an alternative to ploughing down corn stubble. Also, corn stalks that chopped for silage will remove larvae from the field and destroy them in the ensiling process. Dwarf cam seems to be less susceptible to corn borer infestation than tall corn (Martin et al., 1989).

Burning is not a good agronomic practice and is not recommended. Organic matter is needed by the soil and some borers will survive in stalks not completely burnt.

Most commercial corn varieties have some resistance to whorl leaf feeding and sheath collar feeding by corn borers (Barry and Darrah, 1991; Kaster et al., 1991). There is a plant chemical present that affects the growth and digestive processes of the earn borer. Ever more tolerant or resistant corn varieties may be developed and become available in the future.

2. Management Alternatives - Chemical:

Chemical controls may be effective if applied within a day or so of egg hatching and beginning of larval feeding (Showers et al., 1989). Once larvae enter the stalk, the insecticides will not work. If there is an extended hatching period, more than one insecticide application may be required.

Pheromone trapping to determine the time of moth flight and field scouting to determine the presence and hatching of eggs are necessary to ensure the timely application of a recommended insecticide. Too often, insecticide applications are applied too late. Farmers should spray while the larvae are in the leaf whorls.

Because corn is too high for conventional field sprayers when the spraying must be done, high-clearance, row-crop or cam sprayers (e.g., Hi-Boy) or aircraft are required. When using these sprayers, 3 nozzles per row seem to work best (with one aimed into the throat of the corn plants).

Carbofuran (Furadan 480F [0.445 1/acre]) can be used for corn borers on field, sweet, and silage corn. There is a 7-day pre-harvest interval. May be applied by air or ground equipment. Has both systemic and contact action usually with quick results. Apply when insect numbers exceed economic threshold levels in sufficient water for good coverage. Furadan 480F is compatible with fungicides commonly used on the same crops. May be tank mixed with phenoxy ester or amine herbicides and used only on crops listed on both labels.

Carbaryl (Sevin XLR Plus [1.0-1.6 1/acre]) can be used for fie1d and sweet corn. Sevin is of moderate acute mammalian toxicity and is highly toxic to bees. Avoid contamination of food, feed, water supplies, streams and ponds. Do not apply tank-mix combinations unless your previous experience indicates the mixture is effective and will not result in application problems, excessive residues or plant injury.

Deltamethrin (e.g., Decis 5EC [0.1-0.12 1/acre]) is also registered for corn borer control in field corn, using ground application equipment and when temperatures are below 25C. Up to 3 applications per year are allowed. Observe a 15 m buffer zone from environmentally-sensitive areas (e.g., wetlands, sloughs, rivers, houses, farm buildings).

Carbaryl (Sevin XLR Plus) may used to control corn borers on sweet corn. See notes in previous section.

Carbofuran (Furadan 48OF [0.445 1/acre]) can be used for corn borers on field, sweet, and silage corn. There is a 7-day pre-harvest interval.

Cypermethrin (e.g., Cymbush [0.113 1/acre] and Ripcord [0.07 1/acre]) is also registered for corn borer control. Ripcord may be applied by ground or air. Observe a 15 m setback from water bodies and other sensitive areas when applying Ripcord with ground application. Observe a 100 m setback from water bodies and sensitive areas when applying by air. There is a 5-day pre-harvest interval.

Deltamethrin (e.g., Decis 5.0 EC) is also registered for the control of this pest in field corn. Ground applications are made egg masses begin to hatch and before pin-hole feeding on the leaves is evident. It can be repeated up to 3 times at 5-8 day intervals.

Other products registered for use in sweet corn include Ambush 500EC, Malathion 500EC, Lannate-L, and Pounce.

More than one application of an insecticide may be necessary, depending on the length of the adult flight and the subsequent hatching period.