Insects in Corn



Cutworms damage crops by chewing young plants off at the soil surface. They are smooth caterpillars that range in colour from grey to dark brown to almost black. Most cutworms feed at night and most damage occurs between late May and the end of June.
Cutworm infestations are infrequent and unpredictable. Preventative treatments applied at planting times are not recommended. Reliable chemicals are available for controlling outbreaks after cutworms appear. Control is recommended if more than 5% of the plants show damage and the cutworms are still under 1 inch in length. Since cutworms are most active on warm nights, best control can be obtained if spraying is done on an evening preceding a warm night. For recommended insecticides and rates of application, see the Manitoba Agriculture's publication Guide to Field Crop Protection.
For more information and pictures on cutworms, please refer to the following link: Cutworms.

European Corn Borer

The European corn borer was first discovered in North America in 1919 and in Manitoba in 1948. It has been an economic pest in Manitoba corn fields on a sporadic basis. The European corn borer will feed on grain and silage corn. In addition, the European corn borer has a wide variety of other host plants including tomatoes, potatoes, beans, sugar beets and many species of large stemmed flowers and weeds.
In Manitoba, there is only one generation of corn borer per year. In some years, as much as ten per cent of the population will attempt a second generation although this generation will be unsuccessful. Infestations in Manitoba are quite variable from year to year with some areas experiencing heavy infestations while other areas have little or no damage caused by the corn borer.

Life Cycle

The European corn borer has four stages in its life cycle.
Adult moths are tan coloured, with brown markings on the wings. The male moths are darker with darker markings than the female moths. The adult moths emerge from the pupae in late June and early July. The adults are 1/2 in. long with a wingspan of 1 in. and are strong fliers. Newly emerged adults seek out dense areas of vegetation, preferring grassy ditches where they congregate and mate. After mating, females lay eggs during calm, warm summer evenings on the underside of corn leaves near the mid-rib.
Each egg mass consists of 10-40 eggs. Each female can lay two egg masses per evening, for up to 10 evenings. Newly laid eggs are white and overlap resembling fish scales. Just before the eggs hatch, they appear black. The dark appearance is caused by the dark heads of the young borers inside the eggs. The eggs hatch within three to seven days, depending upon temperature.
There are five larval instars of the corn borer. The first instars are whitish with black heads and feed within the whorl and cause shothole and windowpane damage. The first two instars complete development in seven to 10 days. The third larval instar bores into the stalk. Once inside the stalk it is too late to achieve effective chemical control. The later instars (third to fifth) feed within the stalk and ear shanks, disrupting the normal movement of nutrients, which results in decreased yield. Tunnelling and boring may permit secondary infection and damage by rotting of the stalk and ear. Older larvae are flesh coloured with black spots and may also feed on silks, kernels and cobs. Once they are finished feeding the mature larvae overwinter in corn stalks, cobs and plant debris on the soil surface.
In the spring, mature larvae turn into pupae to complete the lifecycle.
The European corn borer causes damage in several ways. The first sign of damage is the shothole and windowpane damage. Economic thresholds for control may be reached when 50% of the plants exhibit shotholes or windowpanes. If the larvae establishes within a plant it may cause several other types of damage. The most severe damage is stalk breakage prior to harvest. Another type of damage is ear drop where cobs fall to the ground. The final type of damage may be the least obvious but can cause significant yield loss. By restricting nutrient flow in the plant yield is affected by the production of smaller cobs.



Cultural Control
Deep plowing of stubble can kill many of the over- wintering corn borer larvae, or cause them to die of exposure on the surface at pupation time in the spring. Mowing corn residue also helps reduce the stalk material that provides the over-winter refuges. These practices will reduce overwintering larvae, but the larvae that are not killed can still cause a severe infestation under suitable climatic conditions. However, soil and moisture conservation must also be considered when managing insect populations. In Manitoba, time of planting does not alter the severity of the corn borer infestation due the much longer oviposition (egg laying) period. Control of alternate weed hosts may help reduce population levels. Mowing of grassy ditches and headlands where adult moths congregate will provide less favourable locations for adults to rest and mate, possibly causing them to migrate to other areas away from the corn field. Rotation will also help control populations.
Biological Control
There are many natural enemies that will feed on corn borer larvae. These include lady beetle adults and larvae, syrphid or hover fly and green lacewing larvae. Natural enemies do not provide control in outbreak situations. In non-outbreak situations it is important to not apply chemical control measures unless the economic threshold is reached so that the populations of beneficial insects will not be affected.
Chemical Control
Insecticides should only be applied when economic thresholds (ET) have been surpassed. Economic thresholds are used to determine if the lost yield (and associated income) caused by the pest is greater than the cost of spraying. To determine whether the population in a given field has reached the economic threshold, it is important to scout the field. Insecticide recommendations can be found in Manitoba Agriculture, Food and Rural Initiative’s publication “Guide to Crop Protection.”
Field scouting should begin in early July. Fields should be prioritized by planting date and relative maturity. Older fields should be scouted first.  In each field, check 10 locations (10 plants at each location) for egg masses and young larvae. The egg masses are most commonly found in the midrib and leaf axil. At each location pull open the whorl to check for larvae feeding within the whorl. If the majority of larvae have bored into the stalk, do not apply insecticides as they are ineffective once the larvae have entered the stalk.  If no larvae or egg masses are found, repeat scouting every five to seven days. Continue scouting until larvae start to tunnel into the stalk or the end of July if no egg masses/larvae are found.
Bt Corn
Bt corn is a type of corn that has been genetically modified. Bt stands for Bacillus thuringiensis, a bacterium that has insecticidal properties when ingested by certain types of insects. Bt is a naturally occurring soil-borne bacterium that produces crystal-like proteins that selectively kill certain groups of insects. The protein crystals are insect stomach poisons that are activated by the insect's digestive enzymes to form the toxin. Through the process of biotechnology, scientists have inserted certain Bt genetic material (DNA) into the corn plant's DNA. DNA is the genetic material that controls expression of plant (or animal) traits. Since the Bt is right in the corn plant, the only way insects are exposed to it is by eating the corn plant. Bt corn does not control feeding by most insects other than the European corn borer. Aphids, spider mites and grasshoppers will not be affected by Bt corn, although some cutworms may be suppressed. Bt corn has little effect on beneficial insects. There are many potential benefits to Bt corn, the most obvious being control of the European corn borer. Other potential benefits include yield protection, reduced insecticide use and cost, proper timing of control, safety to wildlife and insect predators, reduced frequency of disease and reduced stalk lodging. There are also potential limitations to Bt corn. These include the unpredictability of corn borer densities, no guaranteed economic return for the higher seed costs incurred, marketing restrictions as a result of perceptions of transgenic crops, and development of resistance by European corn borer to Bt.
Resistance Management
The European corn borer may have the potential to develop resistance to the Bt proteins found in Bt corn. This is pesticide resistance even though nothing has been sprayed. If all acres of corn were planted to Bt corn it would not take very long before resistance developed and the Bt corn was rendered ineffective. This may occur if a small percentage of European corn borers survive on Bt corn. These resistant moths would then breed with moths that are not resistant and the resistance may not be passed on. If the only corn borers to survive are all resistant to Bt, then the resistance will be passed on to the next generation of corn borers. In time, the Bt corn would no longer be able to control the European corn borer. Resistance is known to occur to Bt sprays in other agricultural areas. Measures must be taken to ensure that resistance does not develop or is delayed. There are several approaches that may be taken. The simplest approach is to not plant Bt corn every year. Use Bt hybrids only in fields where the risk from corn borer infestation warrants the price premium for the seed. In addition to this approach is the preservation of refuges of non-Bt corn for the corn borer to feed on. While it may seem illogical to try to preserve the population, remember that the European corn borer has many hosts and the pest cannot be eradicated. Therefore, the next best option is to ensure that the corn borer remains susceptible to Bt. Refuges are areas where susceptible corn cultivars are planted. Research from corn growing areas of the United States recommend the following ratios of non-Bt corn for refuges:
When no insecticide application will be made to control pests - 20%. When insecticide treatments will be carried out - 40%.
It is critical that all growers retain refuges for corn borer. Planting refuges can be done in several ways: Refuges may be planted in blocks. These blocks must be accessible to the corn borer (do not isolate the refuge in the centre of the field). These blocks may be placed along field margins, headlands and ditches.  It is not recommended to plant strips of Bt corn as this reduces the effectiveness of the refuge.
U.S. studies indicate that reducing the refuge from 10 to 5% increases the probability of resistance developing from 37 to 74%, while increasing the refuge from 10 to 20% reduces the probability of resistance developing from 37% to less than 1%.
For more information and pictures on European corn borer, please refer to the following link: European Corn Borer.


Wireworm larvae are reddish brown or yellow-brown hard bodied worms, up to 0.4-1.0 inches long, depending on species, and each has a flattened, notched tail. This is the stage of the insect that causes damage to crops. They do not curl up when disturbed as do cutworms.
Wireworms will attack most crops although wheat, rye, corn, beans, potatoes, and sugar beets are particularly susceptible to injury. If corn is grown in newly broken light soils, wireworm control is usually necessary. The pests bore into the kernels soon after planting and eat out the contents. Later, they feed on the underground portion of the stem, causing the young plants to wilt and die.


Where it seems evident from previous crop damage that a wireworm population sufficiently large to be economically damaging is present, the use of recommended insecticides as soil broadcast treatments or as combination insecticide-fungicide seed treatments will be profitable.
If possible, corn should not be grown in newly broken, lighter soils because wireworms and other soil pests probably will be present in economically significant numbers.
For more information and pictures on wireworms, please refer to the following link: Wireworms.