During its lifetime, the European corn borer goes through four stages of development (Figure 2): egg, larva (borer), pupa, and adult (moth). These four stages constitute a generation. The larva goes through five instars, or larval stages, of development (Figure 3). During the fifth instar, all larvae either prepare to pupate and become adults or enter diapause. Diapause, a type of hibernation, is a physiological condition resulting in suspended development. It is controlled by day length, temperature, genetic composition of the population, and, in some instances, by the nutritional quality of host plants.

Figure 2. Typical life history of European corn borer in relationship to corn phenology in the central Corn Belt of the United States.

Figure 3. The different developmental stages of European corn borer larvae showing the five instars. (M. E. Rice)

During midsummer to autumn, day lengths shorten and temperatures begin to cool. These environmental changes trigger one or more genes (sex-linked to the male) to alter physiological development in the larval portion of the population, which leads to diapause of fifth instars. Diapause ensures survival through the fall and winter. The diapaused fifth instars remain in suspended development throughout the winter until spring, when diapause ends and the larvae resume development and pupate. The more heat-sensitive European corn borer populations in the southern portions of North America are affected less by shorter day lengths and cooler temperatures than their northern counterparts. In the southern areas, more European corn borer generations are completed before diapause begins.

Generations per season determine three ecotypes (a subunit of a species with most individual insects having similar environmental tolerances) of European corn borer populations in North America: northern (univoltine, or one generation), central (bivoltine, or two generations), and southern (multivoltine, or three or more generations). In addition, the insect has two pheromone types, which will be discussed in the next section.

In northern and cool upland areas, the European corn borer population is usually one generation. Across the central Corn Belt, two generations dominate; but, in warmer years with extended growing seasons, at least a partial third generation may occur. Figure 1 shows the approximate geographic location for the number of annual generations that normally occur. Partial generations often occur in the zones of overlap.

As full-grown larvae, European corn borers spend the winter in cornstalks (Figure 2), corn cobs, weed stems, or in a spun-silk covering located in plant debris. European corn borers that survive the winter in the central Corn Belt end diapause during April or May and continue development as outlined in Figure 2.

The European corn borer begins spring development when ambient temperatures exceed 50¡F (10¡C), which is the developmental threshold (minimum temperature) for sufficient physiological growth to occur. This threshold may be slightly lower in the northern distribution range and slightly higher in southern locations.

During the 1950s, researchers began predicting European corn borer biological events by using accumulated temperature units, called degree-days. Because insects are cold-blooded, degree-days can be used to quantify their physiological development. Accumulations of degree-days were started arbitrarily after January 1 of each year, whenever 50¡F occurred. However, population diversity and environmental variations made these extended degree-day accumulations less accurate for predicting a critical stage of European corn borer that occurs later in the season. Biological events can be predicted more accurately by using a phenological benchmark closer to the event that is being forecast. This is true when predicting egg laying by degree-day accumulations based on the capture of the first moth in either pheromone traps or light traps. Predictions of subsequent European corn borer events over a shorter period, based on the degree-day accumulation following moth flight, have been reasonably successful.

Table 1 shows the first occurrence of life stages or events and general activities of the European corn borer, based on degree-days (base 50¡F) accumulated from initial capture of moths in the spring. Research has shown that physiological time required for development varies according to the host plant and the moisture and nutrient content of the food for the larvae. Greater moisture tends to accelerate development. Consequently, the information in Table 1 represents a generalized situation. Table 1 also gives the average number of developmental days required for most two-generation regions to reach the first occurrence of a stage from initiation of the previous stage listed, based on average daily temperature for the time of year when that particular life stage normally occurs. Variation between generations also occurs in fed upon and invaded plant tissues (Table 1). Second-generation larvae bore into cornstalks at a later developmental stage because the maturer stalks are more difficult for larvae to infest compared to the tender stalks the first generation encounters.

Table 1. Accumulated degree-days (developmental threshold of 50¡F) from initial capture of moths in the spring for first occurrence of life stages and general activity of European corn borer (D. D. Calvin, P. Z. Song, and C. E. Mason).

Accumulated     First occurrence        Days to first   Mean daily
degree-daysa    of stage or event       occurrenceb     temperature     General activity
_________________________________________________________________________________________
       0        First spring moth                                       Mating and egg laying

First generation
   212          Egg hatch (first instar)  16.3            63            Pin hole leaf feeding
   318          second instar             6.6             66            Shot hole leaf feeding
   435          third instar              6.5             68            Midrib and stalk boringc
   567          fourth instar             6.6             70            Stalk boring
   792          fifth instar              10.2            72            Stalk boring
1,002           Pupa                      8.8             74            Changing to adult
1,192           Adult moths               7.6             75            Mating and egg laying

Second generation
1,404           Egg hatch (first instar)d 8.2             76            Pollen and leaf axil feeding
1,510           second instar             4.1             76            Leaf axil feeding
1,627           third instar              4.3             77            Sheath, collar, and midrib boring
1,759           fourth instar             5.1             76            Stalk boringc
1,984           fifth instar              9.0             75            Stalk boring
__________________________________________________________________________________________
aBased on populations from Iowa, North Dakota, Missouri, Delaware, 
 and Pennsylvania that were reared in the laboratory on stalk sections from 
 whorl-stage corn by using a minimum developmental threshold of 50¡F.
bAverage number of days of development in most two-generation 
 regions in order to reach the first occurrence of the stage or event since 
 initiation of the previous stage that is listed, based on the mean daily 
 temperature for the time of year when the previous life stage normally occurs.
cFirst-generation larvae bore into stalks earlier than second-generation 
 larvae because the younger stalks are more tender than those of older, more 
 mature plants.
dPeak egg hatch occurs 10 days or approximately 200 to 250 degree-days 
 later than first hatch.

Degree-days are the mean number of degrees above the developmental threshold temperature occurring each 24-hour period. A simple way of calculating degree-days follows:

1. (Maximum temperature + minimum temperature) Ö 2 = mean daily temperature. The maximum temperature is the highest temperature and the minimum temperature is the lowest temperature recorded during the 24 hours of a day.

2. (Mean daily temperature Ðthreshold temperature) [50¡F for European corn borer] = daily degree-days.

3. Sum of daily degree-days = accumulated degree-days.

Figure 4. Blacklight trap equipped with a 15-watt bulb (M. E. Rice)

Figure 5. Wire mesh pheromone trap with 20-inch base diameter with the base placed below the top of vegetation in an action site. A wire mesh inward skirt at the base has an opening 12 inches in diameter where the lure us centered (C. E. Mason).

If the maximum temperature does not exceed the developmental threshold temperature, no degree-days are accumulated. However, if the minimum temperature is below the threshold temperature, and the maximum temperature exceeds the developmental threshold temperature, then this must be accumulated. This is done by taking (maximum temperature Ð threshold temperature) Ö 2 = degree-days for that day. If the minimum temperature exceeds the threshold temperature, then calculations in 1 and 2 on page 6 are used.

Usually, getting the maximum and minimum temperatures from a nearby weather station is suitable for calculating degree-days. Another alternative is to use a max-min thermometer, which may be purchased from a biological supply company. Temperature readings from a max-min thermometer should be taken between 7 and 10 a.m. every day to acquire a correct reading of the maximum and minimum temperatures for the previous 24 hours. The thermometer should be placed in a white shelter approximately 5 feet above ground level and positioned where no direct sunlight reaches it.

Blacklight traps (Figure 4) or pheromone traps (Figure 5), placed in the field before spring moth emergence during late April (Missouri) to early June (Wisconsin), should capture moths soon after they begin to emerge. Seasonal conditions have a great deal of influence on the dynamics of moth appearance. One example of European corn borer moth population dynamics, as measured by a blacklight trap, is illustrated in Figure 6 for a central Minnesota population during 1991-1992. As shown in Figure 6A, and discussed earlier, the timing of moth flights (and generation peaks) may vary considerably from year to year or by location. Year-to-year variability is mainly due to seasonal degree-day (Table 1) accumulations and weather severity within a season. In this example, 1991 was a very warm year, and 1992 was a very cool year. Therefore, in calendar time, the generation peaks occurred earlier in 1991, with a partial third-generation peak in September of 1991. In 1992, the second-generation flight was nearly 4 weeks later and of much lesser degree than the second-generation flight in 1991. Figure 6B illustrates the usefulness of summarizing the same data on a physiological development, degree-day timescale. Because the degree-day model virtually neutralizes the effect of fluctuating temperatures on insect development, the timing (beginning) of generation peaks on a degree-day scale is more similar.

Use of a degree-day forecasting system continues to be a valuable tool for predicting the timing of peak flights and scheduling sampling activity.

Figure 6. European corn borer moth flights during two years, near Rosemount, Minnesota. Figure 6A illustrates seasonal variation on a calendar timescale; Figure 6B shows synchrony based on a degree-day timescale (W. D. Hutchison).

Figure 7. European corn borer moths: lighter female left, darker male right (M. E. Rice).

The moths (Figure 7) will leave emergence sites in plant debris and fly to nearby areas of dense vegetation, usually brome or other grasses, in conservation lanes or near fencerows. The vegetation in these habitats collects water droplets from dew and rain more effectively than corn plant leaves, so the micro- climate provides water to drink and the proper conditions for resting and mating. Drinking water is very important to moth survival and reproduction. In more arid areas where less dense natural vegetation is available, especially when corn is raised under sprinkler irrigation, moths may fly to and reside in the growing corn, which provides drinking water and suitable resting and mating conditions due to higher humidity.

These locations, which ensure a high degree of moth survival, are referred to as action sites (Figure 5) because of the high level of moth activity. In these action sites, the female moths must drink water before they can begin emitting a sex attractant (pheromone) composed of two isomers of 11-tetradecenyl acetate. This activity usually begins by 10 p.m., peaks at 1 a.m., and ends at dawn. The combination of the habitatÕs microclimate and the emission of pheromone draws large aggregations of European corn borer moths into relatively small areas of dense vegetation.

Female moths, which are lighter in color than males (Figure 7), normally mate during the second night, then leave the action sites to deposit eggs in the target crops for several nights. On warm, calm evenings, these egg-laying activities will begin shortly after sundown and cease by midnight. After laying one or more egg masses, females will leave the target crop and return to action sites to feed, rest, possibly remate (less than 5 percent), and wait for another suitable egg-laying evening.

Figure 8. European corn borer egg mass with numerous eggs on underside of corn leaf (M. E. Rice).

Figure 9. Egg mass in blackheadsatge about to hatch (M. E. Rice).

Figure 10. Recently hatched European corn borer first instars (C. E. Mason).

If the target crop is corn, spring- flight females are attracted to the tallest fields. Egg masses have as many as 60, but average 15 white eggs for first flight and 30 for second flight. The eggs overlap like fish scales (Figure 8). Egg masses are normally deposited on the basal two-thirds of the leaf blade near the midrib on the underside of corn leaves. Egg masses are somewhat flat and approximately 1/4 inch in diameter. Eggs close to hatching have distinct black centers, which are the black heads of the larvae that are visible through the translucent eggshells (Figure 9). The eggs hatch (Figure 10) in 3 to 7 days, depending on temperature and other weather conditions. Each mated female is capable of depositing an average of two egg masses per night for 10 nights. However, the majority of the masses will be deposited during the first 6 nights after mating. Each second- or subsequent-generation female will lay about 400 eggs during her life. Females from the first flight are not as productive.

During the whorl stages of corn development, the majority of egg masses are deposited on the underside of leaves that have fully emerged recently, while a few egg masses will be laid on the leaf sheath and upper surface of leaf blades. Larvae emerging from these egg masses usually move directly into the whorl for shelter and food. Once in the whorl, the larvae tend to stay between the unexpanded leaves until the green tassel begins to emerge from the whorl. Subsequent to tassel emergence, the larvae usually move to other locations on the plant, such as leaf axil, leaf sheath, and stalk. However, some may remain in the tassel for a while before moving.

Figure 11. Feeding on leaf mesophyll by young larva showing windowpane effect (M. E. Rice).

The first and second instars feed on the mesophyll of leaves, which results in areas on the leaves that show a ÒwindowpaneÓ effect (Figure 11). This effect occurs because the mesophyll has been removed, leaving one layer of transparent epidermis. Early feeding can be detected by looking into the whorl for damage to the young leaves. This damage typically consists of small holes and patchy areas lacking leaf tissues. Larval frass often appears on these leaves.

Figure 12. Fifth instar, fully grown European corn borer larva in a cornstalk (M. E. Rice).

Figure 13. European corn borer pupa in cornstalk (M. E. Rice).

If the corn plant is small (usually before the 6th-leaf stage) when eggs hatch, most of the larvae fail to become established. They wander off and die due to several factors, including natural feeding deterrents. In many corn hybrids, a primary factor for behavioral deterrence is a plant aglucone, 2-4 dihydroxy-7-methoxy-1, 4-benzoxazin-3-one (DIMBOA). The concentration of DIMBOA in a given corn hybrid usually decreases proportionally with plant growth. A greater proportion of larvae survive on corn in mid-to-late whorl stage, which is when plants are in the 8th- to 12th-leaf stage. Usually corn plants are mid-to-late whorl stage when most of the eggs are deposited during the spring European corn borer flight in the central Corn Belt. But, even in the presence of optimal corn-plant stage, environmental variables, such as dry weather, high temperature, heavy rainfall, and natural enemies, will kill up to 70 percent of the freshly hatched larvae. Larvae that live normally move to the whorl to feed and develop. Eventually, the larvae crawl out of the whorl and down the side of the stalk to burrow into the stalk of the corn plant where they develop to the fifth instar (Figure 12) and pupate (Figure 13) during the summer, if they occur in areas where there are two or more generations per year. Otherwise, they remain as dia-paused larvae in the stalk and overwinter in the fifth instar.

The moths that emerge in midsummer fly to dense vegetation, primarily foxtail grass in the Corn Belt, to feed, rest, and mate. These mated females prefer to deposit eggs on succulent, recently tasseled corn plants. The eggs will produce the second generation of European corn borer. If corn plants in commercial vegetable areas have progressed beyond the silking and tasseling stage, the females will lay eggs on beans, peppers, or other suitable succulent host plants.

During the silking and tasseling stage of corn growth, approximately 91 percent of the egg masses will be laid on the basal two-thirds of the underside of the three leaves above the ear, on the ear husk, on the underside of the ear leaf, and on the three leaves below the ear. Research in Kansas and discussions with specialists in other states of the central Corn Belt indicate that egg laying during the second moth flight takes place over a 20-day period, with peak egg deposition 10 days after the first eggs are deposited (Figure 14A). However, in cooler regions, the egg-laying period can be appreciably longer. If the egg-laying period is approximated as a symmetric 20-day triangle, the entire egg-laying period can be determined by locating initiation of egg laying. During this time of year, depending on weather conditions, the eggs will hatch in 3 to 5 days. The majority of the small larvae will move to the leaf axils and feed on sheath and collar tissue or on pollen that has accumulated in these sites. Some of these young larvae will feed in other protected areas, such as under the husks on the developing ear. When the larvae reach the third instar, some will bore into the leaf midrib. By the fourth instar, the majority of larvae bore into the stalk, although some will continue to feed within the ear, sheath, and collar.

Figure 14A. Egg-laying period of European corn borer in the central Corn Belt during the second moth flight represented by a 20-day triangle. Shaded area is proportion of total eggs laid if a sample was taken 8 days after the first eggs were deposited (D. D. Calvin).

Figure 14B. Twenty-day, egg-laying triangle for the central Corn Belt during the second moth flight indication the best time to sample egg masses on field corn (D. D. Calivin).

Late in summer, particularly in southern portions of the Corn Belt, a third flight of moths may occur and deposit eggs that produce a third generation. Similarly, during unusually long warm seasons, an additional partial or complete generation can occur regardless of location in the Corn Belt. These eggs may be laid on tasseled corn plants when the kernels have not matured beyond the milk stage. The late-developing larvae may reach the last instar (Figure 12), go into diapause, and spend the winter in plant residue. Those that do not reach the fifth instar before winter will not survive.