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European Corn Borer and Bacillus thuringiensis

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Life Cycle of European Corn Borer

The life cycle of the European corn borer involves four main stages: egg, larva, pupa, and adult. The borer is representative of the larva stage (Fig. 1) and the ECB moth is representative of the adult (Fig. 2). Together, these four different stages of development make up one generation.

Fig. 1 ECB: First instar larvae (Jim Kalisch, University of Nebraska Lincoln. 1995)

Fig. 2 ECB: Adult female moth (Jim Kalisch, University of Nebraska Lincoln. 1995)

The mating process for the first generation of ECB begins in the spring (approximately May to July depending on geographic location). The moths will begin to emerge from diapause and begin mating. The moths find an action site consisting mainly of grasses near the corn field. This area provides water in the form of dew, which is needed for the female moths to emit an attraction pheromone to the males.

The female moth will begin laying eggs in June, after sunset when the wind is calm and the temperature is warm. 'The eggs are laid in masses on the underside of corn leaves near the midvein' (Michigan State University, 1997). 'The masses are approximately ¼ inch in diameter and made up of 15-30 individual eggs (Fig. 3).

Fig. 3 ECB egg mass: This picture shows the egg mass after being laid near the midvein of the corn leaf. (Jim Kalisch, University of Nebraska Lincoln. 1995)

Each female moth is capable of producing two egg masses per night for 10 nights. However, most of the eggs are laid in the first six nights' (Mason, et al., 1996). The egg masses appear white when first laid, but as the eggs mature they will turn a pale yellow and then translucent at the blackhead stage. The blackhead stage occurs right before hatching when the larvae are visible through the translucent egg masses (Fig. 4). The black centers of the eggs are caused by the black heads of each individual larva. 'The egg masses will hatch three to seven days after being laid, depending upon the temperature and other weather conditions' (Mason, et al., 1996).

Once the ECB eggs have hatched, the larval stage of development begins. The emerging larvae move from the underside of the leaves to the center of the plant, the whorl, for shelter and food. During the larval stage there are five developmental stages called instars. Throughout these five instars the larva are growing and preparing to become adults.

At the first instar the larvae are white in color with a black head and are approximately 1/16 inch long. During the second instar, the ECB grows larger and becomes more yellow in color. Both of these two instars feed on the leaf tissue and create holes in the leaves of the whorl. However, the second instar creates larger holes than the first. 'ECB feeding in the whorl lasts approximately two weeks' (Witkowski and Wright, 1997).

Fig. 4 ECB blackhead egg mass: This picture shows the egg mass in the blackhead stage, prior to the larvae emerging. (Jim Kalisch, University of Nebraska Lincoln. 1995)

The third instar begins feeding on the outside before boring into the stalk and creating cavities in the plant. The fourth and fifth instars feed only on the stalk. Once the larvae have reached the fifth instar, their color is grayer with brown spots, outlined in black, present on each section of its body. At the fifth instar, the larvae are approximately one inch in length. The ECB larvae will live and feed inside the stalk.

The second generation of ECB is much like the first, however there are some slight differences. In the second generation, the larvae do not begin feeding on the stalk of the corn plant until the fourth instar, due to the hardness of the maturing stalks. In the first generation the stalks are tender and can be infested more easily by the third instar. As the corn plant reaches maturity the tassel emerges from the whorl of the plant thus eliminating second generation larval feeding on this part of the plant. The second generation larvae tend to feed on the leaf axil, closer to the stalk, rather than the blade of the leaf. The larvae also consume pollen that has collected in the leaf axil.

In certain regions, of the southern United States, there is a third and possibly a fourth generation of ECB. These develop in the same manner as the previous generations. However, the fifth instar must be present in the corn stalk or plant debris in order for diapause to occur after the final generation.

Life cycle animation

Depending upon how many generations occur within a specific region, the fifth instar will do one of two things. In regions where only one generation occurs, the larvae will stay in diapause throughout the winter. Where more than one generation occurs, the ECB will go directly from the larval stage to the pupa stage of development. After the last generation for a region, the ECB will go into diapause for the winter. To see an animation illustrating the entire lifecycle of the ECB, click on the Life cycle animation above.

European corn borer larvae are able to withstand low temperatures during the winter by producing a natural, alcohol-like substance that acts as antifreeze. After a winter of diapause the pupa stage of the ECB life cycle occurs in the spring, once the temperatures have reached 50°F (Mason, et al., 1996). The diapaused larvae form a brown capsule-like cocoon, the pupa (Fig. 5).

Fig. 5 ECB pupa: This picture shows the pupa in a corn stalk, prior to moth emergence. (David L. Keith, University of Nebraska Lincoln. 1971)

This step is necessary for the adult moth to form. Pupation also occurs after each generation. For example, in regions with two generations of ECB, a pupa forms after the fifth instar of the first generation. The pupa stays inside the corn stalk and later emerges as a moth in July and August to start the second generation.

Question : What stage of ECB is the most damaging to crops?



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