Other Early Season Insect Pests

When choosing CRW control, there are other soil insect pests that need to be considered. These other pests include seed corn maggot, seed corn beetle, white grub, wireworm, and black cutworm. Each of these insects can also affect field corn at the same time as CRW, therefore requiring a control method that will control any and all of these corn pests. In order to plan what control methods to use, an understanding of each pest’s life cycle and the damage that can occur is necessary.

The Seedcorn maggot (Delia platura) is only about ¼ inch in length and is light yellow to white. The life cycle of this pest contains an egg, larval, pupal, and adult stage, with the insect overwintering during the pupal phase. The adult flies emerge in the spring and within 2-3 days have mated and laid eggs. The eggs are deposited on seeds, emerging plants, and in decaying material within the field. Once the larvae hatch from the eggs 2-4 days later, full development will occur within the soil. After the larval stage is complete, the insect will pupate. The adult emerges after pupation is complete. This cycle occurs approximately three times to create three generations per season (Bennett et al, 2002). Spring weather conditions, percent organic matter, and the presence of manure influence the likelihood of infestation.

The larvae feed on the seed and on the emerging cotyledons causing damage to germinating corn seedlings. The slow germination of corn seedlings during a wet and cold spring, allows a higher opportunity for maggot attack. The damage caused by seed corn maggots includes killing the seed embryo and feeding on the cotyledons and first true leaves of the plant creating holes in the newly emerging leaves. (FIG. 14)

Seedcorn beetles (Stenolophus lecontei) are light brown in color and have a black stripe on each wing as well as a black head. The beetles are approximately ¼ inch or less in length. The most damaging stage is the adult stage. The adults feed on the germinating seed, which in turn causes reduced plant stand. (FIG. 15) 

The white grub (Phyllophaga spp.) has a C-shaped, white body with a brown head. The adult beetle is known as the Japanese beetle or June beetle. The life cycle of the true white grub lasts approximately three years. During this three-year cycle, the grub will go through four stages: egg, larva, pupa, and adult. (FIG. 16)

In the first year the adults begin to emerge in May and June. The beetles feed, and after mating, return to the originating fields and lay between 35-60 white eggs in the soil. It takes about 30-50 days for the eggs to hatch. When the larvae emerge, they begin feeding on organic matter in the soil and then move on to plant roots. The larvae will reach the second instar before the temperatures drop in the fall and move deeper into the soil, below the frost line, and overwinter (Glogoza, 2005)

The following spring begins the second year of the white grub life cycle. As the temperatures increase the larvae move to the top six inches of soil. During root development and growth the grubs will feed on the roots causing damage to the entire plant. The most damage occurs during this second year of the life cycle. Once the soil temperature begins to drop in the fall, the third instar larvae will move into the soil below the frost line and overwinter.

The third and final year in the life cycle begins the next spring as the larvae move to the top six inches of the soil. The larvae in this year of the life cycle do not cause a great deal of economic damage to the corn crop.

Throughout the three-year life cycle of the White grub there should be only one year where economic injury to corn can be expected due to feeding. The expected year of damage can be determined by taking soil samples and getting a population count of the most prevalent life cycle stage present in the soil.

Wireworms (Limonius spp.) range in size from ¼ to ¾ inch in length and have a hard brown covering. The life cycle of the wireworm lasts from two to three years (depending on the species) and includes the egg, larval, pupal, and adult stages. The larval stage can last for two to three years before maturity is reached. During this time the wireworm can feed on the plant parts below the soil including the seed and stem. (FIG. 17)

The wireworm, also known as click beetles, overwinters in the soil as a larva or adult. After the overwintering adults emerge, they deposit eggs in the soil during the summer months. The maturing larvae feed on the underground parts of the plant and will pupate in late summer. The pupae resemble the adult beetles except that the pupae are white and the adults are black or dark brown in color. The adults emerge from the pupa and remain in the soil, overwintering until the next summer.

The main damage that occurs from the wireworm during the larval stage happens to the seed and emerging seedling. The wireworm can eat the seed itself or cut off the seedling below the soil surface. Once the plants are larger the wireworm will tunnel into the parts below the soil surface. This causes the plant to wilt and eventually die (Gesell and Calvin, 1983).  

Black cutworm (Agrotis ipsilon) larvae are gray to black in color and range in size from ¼ inch to 2 inches when the larvae are fully mature. The life cycle includes the egg, larva, pupa, and adult and can have three generations per year. Each spring, black cutworm adults migrate from the southern states northward to infest fields in the north central Corn Belt (Wright, et al., 2001). (FIG. 18)

The size of the corn plants and the level of soil moisture play a large role in the type of damage black cutworms exhibit. When soil moisture is high, the cutworms will emerge from the soil at night to feed on the above ground plant parts. However, when drier conditions exist the cutworms will remain in the soil and feed on the plant below the soil surface. The effects of below surface feeding cause the corn plant to wilt and eventually die. When black cutworms feed above ground, the effects are evident by plants completely cut near the soil surface. As the plants grow larger, damage by cutworms is less severe.