Biology and Ecology of Corn Rootworm
Because corn rootworm is a significant pest of corn, much research has been done to understand the biology of this pest. Although some data is available for the northern and southern corn rootworm species, a lot of research has been conducted for western corn rootworm since a non-diapausing population exists that makes laboratory research more efficient.
Egg diapause and mortality
Eggs of western and northern corn rootworm, which overwinter in the soil, must undergo a period of suspended development known as diapause. Without diapause, eggs would hatch in the fall or winter when conditions are not suitable for growth and development and a host is unavailable. This would be detrimental to the population. The end of diapause is determined by time elapsed, not any known environmental cues, and occurs during mid-winter in the Midwest. Once diapause ends, eggs stay in a dormant state until soil temperatures have reached 52°F and eggs have absorbed water.
Egg mortality varies and depends on how buffered they are from harsh winter air temperatures. The deeper eggs are laid, the more buffered they will be. Snow cover and residue can provide additional insulation for eggs in the soil. Egg mortality is higher when subjected to freeze-thaw cycles.
Larval survival and movement
Eggs hatch and larvae emerge beginning in mid-May to early June depending on accumulating growing degree days (GDD) in the soil. Research suggests that 50% of eggs hatch when 684-767 GDD have accumulated since January 1 (base 52°F). The average length of egg hatch is reported to be 29 days for males and 32 days for females. Larval survival depends on many factors, including soil type and soil moisture. Sandy soils and very dry soils are abrasive and can scratch larvae, causing them to lose moisture and die. Compacted soils can cause starvation if soil pores and channels are too small for larvae to move through. If soils are flooded or saturated with water, larvae could drown or be unable to locate a host plant. Additionally, if corn is not present in the field at the time of hatching, larvae will starve to death.
Young larvae are attracted to the CO2 released by growing corn roots, and they will orient themselves toward this cue. They likely move through the soil using pores and channels that are larger than their head capsule. It is typical for larvae to move 6 inches (15 cm) to find a food source, but they can move up to 18 inches (46 cm) as needed. First instars typically feed on root hairs and burrow into the roots on the lower nodes. As both the corn plant and corn rootworm larvae grow, larvae will move to consume roots near the base of the stalk where new nodes are developing. Root feeding continues until pupation occurs.
The third-instars pupate in discrete earthen cells. Pupation is a non-feeding stage where rootworms develop wings and reproductive organs before becoming an adult.
Adult emergence typically extends from mid-July through mid-August, depending on the progression of development in a particular year. The duration of emergence varies greatly year-to-year and field-to-field, but is typically 4-8 weeks. The average emergence duration for WCR males and females is 33 and 51 days, respectively. Males emerge before females, and western corn rootworm emerges before northern corn rootworm. Northern corn rootworm males typically begin emerging 5-7 days after western corn rootworm, and male corn rootworms emerge 2-10 days before the females.
Factors that delay adult emergence include delayed planting, conservation tillage, grassy weeds, high larval densities, soil insecticide exposure, and exposure to Bt toxins. Factors that reduce adult emergence include delayed planting, high larval densities, soil insecticide exposure, and exposure to Bt toxins. Narrow rows may increase adult emergence.
Western corn rootworm adults may disperse an average of 20-56 feet (6-17 meters) per day within a field. Most of the time, it is gravid females that disperse to new fields, likely in search of pollen to continue egg development. Long-distance dispersals are uncommon, but WCR has the capacity to fly up to 24 miles (38.6 km) in a day, unless they are carried away by storm systems in which case they could travel much further. It is believed that northern corn rootworm adults disperse more than western corn rootworms, meaning western corn rootworm adults are more likely to stay in the field from which they emerged. Southern corn rootworm beetles are excellent fliers and often migrate large distances.
Adult western and northern corn rootworms are strongly attracted to corn silk and pollen, and clusters of beetles may develop on silking plants. Both species will move away from corn to less preferred hosts once corn pollination is complete. Females need pollen for egg development, even if it is not corn pollen. Southern corn rootworm beetles do not have a strong preference for corn and will feed on any high-quality pollen source.
Egg-laying occurs approximately 14 days after adult emergence, at which time eggs are mature and ready to be oviposited. Adults seek desirable sites, those with sufficient soil moisture and texture, and use cracks and channels to enter the soil to lay eggs. Western and northern corn rootworm typically oviposit their eggs 4-6 inches (10-15 cm) below the soil surface, but in dry years can go as deep as 12 inches (30 cm). 75% of WCR and NCR eggs will be deposited over a period of 30 days in clusters of 50-80 eggs. On average, a western corn rootworm female will lay 400 eggs during her lifetime but may deposit over 1,000 eggs. Northern corn rootworm females lay approximately 300 eggs. Southern corn rootworm oviposits at the soil surface – a female identifies a crevice or soft spot in the soil to lay a few eggs, then moves a short distance to repeat the process. First-generation southern corn rootworm females lay an average of 400 eggs, singly or in clusters, but may lay up to 1,200 eggs.