Life Cycle of Corn Rootworm
The western corn rootworm (WCR), northern corn rootworm (NCR), and southern corn rootworm (SCR) all have a similar lifecycle, which includes four stages: the egg, larva, pupa, and adult. The NCR and WCR both overwinter as eggs in the soil, but the SCR overwinters as a beetle.
Northern and Western corn rootworm
Beginning in late May or early June the WCR and NCR larvae hatch and begin their single generation life cycle. The larvae are immediately attracted to corn roots by the emission of CO2from the root tips and begin feeding. Since corn roots are the primary food cource for CRW larvae extensive damage can occur with a high population of larvae in soil. The CRW larvae go through three instars, or developmental stages, that each lasts seven to ten days. The first instar is less than 1/8 inch in length. By the third instar larva can measure up to 1/2 inch in length. (Willson & Eisley, 2005) (FIG. 4)
After the larvae have completed the three developmental stages (instars), they will pupate in the soil. The pupal stage is a dormant stage when no feeding takes place. During this stage the larva is developing into an adult. The CRW pupa (FIG. 5) is white, somewhat translucent.
The WCR and NCR (FIGS. 6 and 7) adults begin to emerge in July and may continue for one month. Male beetles emerge prior to females. After the beetles emerge from the pupal case, they dig their way up to the surface of the soil. Once the adults have exited the soil they will instinctively crawl upward, often on a corn plant where they will begin feeding.
The female CRW beetles mate soon after emerging and then proceed to feed for approximately two weeks before returning to the soil to lay eggs. In the corn field, female beetles search for cracks in the soil to lay eggs. The NCR will deposit eggs within the top eight inches of the soil, while the WCR may go as deep as 12 inches (Wright et al, 1999). The eggs are laid in and around the outside of the soil crack individually or in groups of 20-30 eggs. Once all the eggs have been laid, the beetles will remain in the soil and die. The NCR and WCR eggs will remain in the soil over the winter and then hatch in the spring to begin the lifecycle once more.
Southern corn rootworm
The southern corn rootworm has the same lifecycle as the NCR and WCR, but the timing and order in which developmental stages occur is different. The SCR is found throughout the Corn Belt, but is most prevalent in the southern states of the United States. Therefore, due to the climate of this area the SCR has two generations per season.
The SCR overwinters in the adult stage (beetle; FIG. 8) in plant material that has not been killed by frost (Southern, 2002). The beetles first become active in mid-March when they emerge in search of food and to begin laying eggs. Eggs are laid individually starting in late April and continuing until early June. Larval hatch is temperature dependent but can occur as early as seven to ten days after the eggs were laid. From September to November, the second-generation adult beetles can be found feeding on clover and alfalfa (Southern, 2002). The SCR not only uses corn as a host but also other field crops, cucurbits (such as cucumbers and squash), vegetables, flowers, weeds, and grasses. The damage caused by SCR to these plants ranges from stem and leaf damage to fruit and flower damage (Grantham, 2005).