Corn Rootworm - Hatch and Larval Stages

• Western corn rootworm (Diabrotica virgifera virgifera) and northern corn rootworm (D. barberi) complete one generation per year.
• Winter dormancy for eggs overwintering in the soil consists of two phases: obligate diapause and facultative quiescence (Krysan, 1978).
• Obligate diapause begins in the fall when embryonic development ceases in eggs that have been deposited in the soil. Embryos remain in this state until diapause ends.
• In the U.S. Corn Belt, the end of diapause often occurs sometime during the winter. At this point, dormancy enters the facultative quiescence phase, in which environmental conditions become the controlling factor.
• Embryonic development remains suspended until soil temperature increases above a threshold at which development can resume.
• In addition to the temperature threshold, eggs need to absorb water to complete development. If the surrounding soil is too dry, eggs will remain in a quiescent state until enough moisture is available to absorb for them to resume development.

• The length of time between the end of facultative quiescence and egg hatch can vary based on soil temperature. In general, warmer and more consistent soil temperatures will enable faster embryonic development, with a constant temperature of 82°F (28°C) being ideal for development (Schaafsma et al., 1991).
• Numerous factors such as soil texture, tillage, and residue cover can influence the soil microenvironment and affect the timing of egg hatch.

• Rootworm egg hatch in the U.S. Corn Belt typically begins sometime between mid-May and early June and continues for around a month. Peak egg hatch may vary as much as 10-14 days from year to year based on differences in soil heat unit accumulation.

• Newly hatched larvae are less than 1/8 inch (3 mm) in length and nearly colorless. Larvae have three pairs of legs behind their head capsule.
• All species of corn rootworm go through three instars during their larval stage. An instar is the period of growth between two molts. Molting is when a larva sheds its skin to allow it to grow larger.
• Third instars are approximately 5/8 inch (16 mm) long and creamy white with a brown head and a brown plate on top of the last abdominal segment (Figure 1).
• At a constant temperature of 70°F (21°C), western corn rootworm first, second and third instars complete development in 6, 5, and 12 days, respectively.
• Northern corn rootworm development is somewhat longer, with first, second, and third instars completing development in 7, 7, and 19 days.
• Newly hatched larvae are attracted by CO₂ released from corn roots and are capable of moving up to 1.5 feet (0.46 m) in the soil to reach roots to feed on.

• Their ability to move through the soil can be limited by soil conditions. Corn rootworm larvae are small and soft-bodied, so they rely on pore spaces in the soil profile for movement. If pore space is restricted, such as in soil that has been compacted by wheel traffic, or highly saturated, larval movement will be reduced. 
• Larvae initially feed on root hairs and smaller portions of roots. As larvae develop, they feed externally and internally on larger roots. Larger larvae tend to move toward the center of the corn root mass, feeding heavily on newer root tissue, including brace roots.
• Feeding is most extensive in early through mid-July in most regions of the Corn Belt.
• After a larva has completed the three instars, it will form a small chamber in the soil in which it will pupate. This is a dormant stage during which no feeding takes place (Figure 3).
• The pupa stage lasts around 10 to 12 days as the larva transforms into an adult.

Proximity to Food

• Newly hatched larvae have very limited mobility in the soil, so the ability to quickly find a proximal food source is essential to their survival. Larvae typically move through about 6 inches (15 cm) of soil to reach corn roots but can move up to 18 inches (46 cm) when necessary.
• If a newly hatched larva does not locate a suitable host within 24 hours, it is much less likely to survive (Branson 1989).
• As larvae grow, they redistribute, moving to younger root nodes that emerge from the stalk. Larvae may also redistribute to other plants when high larval density creates intense competition for food (Hibbard et al., 2004).

Soil Conditions

• Soil conditions can affect the ability of corn rootworm larvae to reach corn roots to feed. Since larvae rely on soil pore space for travel, soils with high bulk density can restrict movement.
• Muck soils generally have lower incidence of rootworm larval feeding damage.
• Dry sandy soils can cause scratching and abrasion to larvae as they search for food causing them to lose moisture and die.
• Silty or loam soils provide the best environment for larvae to move in search of food and generally have a higher survival rate.

Soil Saturation

• Soil saturation and flooding following corn rootworm hatch can dramatically reduce larval survival, causing larvae to either drown or be unable to locate corn roots for feeding (Riedell and Sutter, 1995).
• It’s not unusual for portions of a field that are saturated early in the season to have the least amount of root lodging later in the season when rootworm pressure is high.
• Survival rate is reduced when water is warmer. Larvae that have established feeding on corn root tissue are better able to survive short durations of saturation.
• A wet spell during the spring can be helpful in lowering corn rootworm pressure. However, eggs that have not yet hatched are not greatly affected by soil saturation and hatch can extend for over a month, so the effect of soil saturation on the rootworm population will depend on its timing relative to peak rootworm hatch.
• Soil saturation that persists over a long enough duration to kill off a large proportion of the corn rootworm population may also be detrimental to early corn growth.

• Hibbard, B.E., L.N. Meihls, M.R. Ellersieck, and D.W. Onstad. 2010. Density-dependent and density-independent mortality of the western corn rootworm: impact on dose calculations of rootworm-resistant Bt corn. J. Econ. Entomol. 103:77-84.
• Krysan, J.L. 1978. Diapause, quiescence, and moisture in the egg of the western corn rootworm, Diabrotica virgifera. J Insect Physiol. 24:535-540.
• Riedell, W.E., and G.R. Sutter. 1995. Soil moisture and survival of western corn rootworm larvae in field plots. J. Kansas Entomol. Soc. 68(1): 80-84
• Schaafsma, A.W., G.H. Whitfield, and C.R. Ellis. 1991. A temperature-dependent model of egg development of the western corn rootworm, Diabrotica virgifera virgifera LeConte (Coleoptera: Chrysomelidae). The Canadian Entomologist. 123:1183-1197. doi:10.4039/Ent1231183-6.