C. Richard EDWARDS, PhD, BCE, FRES
Department of Entomology, Purdue University,
W. Lafayette, Indiana 47907-1158, USA
For an insect that spends part or all of its life in the soil, edaphic factors have a significant impact on its ability to move and survive within the soil profile. For the western corn rootworm (WCR), Diabrotica virgifera virgifera LeConte, once the eggs are laid in the soil in mid to late summer various physical and chemical factors impact egg survivability, egg hatching, first instar larval movement, and larval development and mortality within their soil habitat over a twelve month period. Upon hatching, soft-bodied first instar larvae move through the soil in search of food by primarily utilizing a network of interconnected pores. Where the interconnecting pores are greater in size than the width of the first instar’s head capsule, unimpeded movement through the soil normally occurs.
The interconnecting pores not only provide a pathway to their food source, but also serve as a location where oxygen is normally available for larval respiration. These pores also hold water. This can be beneficial, although saturated soils impede larval movement, decrease oxygen availability, and restrict the movement toward corn plants. To what degree the pores fill with water and how this condition relates to the texture of the soil governs to a large degree how successful the larvae will be moving through the soil. Additionally, the movement of carbon dioxide through these pore spaces and the impact of moisture and soil texture are important for larval host finding. Carbon dioxide emitted from the roots of corn has been identified as the primary factor for triggering movement of WCR toward corn roots.
The size and number of pores within the soil profile are governed by soil texture, structure, and bulk density. WCR first instar larvae have been shown to move greater distances in fine textured soils than coarse textured ones. Soils high in clay content have a greater defined secondary structure with a higher number of connected pores than sandy soils. Additionally, larval movement is affected by the interaction of soil moisture with soil texture. In most instances, saturated soils and/or dry soils can impede larval movement and/or cause high larval mortality. In very wet soils, mobility problems and oxygen depletion result in high larval mortality. In dry soils, high mortality occurs due to inadequate humidity levels to support normal larval development and/or, if sandy in texture, the sand particles may scratch the insect’s cuticle resulting in death due to desiccation.
The literature notes that larvae can move up to about 40 cm in search of food before their limited food reserves are used up. At 25oC and 90% relative humidity, first instar larvae can survive a maximum of about 40 hours in the soil (time for 50% mortality). As relative humidity decreases and temperature increases the time of survival is reduced. Soil texture, structure, and bulk density play a role in shortening or lengthening the time available for WCR larvae to find a host plant. Any factor that impedes larval movement, affects larval development, or restricts the movement of carbon dioxide in the soil, will affect WCR larval establishment.