Drowned and diseased corn rootworm.
Drowned and diseased corn rootworm.

2015 is proving to be a perfect example of how environmental extremes can have serious detrimental effects on insect pests. Simply stated…insects breathe air and therefore can drown! This year’s wild swings in moisture and temperature are not unique to Indiana alone, as most Midwestern states are experiencing the same. This has implications for a key pest across the region, the western corn rootworm.

How is corn rootworm larval survival impacted by saturated soils? Rootworm adults have a tendency to lay eggs during late summer (when rain is often scarce) in areas where soil is moist and cooler than surrounding soil. These areas are typically found in soil types and field depressions that tend to collect and hold moisture in late summer. They will often use cracks in the soil to get deeper into the profile. This is why larval injury the following year tends to be clumped and not uniform throughout the field. These same areas that are attractive to rootworm adults for egg laying are where soils tend to become saturated first and remain wet the longest when rain is extensive. Therefore, the impact of drenching rains on rootworm can be significant if the timing is correct. Over-wintering rootworm eggs don’t require much oxygen and are very resistant to flooding and fields would have to be submerged many days before adverse impact would occur. Newly hatched larvae, on the other hand, are particularly susceptible to saturated soil and will die after being denied air for a less than a day. As reported in last week’s Pest&Crop, larvae throughout most of the state starting hatching in late May to early June, just as rains have been saturating much of the state’s soil. Newly hatched larvae that are still embedded in roots will generally not survive in submerged or heavily saturated soil after a day or two. The warmer the soil, the more rapidly these stressed larvae die. In short, it’s a very bad week to be a rootworm in the Midwest. This is a bit of good news we can offer in this very challenging season!