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Figure 1. Adult corn rootworm emergence will begin soon. What will it reveal about weather impacts? Photo: Dave Hansen |
This year’s emergence will be particularly interesting for what it indicates about 3 mortality factors:
- Winter survival – The brutal cold weather in Dec. and early January could have caused egg mortality. Emergence will tell us when and where winter mortality was significant.
- Delayed planting – Later planting dates typically mean smaller root systems at hatch with fewer larvae successfully colonizing root system. Later-planted fields are presumed to have lower larval rootworm densities.
- Spring rainfall – Soils saturated from heavy rains during hatch can cause “drowning” of small, newly hatched larvae. Extreme rainfall events through much of MN in early June could have reduced larval density, especially in low lying and poorly drained areas.
So far, we’ve noticed densities of 20-30 larvae per root system [flotation typically underestimates densities) at heavily infested sites near Rosemount (well drained Waukegan silt loam) and Litchfield (clay loam soil). That’s enough larvae for significant root injury. At these sites, none of these prospective mortality factors did the job. At Springfield, standing water in the field appears to have reduced larval numbers.
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Figure 2. Scout corn rootworm populations early in silking to control silk clipping and prevent pollination problems. Photo: Eric Burkness |
- Ensure pollination success. Scout as field begins to silk. Spray field with insecticide to knock down adult populations only if silks are kept pruned to within 0.5” early in pollination. Quit scouting as browning silks indicate pollination has occurred. Populations pruning any silks after this point are irrelevant from a pollination perspective. Note: Late planted and even replanted fields this year will be at particular risk as beetles are attracted to fresh silks and will congregate in these fields.
- Gauge egg laying populations to adjust management efforts next year. With low corn prices, tailoring corn rootworm management to the population in the field offers the best potential to reduce costs while ensuring adequate root protection and keeping corn rootworm populations under control. Sticky traps and whole-plant counts are the two preferred methods to quantify corn rootworm populations.
Help with the IPM corn rootworm survey
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Figure 3. Scouting corn rootworm populations with yellow sticky traps just got easier. Photo: Dave Hansen |
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