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Issue No. 10, Article 2/May 27, 2005

Corn Rootworm Larval Hatch: Should We Expect a Late Hatch in 2005?

Based on the projected heat unit accu-mulations, the answer appears to be yes. The timing of the corn rootworm larval hatch has varied considerably during the past 10 years. In 1996 and 1997, larval hatch occurred on June 12 and June 13, respectively, across central Illinois. From 2000 to 2003, hatch (for central Illinois) has taken place on these respective dates: May 22, May 16, May 31, and May 29. First hatch was reported over the Memorial Day weekend in 2004.

In most years, corn rootworm larvae begin to hatch during the last week of May. So predicting late May as a projected date for corn rootworm hatch would be a safe bet in most years. However, in some cool springs (1996 and 1997), hatch has been delayed until mid-June. During very warm springs, hatch can occur as early as the beginning of the third week in May. So there is some variation in this annual biological event. Research has shown that after 684 to 767 soil heat units (base 52°F, 4-inch soil profile) have accumulated, from January 1, approximately 50% of corn rootworm larvae should have hatched. We continue to use this published research to project when corn rootworm larvae are most likely to begin hatching. Please refer to this Web site to view maps for current degree-days and projected degree-days (one week and two week) for the corn rootworm larval hatch.

When is the corn rootworm larval hatch expected to occur, based on the degree-day accumulation this season?

Projected heat unit accumulations (base 52°F, 4-inch soil profile) suggest that the corn rootworm larval hatch may not be well under way across central Illinois until June 7 (based on 2-week projections). The cool spring we've experienced is very likely to delay the hatch as compared with the last several years. Once the annual spring hatch begins, it typically lasts for 2 to 3 weeks. Not all of the corn rootworm larvae hatch at the same time. This helps to ensure greater chances of survival for the species.

Does the timing of the corn rootworm larval hatch affect soil insecticide and transgenic corn rootworm hybrid performance?

Early planting (early to mid-April) followed by a late hatch (early to mid-June) creates the most significant challenge to insecticide performance. Under this scenario, corn rootworm larvae may continue to feed on root tissue through most of July. If an insec-ticide has been applied in early April and a late hatch occurs, the insecticide will need to continue offer-ing protection approximately 15 to 16 weeks after application. This may lead to product performance issues, especially if high densities of corn rootworm larvae are present. Likewise, an extended larval feeding period may place more pressure on some transgenic corn rootworm hybrids to provide optimum protection. Based on the very large densities of western corn rootworm that populated the Illinois landscape in 2004, we should anticipate a very impressive hatch of larvae to occur this spring. So far, the dry spring in many areas of Illi-nois also will tend to favor the establishment of corn rootworm larvae. As we begin to receive reports in the coming weeks regarding the corn rootworm hatch and subsequent observations of root injury, we'll share the information with our readers.--Mike Gray

Mike Gray

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