Issue 2, May 8, 2017

Insect Degree Days and Phenology Update

An article in the previous issue of this newsletter two weeks ago indicated that insects were approximately one week ahead in northern Illinois, and two weeks ahead of a normal spring in central and southern Illinois. Due to the cooler weather of the past two weeks, one can see from Kelly Estes' article in this issue on degree days that we are approximately two days ahead of normal on insect development in northern Illinois, four days ahead in central Illinois, but still about ten days ahead in southern Illinois.

As happened earlier this year, high temperatures during the last couple of weeks in the forties and low fifties degrees F have caused insect development to stay relatively static while plants have continued to progress in their development. An even wider gap has opened between insect and host plant development. Insect damage is reduced as their host plants have more leaf area than is usual when insects emerge to feed upon them.

To illustrate advanced plant development, bridlewreath spirea, Spiraea X van houttei, was in full bloom in the third week of April this year in central Illinois. It typically blooms in the second week of May. Another plant we use heavily in phenology is beauty bush, Kolkwitzia amabilis, which started blooming in central Illinois in the last week of April. It typically blooms in the third week of May.

As can be seen above, though plants are considerably ahead in their development over a typical spring, insects are not. In our recommendations, we typically give both a calendar and plant phenology timing for management. This year, it is best to follow the calendar timing to scout for insect stages susceptible to control. They are going to be closer to the correct timing than phenology. (Phil Nixon)

Phil Nixon

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