Weather playing havoc on gardens

If you think this weird weather is playing havoc with your garden,- imagine how the bagworm feels. Or any bug baby that normally hatches in July.

Fluctuating hot and cold temperatures can completely recalibrate an insect's internal calendar. Good and bad bugs play an integral role in the garden; as pollinators, predators and pests. Plants bloom in anticipation of a pollinator's presence, and people plan parties and pesticides around a probable pest's appearance.

If the milkweed blooms before the monarchs emerge, a major pollen source is lost. If mosquitoes hatch before "zappers" are installed, plan for a very itchy garden party.

This synchronicity among plants and insects are part of the natural life cycle studied in phenology. This science, practiced in various forms for centuries, correlates one natural event with another.

In the 1700s, U.K. naturalist Gilbert White recorded 25 years' worth of naturally recurring plant and animal activities (migrations, budblooms, etc.), in his Natural History and Antiquities of Selborne.

Garden diaries and journals before and since have attempted to identify the connections among natural events.

Donna Danielson, the Morton Arboretum's plant clinic assistant, explains that a more recent improvement in phenology is the use of "degree-days." Rather than simply noting the date that plants bloomed or insects hatched, degree days essentially measure the cumulative degrees exceeding a specified base temperature during the season.

With degree days, you get a better picture of the total weather pattern during the season, rather than an arbitrary calendar date. While there are very extensive methods of calculating degree days, a simple estimate can work for home gardeners. Calculating degree days requires a starting point (say, Jan. 1) and a base temperature (typically 50 degrees in the Midwest). Every day, average the high and low temperature and subtract 50. Any positive result gets added to the accumulated growing degree days.

Mathphobics also can go to the arboretum's website at www.mortonarb.org and refer to the Plant Health Care report which keeps a running trend of degree days.

To predict insect emergence, heat is the best indicator. "Water isn't nearly as important as heat for bugs," Danielson said.

Thus, while a season may be exceptionally rainy or dry, insects will emerge closer to schedule than when temperature changes are extreme.

According to the Plant Health Care report, through June 19, we were at 649 growing degree days. This is nine days less than the historical average (1937-2007) and 16 days behind last year.

The colder than usual weather is responsible for late blooms on such plants as roses and peonies, as well as prolonged blooms on early spring flowers like trillium and Virginia bluebells.

Phenology includes "indicator plants" whose bloom is associated with the appearance of certain insects. While there may be no direct causative relationship among these plants and the insects they presage, years of observation has shown that the timing of bloom coincides with the emergence of particular insects. Don Orton, author of Coincide, in fact lists hundreds of insects associated with particular indicatorm plants.

Blooms on Japanese tree lilac, for example, are associated with the emergence of bagworms and euonymus scale. Plan on seeing Japanese beetles when elderberry blossoms. Watch for leaf crumple's and locust borers when Canada goldenrod is in early bloom.

What to do with all this phenological information? Orton's book suggests that, armed with preknowledge of insect invasions, a homeowner can devise an integrated pest management strategy. The timing of some pest control methods depends upon the insect's stage of development.

Or, if you're like Donna Danielson, you can enjoy the experience of observing nature such as the hatching of European pine sawflies. "It was really cool," she asserts. "The eggs are like little gold dots."

She pauses and adds, "Maybe you have to be a bug person." Bug people like Donna will enjoy the Big Bugs exhibit at the Arboretum through July 20.