Why Things Happen When They Do

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This column has covered growing degree days, chilling hours, planting based on our average frost free date and growing season extender methods. One more to add to the list when it comes to insect infestations on our favorite plants is something called Phenology. What a plant looks like and very specifically what stages their flowers are in can tell us it may be time to be on the look- out for and perhaps the need to treat for certain insects. This is not limited to the just the plant in flower and its pests, but to lots of insects developing on other plants too. By getting to know the stages of flower development on just one landscape plant you can predict and be prepared for a whole host of insects in the landscape

Phenology is actually more accurate at predicting an insect outbreak than using a calendar. A strong change in the weather may delay or accelerate plant growth, the insects are also exposed to the same conditions and the both develop together. Plants and insects have never been too good at reading a calendar. One of the neat things about using Phenology is that not only does it let us know that the potential for a pest outbreak is near, it also can guide us to treat those harmful insects at their most vulnerable stage, making control easier and allowing the least toxic product to be used that will manage the insect population.

Dr. Phil Nixon, Entomologist with the University of Illinois recently provided an example recently of how Phenology works. Dr. Nixon, using a reference book called "Coincide" by Don Orton used just one plant to show just how many ornamental insect pests could be monitored. Excerpted from a recent issue of the home yard and garden newsletter as follows:

Following are the most common pests that are in susceptible treatment stages during vanhoutte spirea bloom.

Full bloom: Birch leafminer young larvae; elm leaf beetle young larvae; European pine sawfly feeding larvae; gypsy moth feeding larvae; pine needle scale crawlers (first generation), black turfgrass ataenius (first generation).

Full to late bloom: Lilac (ash) borer newly hatched larvae; oystershell scale (brown) crawlers; emerald ash borer adult beginning emergence.

Finishing bloom: Bronze birch borer newly hatched larvae.

Most blossoms brown, still a few white: Flat-headed appletree borer larval hatch; peach tree borer newly hatched larvae; viburnum borer newly hatched larvae.

Bloom finished: Oystershell scale (gray) crawlers.

If you began to connect insect feeding problems in your yard with the flowering stage of either the host plant or another landscape plant in your yard you could easily begin to predict the emergence of insects. I looked up another favorite of gardeners, the Bagworm, to find out the best time to expect young larvae to be present. According to Don Orton, the best indictor plants are Catalpa and Tree Lilac when they are in full bloom. I will be watching and waiting, Tree Lilacs are going to be in bloom in 2-3 weeks.

About the author: Richard Hentschel’s expertise extends across several subject areas with specialties in lawn care, fruit tree production, woody ornamentals, and home and community gardening. During his 45-year career in horticulture and agriculture, Hentschel became a well-known and respected expert for commercial and homeowner audiences, industry organizations, and media. He retired from University of Illinois Extension in April 2022 with nearly 30 years of service as a Horticulture Specialist and Educator in northern Illinois.