Bridal wreath spirea, or Vanhoutte spirea (Spiraea x vanhouttei), is blooming in southern and central Illinois--at least as far north as Peoria. This is a major phenology plant in Don Orton’s book Coincide, available from the publisher, Labor of Love Conservatory, 468 S. President, Suite 103, Carol Stream, IL 80188-2894; (630)668-8597. With phenology, stages of plant development (usually bloom time) are used to predict stages in pest development. This method is more accurate than using calendar dates because the plant is exposed to the same climatic conditions as the insect. Thus, “early” and “late” springs associated with unusually high or low temperatures, respectively, cause similar responses in both plant and insect.
Phenology helps predict when pest stages susceptible to control are likely to be present, but it is not a spray guide. When a phenological event predicts that a pest is susceptible to control, one needs to scout to verify that the pest is present and in a susceptible stage before using a control measure. 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). Full to late bloom: Lilac (ash) borer newly hatched larvae; oystershell scale (brown) crawlers. 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.