Everyone has heard the advice to look for morels when the lilacs begin to bloom or it?s time to plant corn when the oak leaves are the size of a mouse?s ear. Those statements are based on the concept of synchronous phenology. The same concept applies to pest organisms and can be used to monitor their occurrence. Most plants and animals have temperature thresholds, below which they cease to develop. Below those temperatures, they lay dormant, hibernate or die. Because plants and animals are often dependent on each other for various stages of the life, their development has become synchronous with one another, their host or pollinating species for instance. Since plant development is often more easily observed in the field, their development can be used to help time the development of associated insects, or unrelated insects that have similar temperature thresholds.

These thresholds are measured and reported in ?degree days?(DD). Degree days are calculated by taking an average of the minimum and maximum temperatures for each day and then subtracting the threshold temperature for the organism in question. For example, if the warmest it got that day was 78 and the coldest it got was 62, than the DD for an organism with a threshold of 50 (DD50) would be 20 or (78 + 62)/2 ? 50. Many trees and shrubs begin to break bud when the accumulated DD50 in their area is between 50 and 100. This is also when development begins for several important pest species. The emergence of young succulent shoots and leaves coincides with the emergence of the insects that dependent on them. Since temperature thresholds vary for each organism, the use of common thresholds, 30 and 50 (DD30 and DD50) have become the norm for reporting accumulated degree days for specific organisms.

To effectively control pest populations, pest managers need to be familiar with:

  • The host plant; its life cycle, vulnerable stage of development, cultural needs, symptoms of stress and common pest problems
  • The plant pests; their life cycle, vulnerable stages of development, cultural needs, and natural enemies
  • Site conditions that favor one or the other
  • Land use objectives and the economic threshold of damage

Pest management only becomes necessary when population levels outpace natural enemies such that the level of damage exceeds that tolerated by valuable host species or when the damage impacts land use objectives. The presence of a pest organism alone is not justification to treat. However, when pest management is necessary, the most effective approach integrates a combination of management tools, including monitoring activities, to target the most vulnerable life stage of the pest organism. For insect pests, the larval stage is often the most vulnerable because it is soft-bodied, and its feeding and/or dispersing behavior leaves it more exposed. For many insects, the larval or nymph stage is the only stage that can be controlled, so knowing when and where it will occur is critical. Yet monitoring for these life stages can be next to impossible. Knowing which plant species coincide with the most vulnerable life stage of a particular pest, can be helpful in achieving effective control.

Good indicator plants are those that are commonly found in the landscape, and whose development is easily observed on drive-by surveys. A good example is bridal wreath spiraea. Almost everyone is familiar with the shrub and because of its size and bloom habit, its development can be easily monitored in the course of daily business. By locating and periodically checking the stage of development of certain common tree and shrub species, pest managers can better predict the emergence of common pests. The information can then be used to monitor potential damage and determine the need for management. See the table for indicator plants useful in monitoring some of our mid-west pests.

The phenology and associated DD50 has been outlined for relatively few of our common tree pests. If there are some pests not listed here that regularly occur in your area, keep a diary of weather variations, plant development and when the pests emerge each year. With careful observation, you can expand the table below to monitor pest occurrence and time management future efforts.

Pest Life Stage DD50 Silver maple, Acer saccharinum Serviceberry, Amelanchier laevis Bridal wreath Spiraea, S. X vanhouttei Lilac, Syringa vulgaris
European pine shoot moth Larvae 50-100   Blossom buds show
Spruce gall adelgid Adult Female 50-100 First leaves separate from emerging shoot ?Grandiflora? blossom buds show    
Cankerworms Larvae 100-200   Blooming
European pine sawfly Larvae 100-200   Blooming Blooming Late to finished bloom
Spruce budworm Larvae 100-200   Blooming
Eastern tent caterpillar Larvae 100-200 Leaf blades 1-2? long Blooming    
Birch leaf miner Larvae 275-500 Dropping seed   Blooming Late to finished bloom
Lilac borer Larvae 275-500 Seed ripe, many dropping   Full to late bloom Late to finished bloom
Bronze birch borer Larvae 400-600     Finishing bloom
Elm leaf beetle Larvae 400-600     Finishing bloom  
Bagworm Larvae 700-800   Some fruit ripe

Taken from COINCIDE, The Orton System of Pest Management. D. A. Orton & T. L. Green. Plantsmen?s Publications. 1989.