Issue 1, April 22, 2013

Phenology and Insect Management

Have you ever heard sayings such as "plant corn when oak leaves are the size of squirrels ear" or "apply crabgrass preventer when forsythia are blooming"? Sounds like old folk-lore but actually there is a science behind these statements. Phenology is the study of periodic plant and animal life cycle events and how they relate to seasonal/environmental changes. Rather than planning annual gardening tasks solely by the calendar, scientists have found correlations between temperatures and certain events by observing such things as bird migration, plant budding, flowering and fruiting and insect activities. American Indians observed nature and determined that soil was warm enough to prevent corn seeds from rotting at the same time oak leaves were emerging in the spring. Unknowingly they were using a phenological indicator. Oak leaves were a visual cue that told them it's time to plant corn.

Horticulture uses plant development to predict insect-pest activity. This is very useful as part of an integrated pest management (IPM) program because it helps to properly time pesticide controls to target the most susceptible life stage of a pest. Insects are cold blooded and their growth and development is directly aligned with weather conditions, particularly temperature. Plants and insects are likely to be similarly affected by cloud cover, rainfall, and the number of hours at various temperatures. The observation of visual cues such as plant bud break and bloom time, lets us know when certain pests are likely to be present and in a vulnerable stage to control. Pest populations vary from year to year so scout the pest-prone plant to make sure the insect is present. Once identified, correct pesticide applications should result in a high percentage of control with the least amount of chemical compared to the calendar method that does not take into account seasonal temperatures.

We know that insects emerge earlier in warm years than in cool years – but how to predict? Scientists monitor growing degree-days. This is a measure of the amount of heat that accumulates over a specified base temperature during a 24-hour period. A base line temperature of 50°F has been commonly agreed upon for landscape/turf calculating the development of insect pests in landscape/turf. One degree-day accumulates for each degree the average temperature remains above 50° over those 24 hours. In a 24-hour period several degree-days can accumulate. If the temperature does not rise above 50 in that 24 hour period, no degree-days are reported.

There are several ways to calculate the number of degree-days but the easiest is the Average Method. Simply add the daily maximum and minimum temperatures and divide the sum by two to get the average temperature for the day. Subtract the base temperature (50) to get the number of degree-days for that 24-hour period. If the result is 5 degree-days, add them to the running total for that season. If the result is a negative number don't add to the accumulated number.

The Illinois Water Survey keeps track of growing degree-day reports from across the state. You can select a base temperature of either 40 or 50 degrees F. Although most insects do not develop at temperatures below 50 degrees F, many plants start developing at 40 degrees F. Generally, the number of days when high temperatures hover in the 40's degrees F is similar from spring to spring, so plant phenology works to predict insect development. Occasionally, there are extended periods of high temperatures in the 40's degrees F, resulting in plant phenology being less precise in predicting insect development. This is one reason for scouting before treating when using any of these methods.

The green-industry has phenological indicators for some of our insect pests. For example newly hatched Eastern tent caterpillars appear at the same number of degree-days when Saucer Magnolia (Magnolia x soulangiana) is in pink bud to early bloom. Bridal Wreath spiraea or Vanhoutttei spirea (Spiraea x vanhoutteii) is an indicator for various pest life stages from blooming through finished bloom.

Euonymous scale is a real problem to control since this pest protects itself with a hard outer shell. It has piercing/sucking mouthparts so it removes plant liquids while completely protected from chemical contact sprays. But there is a time in its lifecycle when it is not protected. This is when the eggs hatch and the crawler stage is present. The crawlers are spreading out to find a spot where they can anchor in and form their armored coat. They are very small and often go unnoticed. The visual cue is when Northern catalpa (Catalpa speciosa) is in early bloom. Never assume they are there, verify that the crawlers are present before applying a chemical control. The most vulnerable stage of bagworm is present when Northern catalpa is in full bloom.

Crabgrass seeds germinate when the soil temperatures are above 55°F for 7 - 10 days. Frequently, the soil has warmed to this temperature when we have had the correct number of degree-days for forsythia to bloom. Don't apply every year on April 15 – when forsythia blooms, check soil temperature data to verify that the timing is right. (Martha Smith, Horticulture Educator, Henry/Mercer/Rock Island/Stark counties, Phil Nixon)

Martha Smith
Phil Nixon

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