Insect and Mite Control on Woody Ornamentals and Herbaceous Perennials

Bulletin 504

Detection and Monitoring of Insects and Mites
on Woody Ornamentals and Herbaceous Perennials

Using Plant Phenology to Predict Pest Activity

Difficulties in detecting and monitoring the tremendous diversity of insect pests of ornamental plants make accurate timing of pesticide applications challenging. As a result, pesticide applications frequently are scheduled on a calendar-day basis. However, year-to-year variation in patterns of degree-day accumulation makes calendar-based scheduling inaccurate. The use of plant phenology provides an alternative approach for predicting insect activity. Phenology is the study of recurring biological phenomena such as bird migration and breeding patterns, blooming of plants, and the seasonal appearance of insects. Since the development of both insects and plants is temperature dependent, plants can be used to accurately track the environmental factors that affect insect development. Therefore, the sequence of plant phenological events, such as flowering time, can be used to predict when insects will become active.

The following table presents the phenological sequence for numerous ornamental plants and key pests for Wooster, Ohio. Average degree-day data for each phenological event are also shown as well as the average calendar date of the event. Calendar dates are not recommended for actual timing of controls, because the actual date of an activity can vary much from year. These dates are presented merely as a guide of when biological events have occurred. The order in which phenological events occur is the important consideration.

Research has shown that the sequence of phenological events is remarkably constant from year to year, even when weather patterns differ dramatically. The easily monitored plant sequence can be used as a biological calendar to predict the order and time pests reach vulnerable stages. This can greatly facilitate logistics and increase the efficiency of monitoring and pesticide application schedules. For example, from the following listing we see that when common lilac is in full bloom:

1. crawler emergence of pine needle scale is underway and sprays should be made very soon;
2. it’s too late to treat first generation birch leafminer;
3. lilac borer pheromone traps should be deployed promptly;
4. it’s still a little too early to see any euonymus scale or oystershell scale crawlers; and,
5. there is still time to plan for bronze birch borer.

A useful attribute of phenological sequences is that they can be readily expanded and customized by users. Because research has shown phenological events to occur in basically the same order every year, any phenological observation is very likely to be valid in future years. Recording which plants are in bloom when pesticide applications are made can be very valuable in future years. For example, if a treatment was particularly successful, any plants in bloom at the time can be noted and used to duplicate the timing in future years. Or, if follow-up monitoring shows that a pesticide application was made too late, the application can be scheduled earlier in the phenological sequence in future years. Once the basic sequence is in place, any new plants or pests can be added as the need occurs.

One word of caution: research is showing that phenological sequences and indicators developed in one region are not always accurate when used in another region. For example, a plant phenological sequence developed for central Michigan accurately predicts many but not all of the insects monitored in Wooster, Ohio. The Wooster sequence is probably accurate for much of Ohio. In regions where patterns of degree-day accumulation vary dramatically from those in Wooster, such as very near Lake Erie, then this sequence should be used with caution until it can be validated.

There are a number of reasons why geographic variation in phenological patterns can occur. Differences in snow cover can dramatically affect the phenology of insects that overwinter in the soil. Origin or provenance, of indicator plants can affect their accuracy as phenological indicators as different provenances can respond differently to day-length and degree-day accumulation. Furthermore, it has been shown that the same genotype can display different phenological patterns when planted in different geographic regions. Substantial geographic variation has also been documented in the phenological patterns of insects, and research has shown that for some species different degreeday models are necessary for the same insect in different regions.

Table 2. Phenological Sequence for Wooster, Ohio, based on data collected from 1997 through 2001. Insect names are indicated in bold type. "First bloom" is defined as the date on which the first flower bud on the plant opens revealing pistils and/or stamens. "Full bloom" is defined as the date on which 95% of flower buds on the plant have opened (or 1 out of every 20 buds has yet to open).

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