Ornamental Plants -- Annual Reports and Research Reviews 1998

Special Circular 165-99

Plant and Insect Phenology in the Year of El Niño: Comparison with 1997

Daniel A. Herms

Summary

Since the development of both insects and plants is temperature dependent, plants may accurately track the environmental factors that affect insect development. If a sequence of plant phenological events, such as flowering time, can be shown to correspond with the appearance of insect pests, pest managers could use the easily monitored plant sequence to predict the order and the time pests reach vulnerable stages, greatly simplifying the logistics required to effectively manage the large number of pests infesting the great diversity of ornamental plants.

This report presents the phenological sequence of 82 plant and 40 insect and mite taxa for Wooster, Ohio, in 1998. Degree-day data for all phenological events in the sequence are also presented. The fundamental assumption behind the use of phenological indicators for predicting pest activity is that phenological events occur in the same (or nearly the same) order from one year to the next. The phenological sequence for 1998 was found to be very similar to the 1997 sequence, even though patterns of degree-day accumulation varied dramatically between 1997 (cool spring) and 1998 (warm spring). In general, plant phenology in 1998 was predicted more accurately than insect phenology by the 1997 sequence, although the phenology of most insects was predicted quite accurately. The phenology of all insects was predicted more accurately by plant phenology than by calendar days.

Introduction

Difficulties in detecting and monitoring the tremendous diversity of insect pests of ornamental plants makes accurate timing of pesticide applications and other pest-management tactics challenging. Consequently, pesticide applications frequently are scheduled on a calendar-day basis. However, variation in patterns of degree-day accumulation from one year to the next frequently 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 the blooming of plants and the seasonal appearance of insects. Because plant and insect phenology are both temperature-dependent, it may be possible to use plant phenology to accurately predict insect appearance. If a sequence of plant phenological events, such as flowering time, can be shown to correspond with the appearance of insect pests, pest managers could use the easily monitored plant sequence as a biological clock to anticipate the order and time pests reach vulnerable stages. This would greatly facilitate the logistics of serving many clients with a variety of pest problems.

The critical assumption underlying the accurate use of phenological indicators for predicting pest activity is the existence of a high level of consistency in the sequence from region to region. In recent years, several studies have been published that permit scrutiny of this assumption, including an extensive list of observations made in Illinois (Orton and Green, 1989) and phenological sequences from three regions that were developed using the same protocols -- Midland, Michigan (Herms, 1990); Lexington, Kentucky (Mussey and Potter, 1997); and Wooster, Ohio (Herms, 1998). An analysis of the Michigan and the Ohio sequences revealed that reliance on the Michigan sequence for timing pesticide applications in Ohio in 1997 would have been effective for many but not all pests (Herms, 1998). There are a number of reasons why discrepancies may occur in phenological sequences from region to region, some of which the author has discussed previously (Herms 1990, 1998).

Another critical assumption is that phenological patterns will remain constant from year-to-year in the same region when weather patterns differ greatly. A comparison of 1997 and 1998 provides an ideal opportunity for testing this assumption; 1997 was characterized by a delayed, cool spring, while 1998, the year of El Niño, was characterized by an early, warm spring.

The objectives of this report are to:

• Report the phenological sequence for Wooster, Ohio, for 1998.
• Present degree-day data for plant and insect phenological events in 1998.
• Compare patterns of degree-day accumulation in 1997 and 1998.
• Compare the 1998 Wooster sequences with that of 1997.

Methods and Materials

During 1997, the phenology of 56 plant species and/or cultivars and 22 species of insects on or near the campus of The Ohio State University's Ohio Agricultural Research and Development Center (OARDC) in Wooster, Ohio, were monitored. In 1998 this list was expanded to 86 plant and 36 insect taxa. (See Table 1). For clarity, only common names are listed. To achieve standardization in nomenclature, common names of plants follow Dirr (1983), and insect names are official common names as approved by the Entomological Society of America.

Plants were chosen to represent a range of blooming times from early March through late July. This time period corresponds with the activity of most of the important insect pests of ornamental plants. Four individuals of each species or cultivar were monitored. All individuals of a species were located either in uniform sun or shade, depending on the environment to which the species is adapted. Plants in microenvironments obviously altered by buildings, parking lots, and other factors were not used.

Plants were monitored at least three times each week, and the dates of "first bloom" and "full bloom" were recorded. "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 the flower buds have opened (i.e., one bud out of 20 has yet to open). These phenological events can be identified and recorded with precision.

The phenology of 40 insect and mite species with diverse life histories was also monitored in 1998, including defoliators, scales, gall formers, wood borers, leafminers, and butterflies. As opposed to methods used to monitor plant phenology, which were designed to minimize variation in order to increase predictive power, sampling protocols for insects were designed to characterize the phenology of the entire population.

Degree-days for 1997 and 1998 were calculated using the double sine wave method (Allen, 1976) from daily maximum and minimum temperature data for Wooster (OARDC Weather System, Wooster Station) and a base temperature of 50°F and a starting date of January 1.

Results and Discussion

The Phenological Sequence

The phenological sequence observed in Wooster, Ohio, in 1998 is presented in Table 1. A comparison of plant and insect phenology in 1997 and 1998 is presented in Table 2. In general, plant and insect phenology was substantially accelerated in 1998 relative to 1997. A comparison of degree-day accumulation in 1997 and 1998 confirms that 1998 was warmer (almost 3,200 degree-days had accumulated by October 1 in 1998, compared with 2,500 in 1997). (See Figure 1.) However, a couple of patterns are worth noting.

The winter of 1997-98 was among the mildest in recorded history, yet more degree-days accumulated prior to March 26 in 1997 than in 1998 (Figure 1, inset), and several plant species actually bloomed at about the same time in both years (e.g., corneliancherry dogwood, red maple, forsythia). To understand this apparent paradox, it is important to realize that degree-days are calculated above a lower threshold temperature, in this case 50°F (below which many insects and plants do not develop). So it does not matter if the high temperature for the day is 48°F or 0°F; in both cases there is no degree-day accumulation. However, when 32°F is used for the lower threshold, it is apparent that the winter of 1998 was warmer; 397 degree-days accumulated by March 1 in 1998 compared with 298 in 1997.

Figure 1. Comparison of patterns of degree-day accumulation in Wooster, Ohio, in 1997 and 1998. Inset figure provides fine-scale resolution between March 15 and May 15.

The warm spring of 1998 began in earnest in late March. Between March 26 and March 31 the high temperature in Wooster reached at least 75°F every day. This warm spell catapulted 1998 degree-day accumulation and plant phenology well beyond that of 1997 (Table 2, Figure 1). For example, Bradford Callery Pear began blooming on April 1 in 1998, but not until April 21 in 1997. Degree-day accumulation between mid-May and mid-August was very similar during 1997 and 1998. However, because of the warmer weather during the early part of the spring of 1998, plant and insect phenology remained about three weeks ahead of 1997 throughout the growing season.

Despite the dramatic differences in the springs of 1997 and 1998, the phenological sequence was, for the most part, quite consistent during the two years. In Table 2, the order in which phenological events occurred in 1998 is compared with the order in which they occurred in 1997. The magnitude of any disparity in the order in which a particular event occurred in the two sequences is also shown. A positive value indicates that the event occurred earlier in the sequence in 1997, while a negative value indicates that the event occurred earlier in 1998.

The correspondence in the flowering sequences of the plants common to both studies was quite close. Flowering of silver maple occurred first during both years, followed by flowering of corneliancherry dogwood, red maple, 'Northern Lights' forsythia, border forsythia, manchu cherry, and star magnolia. In fact, in no case did the order of flowering by any species vary by more than the five places between the two years, and only common flowering-quince, sargent crabapple (which bloomed very lightly in 1998), and bumald spirea varied by five places (Table 2). As more plant species are included in the sequence, the chance that one will be led astray by any one species that departs from the pattern will diminish.

In general, the sequence of insect phenological events also corresponded closely between the two years. For example, the phenology of eastern tent caterpillar, eastern spruce gall adelgid, pine needle scale, oystershell scale, bronze birch borer, juniper scale, potato leafhopper, fall webworm, spruce budscale, and magnolia scale were all predicted with great accuracy. None of these species deviated by more than one position in the sequence between the two years.

The phenology of other insect species did not correspond as closely. Inkberry leaf-miner, lilac borer, and lesser peach tree borer deviated by more than five places in the sequence. A number of factors may affect the accuracy of plant phenological indicators as predictors of insect activity. The assumption that a given phenological correlation will occur from one year to the next requires that all organisms included in the correlation have the same upper and lower temperature thresholds for development, as well as the same developmental responses to changing temperature. These traits are known to vary widely among both plants and insects (Tauber and Tauber, 1981; Rathke and Lacey, 1985).

When temperatures differ widely among years, as they did in 1997 and 1998, then variation in these traits will cause discrepancies in the phenological sequence, especially if temperatures frequently drop below or hover near the lower threshold of 50°F, as they did during the spring of 1997. However, the phenology of all insects was predicted more accurately by plant phenology than by the calendar, and the use of plant phenological indicators for timing pest activity holds tremendous potential for improving the effectiveness of integrated pest management programs in the landscape.

Literature Cited

Allen, J. C. 1976. A modified sine wave method for calculating degree-days. Environmental Entomology. 5:388-396.

Dirr, M. A. 1983. Manual of Woody Landscape Plants: Their Identification, Ornamental Characteristics, Culture, Propagation, and Uses. 3rd Ed. Stipes Publishing Co., Champaign, Ill.

Herms, D. A. 1990. Biological clocks: using plant phenology to predict insect activity. American Nurseryman. 172(8):56-63.

Herms, D. A. 1998. The flowering sequence of ornamental plants as a tool for predicting the phenology of insect pests. In: M. A. Rose and J. A. Chatfield, Eds. Ornamental Plants: Annual Reports and Research Reviews. Special Circular 157. The Ohio State University, Ohio Agricultural Research and Development Center.

Mussey, G. J., and D. A. Potter. 1997. Phenological correlations between flowering plants and activity of urban landscape pests in Kentucky. J. Econ. Entomol. 90:1615-1627.

Orton, D. A. and T. L. Green. 1989. Coincide. Plantsmen's Publications, Flossmoor, Ill.

Rathcke, B. and E. L. Lacey. 1985. Phenological patterns of terrestrial plants. Annual Review of Ecology and Systematics. 16:179-214.

Tauber, C. A. and M. J. Tauber. 1981. Insect seasonal cycles: genetics and evolution. Annual Review of Ecology and Systematics. 12:281-308.