If Winter Doth Not Come, What Shall Springtime Bring?
The winter of 2011-2012 is now past, although if you blinked you missed its less-than-mighty roar. Enquiring minds want to know what this will mean relative to pests, pathogens and plants for the upcoming seasons. There are no easy answers, but here are some considerations.
It ain't over 'til it's over
Listen to your inner Yogi (Berra). Even though it is now officially April and spring, do not be fooled. The cold season may not yet be past. As you know, our springs have trended earlier and our winters milder for some time. Nevertheless, that is a trend, and though trends are very useful as we think of the implications of climate change, as we develop new Hardiness Zone maps, as we consider frost-free dates, these maps and dates are not laws - and it all depends on the vagaries of each season.
Crocus emerged in late February in the Chatscape in northeast Ohio.
What will happen in May, for example, is best predicted by what happens in April. Cold temperature injury due to extremely low temperatures may not be a big problem this year, but frost injury to flower buds that emerge earlier due to heat units that have built up may be a major problem. Tapping maples came to an early end and a short season this year due to the early warmth, and the fact that plants are so active can make them more susceptible to damage if we get sudden and severe temperature drops in April.
Although some plant pathogens survive better in milder winters, such as the powdery mildew fungus that occurs on apple and crabapple in which the fungus overwinters in buds, most pathogens make it through all winters. How bad infectious diseases are in a particular year often depend largely on the environmental conditions once the growing season starts, especially spring moisture. We can predict that about as well as we can predict May and June weather.
Viburnum leaf buds and flower buds are seen in Wooster, Ohio, as they were about to open on March 13.
Photos by Jim Chatfield
How warm was the winter?
It is also important to keep things in phenological perspective. Phenology is the relationship of biological events, such as insect development and flower development, to environmental factors, such as temperature. An important aspect of what drives insect activity and plant flower development is that growing degree-day heat unit models are good predictors based not strictly on how mild the winter is, but on temperatures that exceed a certain minimum. So, there are various models (for example, some use 50ºF as the lower base temperature while others may use 40ºF ) that turn out to predict biological activity quite well. A hypothetical, exceedingly mild Ohio winter in which the temperature never fell below 30ºF, but never rose above 45ºF, would not accrue any growing degree-days when using 50ºF as the base temperature. Thus, flower development would not be initiated even in this mild winter.
So, what was the winter of 2011-2012 like? As an example, many in northeast Ohio were surprised and concerned that corneliancherry dogwood (Cornus mas) was blooming on March 13, and that this was a result of a remarkably mild winter. However, though the winter was mild - and degree-days by then were somewhat higher than average and certainly have trended upward in recent decades - those degree-days accumulated not from the fact that we had almost no bitter cold, but that we had a few days over 50ºF. In fact, C. mas first blooms (1 of 20 blossoms) at 40 degree-days and reaches full bloom (19 of 20 blooms), on average, once 98 growing degree-days have accumulated.
On March 13 at the Secrest Arboretum of the Ohio Agricultural Research and Development Center (OARDC) of The Ohio State University in Wooster, Ohio, we had accumulated 50 degree-days and so C. mas was in bloom - for the third time in the past 10 years (also in 2009 and 2006).
Of course, to reinforce Yogi's aphor-ism, March 13 was followed by sustained, unseasonably warm weather and degree days accumulated rapidly, and phenological events arrived quickly, from cherries blooming quite early in Washington, D.C., to early insect emergence throughout much of the country. But this was really only because of those March madness temperatures, not the earlier "mild" winter. Check out phenology and degree-day information at www.oardc.ohio-state.edu/gdd.
We are not alone
As we consider the survival of a particular pest, remember that these pests do not exist in a vacuum. As Jonathan Swift said: "So nat'ralists observe, a flea/Hath smaller fleas that on him prey,/And these have smaller fleas that bite 'em,/And so proceed ad infinitum." Or, as John Muir has said: "When we try to pick out anything by itself, we find it hitched to everything else in the universe."
So, even if a pest survives a winter better than usual, it may also mean that its predators or parasites may also survive better. Or worse. Or some combination thereof. Furthermore, temperature is not the only environmental factor to affect pest populations. A good example is gypsy moth and one of its parasites, the Entomophthaga maimaga fungus. For this combination, one of the relevant keys is how wet it is in May and June.
The bottom line is that population dynamics of pests are not truly straightforward and are subject to many complexities, including such factors as where the pest survives; and if in the soil, whether or not there was an insulating factor of snow.
Home on the range
Remember that insects and mites that overwinter in a particular location have had hundreds to thousands to millions of winters as a species to adapt to winter conditions there. Whether the winter conditions are severe or mild, the species have experienced these fluctuations in the past and are adapted to them, otherwise they would not survive in that location as a species. With that being said, change, in the form of range expansion, may occur.
In Wooster, Ohio, corneliancherry dogwood (Cornus mas) blossomed out on March 13.
For example, in Ohio it has been observed that some species (such as bagworm) have had their ranges altered by recent milder winters. Since the 1980s to the present, bagworm survival, which is influenced by cold low-temperature survival, has progressively resulted in greater survival and higher populations further and further north in Ohio.
In the early 1980s, bagworm was not found in any great numbers beyond central Ohio, but by the late 1990s, the pest was commonly found throughout the state and even into parts of southern Michigan. Global worming? This pattern of range expansion and changes in population dynamics is well-chronicled in other areas of the United States, for example with expansion northward of southern pine beetle in the Eastern U.S., and northward and at higher elevations for mountain pine beetle in the Western U.S.
David Wiesenberg, owner of the Wooster Book Company, Ohio's largest independent bookstore, examines filbert flowers as they made their appearance on February 27 in Wooster, Ohio.
So, what will the spring and summer of 2012 bring? Stay tuned and do not let your predictions paralyze your perceptions. To invoke another All-American wag, Mark Twain: "What gets us in trouble is not what we don't know, it's what we know for sure that just ain't so."
Jim Chatfield is state extension specialist in the Department of Plant Pathology and Department of Horticulture and Crop Science at the OARDC in Wooster. He can be reached at firstname.lastname@example.org. Dan Herms is professor and interim chair of the Department of Entomology, OARDC/OSU; he can be reached at email@example.com. Curtis Young is extension educator and chair, OSU Extension, Van Wert County. He can be reached at firstname.lastname@example.org.