The Bug-Bloom Connection
March 27th, 2003
Tree buds and spring bulbs aren’t the only things starting to wake up this week after their winter snooze.
Bugs and plant-disease organisms also know it’s time to get busy.
They know this not because the calendar says it’s the end of March.
They know it because all overwintering living things have biological clocks that trigger certain actions when rising temperatures and increasing day-length say the time is right.
It’s a remarkably accurate system, too… much better than how we gardeners keep getting faked out by a calendar that never seems to be in sync with our erratic weather.
But some savvy gardeners are beginning to wise up and use that predictability against the creepy, crawly enemy.
The idea is a science called “phenology,” which is the study of how living things (plants, bugs, birds, etc.) respond to weather events.
It’s actually a very old science that was commonly used by America’s earliest farmers and gardeners. Those people didn’t know much about fancy-schmanzy biophenometers or “growing-degree-day” calculations, but they did know it was time to plant corn when new oak leaves had grown to the size of a squirrel’s ear.
For a long time, more “sophisticated” modern gardeners wrote off that kind of thing as an old wive’s tale.
But there’s actually some very good scientific data to back up that and other such “superstitious” connections.
Plant scientists around the country have been monitoring plant/bug connections and weather data for more than 40 years.
They found that plant bloom times and bug emergence can vary by weeks from year to year, but no matter how screwy the weather, the sequence of living events is amazingly consistent.
In other words, gypsy moths might hatch in mid April or they might hatch in early May, but they always do it at exactly the same time redbud trees are just starting to bloom.
This kind of revelation can be pretty helpful to a gardener, says Eric Vorodi, a former Penn State Extension agent who has studied phenology for years.
He says that by watching when certain plants do certain things, you can accurately predict when certain pests might be primed to cause trouble.
That’s a big edge, he says, “because by the time most people notice an infestation, the damage already has been done and it’s too late to do anything.”
Case in point: Eastern tent caterpillars on crabapple or cherry trees.
These web-forming leaf-munchers overwinter as eggs on tree branches and normally hatch anywhere from late March to late April.
“They get big very quick,” says Vorodi, owner of the Boiling Springs-based About Trees consulting business. “If you wait until they’re mature, they not only have eaten more leaves, but they’re more difficult to kill at that stage.”
If you know that these eggs always hatch shortly after star magnolias bloom or when forsythias are in full bloom, you can time treatments perfectly – or at least know when to start looking for them.
Vorodi says a big reason phenology is making a comeback is because farmers and gardeners alike are trying to cut down on pesticide use.
A well timed spray may do the job in one or two treatments instead of three or four that are applied according to the calendar. Also, less toxic products such as soap sprays and horticultural oils generally work well only when they hit bugs at vulnerable life stages, which makes timing more critical.
Besides being a bug- and disease-fighting tool, phenology is useful for knowing when to plant or do other jobs around the garden.
For instance, crabgrass begins to germinate once soil temperatures 4 inches deep stabilize at 55 degrees. That normally happens when it’s been warm enough to cause forsythia to bloom.
Lesson: Put crabgrass preventer on the lawn when forsythias are in bloom.
Similarly, when daffodils begin to bloom, that’s a signal that enough warmth has occurred to plant peas. And it’s warm enough to plant tomatoes, beans and most annual flowers when lilacs are in full bloom.
Plant scientists were able to connect the dots by linking events according to “growing degree days.”
A growing degree day measures the number of degrees a day’s average temperature exceeds 50 degrees. The numbers start accumulating as of March 1.
Example: If today’s average temperature is 55 degrees, that counts as 5 degree days. If tomorrow’s average is 60 degrees, that’s 10 degree days. Together, that would add 15 degree days to the running total that began March 1.
By putting it all together, it’s possible to develop charts that really make the data useful to gardeners.
That’s exactly what Illinois plant inspector Don Orton did in his book “Coincide: The Orton System of Pest Management” (Plantsmen’s Publications, 1989, $26).
A good online source for phenology links is the National Center for Appropriate Technology at www.attra.org/attra-pub/phenology.html.
Armed with this information, you’ll never have to worry about bugs getting the upper tentacle on you again.
Known links between plant events and bug emergence:
* Eastern tent caterpillars: early bloom of star magnolia or full bloom of forsythia.
* Pine sawflies: early bloom of ornamental pear or weeping cherry.
* Spruce spider mites: full bloom of serviceberry or ‘PJM’ rhododendron.
* Andromeda lace bug: early bloom of flowering crabapple.
* Gypsy moth: early bloom of redbud or full bloom of Korean spice viburnum.
* Birch leafminer: early bloom of silverbell or full bloom of flowering quince.
* Spruce gall adelgids: early bloom of weigela or full bloom of lilac.
* Lilac borer: early bloom of deutzia or doublefile viburnum.
* Holly leafminer: early bloom of black cherry or sweetshrub.
* Euonymus scale: full bloom of honeysuckle or early bloom of ‘Miss Kim’ lilac or beautybush.
* Dogwood borer: full bloom of Washington hawthorn or mock orange.
* Bagworms: full bloom of Japanese tree lilac.
* Rhododendron borer: early bloom of winterberry holly or full bloom of catalpa.
* Fall webworm: early bloom of oakleaf hydrangea.
* Japanese beetle: early bloom of goldenrain tree or full bloom of Japanese spirea.
Source: Dr. Daniel Herms, Ohio State University
Some gardening “old wive’s tales” that really have scientific backing:
* It’s time to plant potatoes when the serviceberry trees bloom.
* It’s time to put crabgrass preventer on the lawn when forsythias bloom.
* Plant corn when oak leaves are the size of a squirrel’s ear.
* Plant corn when the apple blossoms fall.
* Plant lettuce, peas and other cool-weather crops when lilacs first leaf out.
* Plant tomatoes, corn and peppers when dogwood trees are in peak bloom.
* Japanese beetles will arrive when morning glory vines begin to take off and climb.
* Plant beets, spinach and carrots when the dandelions are blooming.
Source: Golden Harvest Organics, Fort Collins, Colo.