Horticulture Forum: May 1998, Extension Agent Weather Reports

Making predictions about the potential effects that past weather can have on future pest and disease activity can be an elusive proposition for university research and extension people who regularly discuss these issues with lawn and landscape professionals and the public. Weather can turn on a dime, and the activity of damaging pests and diseases can confound even the most experienced people in the field.

"We can usually get an idea as to whether the insects will be out earlier in any given year, but in terms of abundance, we really have a hard time making any predictions," noted Dan Potter, professor of entomology, University of Kentucky, Lexington.

Still, Lawn & Landscape asked some of these respected individuals to offer their best predictions about what might happen in the future. This article reviews some of the points they made.

EL NINO EFFECT. For much of the country, the El Nino weather pattern that ruled the air this winter brought milder than normal temperatures. Rick Brandenburg, entomologist, North Carolina State University, Raleigh, said that the warmer temperatures could generate greater populations of insects.

"We’ve certainly had good weather for overwintering," noted Brandenburg, who added that mild temperatures combined with rainy weather in many areas of the Southeast could result in very high populations of fire ants in urban areas. He tempered the prediction by also mentioning that the positive environmental conditions "also hold true for the natural enemies as well as the pests."

What the mild winter will do to the populations of other insects remains to be seen. David Shetlar, entomologist, The Ohio State University, Columbus, said that different species of insects have varying degrees of tolerance for winter conditions.

Grubs, for instance, may come up to the surface during extended periods of warm weather to begin feeding. If the temperatures take a sudden dive, the grub may not have enough energy reserves left to tolerate the cold snap. In addition, grubs develop a natural "antifreeze," according to Shetlar, that keeps their body fluids from forming fatal crystals that can kill the insect. When the weather warms, their bodies convert the fluid back into sugars, leaving them vulnerable to crystalizing.

"Have we had enough warm weather to cause insects to break dormancy and eliminate their antifreeze? Have the grubs expended too much energy? I certainly don’t know for sure, and only time will tell," said Shetlar.

TREE TALK. Because trees often don’t reveal the effects of environmental stress for months or even years after their occurrences, making any predictions that should raise immediate concerns is difficult. Bob Olive, superintendent at Auburn University’s Mobile, Ala., experiment station, noted one thing to look for. In his area, there was a lack of rain during the fall as deciduous trees went into dormancy. "We may see some stress symptoms, such as some unusual dieback, during the spring," he said.

In areas where flooding rains occurred, trees can be damaged at the roots as the water robs them of oxygen. Another factor at work according to Jeff Iles, assistant professor of horticulture, Iowa State University, Ames, is the tree’s response to flood stress. Trees in this environment will release carbohydrates and sugars, which "seem to act as an attractant to problem pests," he said.

In these cases, it can be difficult to get the tree to recover. Iles suggested applying a low nitrogen fertilizer, aerating the soil and mulching in the root zone. Also, don’t forget to water during extended dry periods to help the tree regenerate roots and recover, he noted.

The warmer winter may have one other tree effect - an increased incidence of disease. Jean Woodward, extension plant pathologist - ornamentals, University of Georgia, Athens, explained that the early leafing combined with a wet, cool spring could give diseases a jump start on the season.

THE WEEDS. University researchers indicated that the timing of the emergence of weeds seems to be pretty close to on schedule, although it appeared earlier in the season that crabgrass could emerge as early as two weeks before the average.

Fred Yelverton, professor of weed science at North Carolina State University, said that the warmer than normal winter kicked emergence in early but that a cold snap that struck the country in March acted to reset the clock. "Weed emergence is essentially back on schedule," he said.