Winter’s deep freeze kills garden pests


The recent cold snap may suppress but not eliminate Asian tiger mosquitoes this year. (Bigstock)
Adrian Higgins
Gardening columnist January 15

For all the forecasters’ frothing about last week’s deep freeze — it got down to 7 degrees where I live — the chill simply recalls the winters of yore for longtime Washingtonians.

Its effects will be felt in the garden come spring, though not that much, I suspect, and not all negatively. This might be the year that you wished you had dug and brought inside the dahlia tubers and the canna rhizomes. Many plants that now look awful will flush green and vital in a few weeks. Some plants that might have made it through a mild winter are kaput. Losses bring disappointment, but also a better understanding of your garden’s singular microclimate.

Adrian Higgins has been writing about the intersection of gardening and life for more than 25 years, and joined the Post in 1994. He is the author of several books, including the "Washington Post Garden Book" and "Chanticleer, a Pleasure Garden." View Archive

What I’m hoping to get out of this event is a break from some of the awful arthropods that stand in the way of total gardening nirvana. Deep freezes have an ability to knock back populations of insect pests we could do without. The worst of them are quite new, transported to our shores as part of the hidden costs of globalization. One wishes they were more hidden.

I’m thinking of highly destructive or nuisance pests such as the brown marmorated stink bug, the emerald ash borer and the Asian tiger mosquito.

The stink bug splits into two basic types: the hipster, urban stink bug that squats in our (heated) houses, specifically the spaces between the walls. The other is the granola bar, Birkenstock kind of stink bug, which stays in the woods and spends the winter beneath the bark of dead trees. This lifestyle choice might prove costly.

“The ones in the woods are probably suffering some mortality,” said Tom Kuhar, a professor of entomology at Virginia Tech. That would be good, because this pest does a lot of damage to fruits and vegetables.

More invidious, arguably, is the emerald ash borer, which has now spread to 22 states and threatens to do to the American ash tree what the chestnut blight did to the American chestnut — that is, virtually wipe it out.

The EAB, as it’s known, “is the worst forest problem in our lifetime,” said Whitney Cranshaw, an entomologist at Colorado State University.

The pest kills the tree by tunneling into the sapwood as a grub; it is in this state that it spends the winter, protected by predators and against the cold. “Even if it might be 5 degrees outside, a wood borer in a tree may not get to that temperature unless it persists for three, four, five days,” he said.

Other factors come into play: Insects are chemically at their most naturally cold-hardy now; they would be less so in November or March. And as with plants, a blanket of snow forms a protective blanket against colder air temperatures, whether the insect spends the winter as an egg, a larva, pupa or adult.

If the polar vortex genie gave me three insect (death) wishes, well, I have a little list.

The first horror doesn’t afflict plants, but gardeners: It’s the noisome Asian tiger mosquito, which since its spread in recent years has become a constant irritant from June to October. It’s small, black and white, sneaky and persistent. It is not a mosquito of the swamp or forest, but another urban beast and one that can reproduce in something as small as a discarded bottle cap.

It doesn’t like extreme cold, so will this month’s freeze reduce our repellent use in 2014?

Dina Fonseca, a scientist with Rutgers’s Center for Vector Biology, told me that in the laboratory, at least, the eggs of the tiger mosquito perish below 10 degrees Fahrenheit — they simply dry out at that temperature.

“In principle, if all the eggs were exposed to the air temperatures we had, they would die,” she said. But eggs protected by snow, soil or leaf litter might not die, she said. If we get a particularly wet spring, and if people are not diligent about removing sources of standing water — too few are — a decimated population of the tiger mosquito could quickly rebound.

“It would be great if this kind of weather could kill off this mosquito,” she said, “but my fear is that once they become established, it would take an extreme event for them to disappear.”

Like what, I wonder — an asteroid destroying the Earth? A new Ice Age? The Redskins winning the Super Bowl? I’ve stocked up on DEET and less effective botanical concoctions.

The second member of my despised threesome is the Mexican bean beetle, which in spite of its name seems perfectly okay with a Washington winter, hibernating and dreaming of a steamy August. Once you have a nice display of beans — green beans, lima beans, asparagus beans, it doesn’t seem to matter — the pest can just show up and quickly explode in population. The soft-bodied larvae do the most damage, reducing leaves to skeletons. Even diligent handpicking of beetles and egg-mass squashing don’t seem to hold back the tide. I doubt the recent frigid spell will bother them too much. The pest is found all the way to the Canadian border and beyond. The adult beetles have a measure of antifreeze in their veins, and they are good at finding winter shelter.

We might have more luck with a Southern dandy, the harlequin bug. It is defiantly beautiful in its calico markings of black, yellow and orange, because it wants predators to know that it is distasteful. How does it become distasteful? By feeding on cole crops and absorbing the plants’ chemicals.

This is another creature whose numbers can build up quickly to defy the most dutiful efforts of the organic gardener. The bugs are drawn to kale and broccoli plants more than cabbages, in my experience, but they will bother whatever brassica they can find, including collards, arugula and rutabaga.

This cold winter, one hopes, will set them back. But as with the bean beetle, it is a good idea to do a thorough cleanup of the garden before spring arrives to deny the pests their shelter. The remnants of bean vines, the last of the winter kale, the lingering mustard greens, all these should be pulled and bagged. Whatever else this winter has in store, it only has nine weeks left to run.

Kuhar, of Virginia Tech, says the freeze may well have knocked back another pest, the thrip, which is skilled at sheltering in the tightest folds of leaf and stem, or within flower petals. Thrips, like aphids, have a way of spreading viruses to plants. Some thrip species infect tomatoes with the tomato spotted wilt virus, which can be a major, if localized, problem in some years.

Kuhar pointed out that the same polar vortex that kills off bad insects can also affect good insects such as ladybugs, which feed on aphids. Nature might be beautiful, but like the winter weather, it’s also complex and unpredictable.

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