With recent winter temperatures reaching single-digit lows across Virginia, a chorus of begrudging gardeners might be thinking, “Well at least these temperatures will kill garden pests!” But do cold temperatures really have an impact on pests?
We asked Eric Day, manager of the Virginia Tech Insect ID Lab and Virginia Cooperative Extension Entomologist what effect, if any, cold weather might have on the populations of insect pests that inhabit the Virginia landscape.
“Generally cold weather has a minor impact on pests in the Mid-Atlantic region. Although this cold spell was lengthy, the temperatures were not record breaking,” says Day, though he adds that some scales, adelgids, and mites may be reduced in number.
According to Day, gardeners can expect to see some mortality among insects that overwinter in exposed locations, for example bagworms overwintering on conifer branches, but our recent cold temperatures will not wipe out entire populations of these insects.
Invasive species like the hemlock wooly adelgid and emerald ash borer will also be relatively unaffected. Day notes that hemlock trees in exposed locations often already carry a reduced load of the wooly adelgid, and this winter’s cold temperatures will likely kill adelgid nymphs (or immature adelgids) overwintering on exposed branches, so these trees might get further relief.
“The biggest mortality factor for insects is late frost,” says Day, “and unfortunately a late frost is also bad for plants.” The kinds of temperatures we’d need to kill large numbers of insects range around -20 degrees Fahrenheit–which is also cold enough to damage many Virginia plants. According to Day, a good rule of thumb is that whenever it gets cold enough to substantially impact insect populations, plants also suffer cold damage.
“People still need to plan to treat their trees and remain vigilant,” says Day, noting that winter is the best time to inspect plants for signs of infestestation.
While the cold temperatures might not kill the pests in your garden, there’s still plenty of time to bundle up and head outside to check for signs of bagworm, scale, and mites before spring foliage fills in providing these insects protective cover!
For more information on bagworm, scale, and mites, please reference the VCE factsheets below:
Spruce Spider Mite: