Stink Bugs: An Autumn Ritual

This column was published in the Cranbrook Daily Townsman, Thursday October 21, 2010. 
 
Autumn. Time for one of those Kootenay traditions that celebrates warm afternoons and cool nights. That’s right, it’s the season again to patrol the outside of the house and flick stink bugs off the deck, chairs, door, walls and apparently every other possible surface. Those that find their way indoors need to be gently evicted. Careful moving them, they’ve got a loaded rear end to make any warthog or skunk jealous.

For good fun, watch the dog catch one, get a mouthful of the bug’s foul discharge then try to clean it out. A smart pet will only do that once. One of our own spring rituals is to open the deck box that holds our outside chair cushions and shake out the dead stink bugs that have lain entombed and frozen throughout the winter. Deck boxes make a poor hibernation choice for insects.

Stink bugs are more properly called Western Conifer Seed bugs (Leptoglossus occidentalis) and are native to the western United States but have been increasing their range through the last century. They are now found throughout eastern North America as well as accidental introductions into Europe (and probably elsewhere), hitching rides on Christmas trees and other forest products.

They move through a single life cycle each year, overwintering as adults before emerging in the spring to feed on developing seeds in green cones. Females then lay neat rows of eggs on conifer needles as their final act. The eggs hatch and successive larval stages continue to feed on the cones and needles of the trees until the adult stage is reached by late August.

And then the fun begins anew. South and west-facing walls and homes make welcoming places to adult stink bugs looking to keep warm. As the nights cool, the bugs will push farther in their search for warmth. No crack or opening seems to narrow for them to fit through. Their body seems almost two-dimensionally thin and they are able to squeeze through any seal not quite air-tight. Think of them as friendly reminders to check your door and window seals.

There is no need for alarm if they are found inside. They do not bite, for their mouth parts are adapted to sucking tree sap from cones, not biting or chewing. Their only downside, apart from those squeamish about insects in the home, is the foul odour they emit if disturbed. Thus the ‘stink bug’ moniker, one well earned. Crushing them is not advised, even picking them up can result in an unfortunate olfactory experience. Encourage them to move onto a piece of paper, then release them back outside.

The insects are entirely harmless, unless you’re running a tree nursery and trying to gather seeds from conifer trees. They can cause significant losses in seed production by adult trees by feeding on developing cones and occasionally killing the tree itself.

How they find these cones was described by Simon Fraser University biologist Stephen Takács. His team found that growing conifer cones are much warmer than the surrounding vegetation, up to 15º C! (see paper here.) To infrared sensors, a conifer with growing cones appears like a Christmas tree, with each cone brightly “lit” relative to the adjacent greenery.  

Turns out, stink bugs have highly sensitive infrared sensors in their abdomens, directly connected to their brain. When biologists blocked these sensors, seed bugs could no longer find the trees. This attraction to high intensity infrared radiation also explains why the bugs are so drawn to warm, sunlit walls during the autumn.

Ultimately, all the bugs are doing is looking for a place to spend the winter where they won’t freeze to death. As the weather cools and winter looks to set in, that’s something we can all relate to.

Conifer tree under visual and infrared light showing warmer temperature of cones