Bruce Spanworm - Bob Armstrong`s Nature Alaska

Bruce Spanworm
Moths
Bob Armstrong
Bruce spanworm moths typically mate near the end of twigs in an
upside down position.
Have you ever wondered about the numerous moths you see flying about at night in
late fall and even into early winter when
there is snow on the ground? You are most
likely seeing male Bruce spanworm moths (Operophtera bruceata). The males are seeking the
wingless females that typically crawl up to the tips
of bushes or the base of a tree and emit pheromones in hopes of attracting a male for mating.
Flying and mating in late fall and early winter has
the advantage of fewer insect eating birds being
around. Also, this mating ritual occurs under the
cover of darkness which further decreases the
chances of being eaten.
Most female moths have wings. So what would
be the advantage of being wingless? I can think of
a few: the females don’t have to expend energy,
get hit by cars, can produce more eggs (being too
heavy to fly is not important), they are close to the
bushes and trees that their young need to feed on,
and they don’t have to travel far to lay their eggs.
After mating, the females crawl into the bark crevices or other protected areas of the tree or into the
moss at the base of the bush or tree to lay their
eggs. The eggs are laid in late fall and early winter
and remain dormant until the following spring.
The eggs typically hatch in late spring when the
leaves of the bushes and trees that the caterpillars (larvae) feed upon have started to grow. If cool
weather delays the growth of the leaves the young
larvae will bore through the buds to feed on the
soft tissues within the bud.
The young caterpillars go through about five
growth changes called instars or molts. Early in
their growth the caterpillars are a pale yellow color.
Later they may change to a dark olive or brown
color with conspicuous lines along their body.
This caterpillar period may last from 5 to 7 weeks.
During this time they feed openly on the leaves or
within the shelter of a leaf they have loosely rolled
together with silk they can produce. By living, feeding, and growing inside a rolled leaf they are less
conspicuous to predators. However, some birds,
such as chickadees have learned to target these
rolled up leaves.
Eventually the caterpillars lower themselves to the
ground, usually on a thread of silk they produce.
They then pupate within thin silken cocoons they
make in the soil or in the moss under the trees and
bushes. Studies have shown that wind can disperse
the very young larvae, hanging from their silken
threads, quite long distances up to about one-half
mile. This certainly helps the wingless females disperse to other areas.
The adults emerge from the pupal stage in late
fall to begin the cycle again. Typically the males
emerge one to two days before the females.
One trick to find the wingless females is to look for
bushes or trees with a lot of defoliation. They seem
to really target blueberry bushes in the Juneau
area. So find an area where the leaves have been
obviously eaten or chewed upon. Wait until you
see male moths flying about. Then visit these defoliated areas in early darkness. With a headlamp it is
pretty easy to locate the mating moths by looking
carefully at all the ones with wings and you may
see one mating with a wingless female.
As caterpillars they can do a lot of defoliating to
their host bushes and trees. This can cause problems with the plants, especially over the short
term.
Of interest is some studies have shown that the
defoliation by Bruce spanworms on blueberries
may, over the long run, actually benefit the plants.
It appears that the caterpillars’ poop (technically
called frass) provides more nutrients than the decomposing leaves and the blueberry plants actually
do better in the long run.
The male Bruce spanworm moth typically flies at
night in search of females.
This is a female Bruce spanworm moth waiting at
the end of a blueberry branch for a male to find her.
The eggs of the Bruce spanworm are quite tiny,
less than 1 mm in diameter.
When their caterpillars first come out they are a pale
yellow color.
As the caterpillars grow they turn a darker color and form
stripes along their body.
The caterpillars often lower themselves on silk
thread that they produce. At this time they
can be easily distributed long distances by the
wind.
Bruce spanworm caterpillars provide good food for
birds such as this Chestnut-backed Chickadee.
The Bruce spanworm pupae start out a golden brown
or green, becoming a dark gray-brown as the adult
develops.
For More Information
Brown CE. 1962. The life history and dispersal of the Bruce spanworm, Operophtera bruceata (Hulst) (Lepidoptera:
Geometridae). The Canadian Entomologist 94: 1103-7.
Eidt, D. C.; Embree, D. G.; Smith, C. C. 1966. Distinguishing adults of the winter moth Operophtera brumata (L.), and
Bruce spanworm O. bruceata (Hulst) (Lepidoptera: Geometridae). Canadian Entomologist 98: 258-261.
Fitzpatrick, S. M.; Troubridge, J. T.; Peterson, B. 1991. Distribution of European winter moth, Operophtera brumata
(L.), and Bruce spanworm, O. bruceata (Hulst), in the lower Fraser Valley, British Columbia. Journal of the
Entomological Society of British Columbia 88: 39-45.
Htbert C, St-Antoine L. 1999. Oviposition Trap to sample eggs of Operophtera bruceata (Lepidoptera: Geometridae)
and other wingless geometrid species. The Canadian Entomologist 131: 557 - 565
Walther-Toews, David, 2013. The origin of feces: what excrement tells us about evolution, ecology and a sustainable
society. ECW Press, Toronto, Ontario, 200 pp.
Marden, J.H. 1995. Evolutionary adaptation of contractile performance in muscle of ectothermic winter-flying moths.
The Journal of Experimental Biology 198, 2087–2094
Mols, C.M.M. and M.E. Visser 2002. Great tits can reduce caterpillar damage in apple orchards. Journal of Applied
Ecology 39: 888-899.
Weiss, M.R. 2006. Defecation behavior and ecology of insects. Annu. Rev. Entomol. 51: 635-61