Hello, i’m Elizabeth Graham, an entomologist with the Forest Service based in Juneau, Alaska.
i’m here today in a forest north of Juneau to talk about hemlock sawfly,
an important natural component of our forests. for many of you living here in southeast,
you may already be familiar with this native pest and not even know it.
an outbreak of the native defoliator began last year
in the central and southern parts of southeast, especially on Mitkof, Kupreanof, admiralty
and prince of wales islands.
this year we are seeing damage extend as far north as Juneau.
hemlock sawfly larvae hatch in June and begin feeding
on the older needles of hemlock trees.
the larvae will feed throughout June and July, with the damage becoming more apparent as
the larvae become older and bigger and consume more needles.
hemlock sawfly larvae only feed on the older needles, leaving
the new growth behind.
they are referred to as wasteful feeders because they do not
fully consume the needle, leaving behind the small midrib vein.
larvae go through 4 or 5 growth stages called instars.
between stages, larvae shed their exoskeleton in a
process called molting.
hemlock sawflies pupate in late summer, producing small golden pupal cases,
which can be found on the hemlock trees as well as
lots of the understory plants beneath.
the adults emerge from pupal cases in the fall
to mate and lay eggs,
which are laid on the underside of the needles.
sawflies are classified as wasps,
but sawflies do not sting and are not bothersome to people.
from a distance, forests affected by hemlock sawfly
appear yellow-brown and the tree crowns appear much
thinner than normal due to needle loss.
within these forests, there may be so many sawflies present
that the frass, which is a nice way of saying sawfly poop,
can be heard raining down on understory plants, such as devil’s club, blueberry and skunk
cabbage.
you can even find piles of it collecting there.
outbreaks typically last 2 to 3 years.
this native insect is always present in the forest,
just at lower levels during non-outbreak years, providing an
abundant food source to other animals.
and they cause negligible damage when they are not in
outbreak mode.
in 2019, the forest service’s forest health protection
team conducted surveys throughout southeast Alaska
to assess hemlock sawfly populations and damage to
the trees.
we use a simple tool, called a beating sheet,
along with a stick, to tap branches, and count how many sawfly larvae
fall out onto the sheet, as well as other defoliators.
some people thing we’re doing really weird medieval
role-playing games,
or perhaps flying a poorly designed kite;
however, we’re scientists here, hard at work.
we also use the beating sheet method to assess the
presence of natural population controls in the forest.
there are fungi that infect and kill
hemlock sawfly larvae. and so we use this method to
determine the proportion of healthy to infected larvae.
we’ve noticed that in forests where the outbreak has
persisted longer, fungal disease is more prevalent.
that indicates that the populations will start to decrease,
and the outbreak will be ending in these forests.
warm, dry weather reduces fungal infection, and it is
likely that the abnormally dry summers
we’ve experienced the last couple years contributed to our outbreak.
parasitic wasps that invade sawfly pupal cases are
another natural population control
that can help to end the outbreak.
topkill and tree mortality can occur when there is an
outbreak of both the hemlock sawfly
and another native defoliator, the western blackheaded budworm.
western blackheaded budworm feed in the new buds and
the new foliage of hemlock trees,
whereas hemlock sawfly feed on the older needles.
when both are in outbreak mode at once, you can
have complete defoliation of the tree
resulting in mortality.
survey work in 2019 found low populations of the
western blackheaded budworm throughout southeast Alaska,
so we do not expect to see a lot of mortality this year.
we monitor the extent of the hemlock sawfly outbreak
during our aerial detection surveys.
every year, the forest health team conducts surveys
across 15% of the forested area of the state,
looking for insect, disease and other damages to the forest.
aerial surveys for southeast were just recently completed,
and our preliminarily results have shown over 200,000 acres
of hemlock sawflying damage, indicating that we are in the middle of a significant
outbreak.
we also are now developing new tools as a way to monitor
the hemlock sawfly outbreak
using satellite imagery and remote sensing tools.
this will enable us to map the entirety of the outbreak,
not just the areas that we were limited to fly.
although most people think of insect outbreaks in a
negative way,
there are also several ecological benefits to this
natural part of our forest.
hemlock sawfly larvae provide an abundant food source for
birds and other animals.
the frass that rains down to the forest floor is
a nutrient pulse to the soil.
the more open canopy increases light to the understory
for the plants and trees that are competing with the
western hemlock.
this can then lead to an increase in browse for herbivores
like deer.
and, the pupal cases…
well, they’re delicious.