Dragonfly beach Armageddon
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Viral
social media videos showed beachgoers ducking under beach towels, gazing in
disbelief or quickly deploying their cell phones as thousands of dragonflies
descended on local beaches such as Green Hill in South Kingstown and
Misquamicut in Westerly. The spontaneous invasion lasted only minutes.
Rhody Today turned to local expert David W. Gregg, executive director of the Rhode Island Natural History Survey, which has been housed at the University of Rhode Island for 30 years.
While the Survey is an independent nonprofit organization,
URI faculty and staff sit on its board of directors and it is loosely
affiliated with the College of the Environment and Life Sciences and its
Department of Natural Resources Science.
Rhode
Island is home to 140 species of dragonflies and damselflies, a total of 92
species of dragonflies and 48 of damselflies, said Virginia “Ginger” Brown, the
state’s leading dragonfly expert. They occur in all parts of the state, even on
islands, with most found in still water bodies such as ponds, lakes,
reservoirs, bogs and swamps.
Gregg,
who served as a source on the dragonfly invasion for numerous media outlets,
said the weekend’s swarming may have been caused by blue dashers, a feisty
species of dragonflies. Here’s what he had to say about the unusual occurrence.
Was this weekend’s invasion of dragonflies on area beaches an unusual event for dragonflies? Do they normally swarm?
This weekend’s dragonfly swarm is best described as unusual but not unheard of, and it is probably not technically a migration. Some dragonfly species migrate, and that’s usually what attracts people’s attention.
Dragonfly
migrations usually happen along the shore in late August or September and
typically involve a dragonfly called the green darner (Anax junius),
sometimes one called the wandering glider (Pantala flavescens).
Ecologists tend to describe a migration as a group moving in a seasonally
predictable direction over long distances and ultimately ending up back where
they started annually. This swarm seems to have a large number of
dragonflies called blue dashers, which is not a migratory species, though it is
known to swarm occasionally.
What
do you think caused it?
The
swarm of dragonflies may have been a result of blue dashers responding to
changes and crowding in their habitat.
Blue
dashers are one of the most common and widespread dragonflies in the U.S. They
prefer quiet ponds with emergent vegetation like lily pads, reeds, and
overhanging bushes. This is a common feature throughout our region.
Blue dashers are known to be very territorial. Many dragonflies will defend a territory, but blue dashers are especially known for it – partly because they’re relatively small and pugnacious, partly because they are especially energetic and feisty.
When the adults emerge from the pond, they will stake out
a territory, guarding it from a perch and zooming after other blue dashers that
come into it. The males are guarding females that may be around but also
foraging territory. They also stake out areas – for instance in small
clearings, the edge of woods or yards – where they forage for flying insects,
often mosquitos.
In a good year, blue dashers can be extremely abundant, which would already make for a tight situation in the habitats they prefer. But when the hot summer weather hits and starts to dry up pools and ponds, or the mosquitos start to run out in a certain area, there’s even less area for them.
Once a few reach
their breaking point and decide to head off in search of better habitat, they
can push others to move and as they push more, pretty soon it’s a cascade. This
can be more or less abrupt depending on how fast the habitat is changing.
I suspect what we have is a particularly good year for blue dashers combined with a particularly abrupt change in conditions of habitat inland of us, in inland Rhode Island, Connecticut and central Massachusetts. A cascade started and became self-accelerating and the dragonflies moved out in a swarm.
They reached
the coast and kind of stopped and piled up because they ran out of
room. The blue dasher is not a sand dune or salt pond
type of dragonfly (there are a few that are, but they’re not one of them), so
that’s not where they’re going to stay. They’d have to move up into a
wind stream to help them be carried farther south and west or turn right and
move along the coast.
It’s
probably best to think of this as an unusually big swarm spreading out in
search of better digs than where they were. Dragonflies are nothing to be
concerned about, they don’t bite or sting. The worst that could happen is they
eat up all your mosquitos.
