Fireflies

by Eoin Carroll

Fireflies, lightning bugs, glow worms, whatever you call them are for me one of the highlights of the Summer when I see their almost magical flashing in the evening.  Of course, as a beginner naturalist, the first thing that I have to point out is that they are neither flies, bugs, or worms.  Instead they are a type of beetle. There are more than 2400 species of fireflies in the world and about 30 in Pennsylvania including one that is our official state insect.

They are not threatened but their numbers have been declining due to the very familiar problems of habitat loss and increased pesticide use.  Before I talk about what you can do to help fireflies, it helps to understand more about their lifecycle.

The fireflies you see flying around and perched on vegetation are the adults.  The flyers are the males who are flashing in order to find a female to mate with.  The perching firefly is a female who will respond to a flashing male with a flash of her own.  Each species of fireflies has its own specific flashing pattern which is how the female can detect whether the flash she sees is from a suitable male.  This mating time is the shortest part of their life cycle lasting only a couple of weeks.  Many fireflies will not feed at all during this time but some will occasionally feed on nectar.

Once mated the female will lay her eggs on wet ground or on organic matter.  The eggs too are bioluminescent although they emit a much softer glow.  The eggs then hatch in a few weeks to a couple of months and the newly hatched larva then burrows down into the ground.  The larva also glows giving it the name of glowworms.  This is the longest part of its lifecycle as it stays in this form for between six months and two years and is the stage when it needs a steady supply of food.  The larvae mostly eat the decomposers of the food chain; worms, snails, slugs, and sometimes other firefly larvae.  Finally the larva pupates and from the pupae an adult emerges with a very short time to find a mate.

We can help the fireflies by improving their habitat.  An important step is to avoid over tidying of our landscape.  When we remove all of the leaves and plant matter then we are removing the food that the decomposers (the worms, slugs, snails) eat.  This eliminates the larva’s food source.  We are also sometimes inadvertently removing the firefly eggs and larva.  So leave the leaves when you can.

We are also killing the fireflies with pesticides.  Spraying for mosquitos is not only ineffective but it is very indiscriminate in its killing of all sorts of insect life.  This also applies to lawn chemicals and agricultural pesticides which can be quite deadly to larva.

The last major issue is light pollution.  The firefly’s mating signaling of flashing can be easily outshined by night lighting which is magnitudes brighter and blinds the female from responding to the male’s flashes.  Installing timers or motion sensors on your outdoor lights can help reduce this problem.