Tiny tigers in our midst

 

A six-spotted tiger beetle. PHOTO BY JOSHUA ROSE

A six-spotted tiger beetle. PHOTO BY JOSHUA ROSE

By Joshua Rose Gazette Contributing Writer

Published in print: Saturday, June 8, 2013

Science fiction is full of tales of humans who shrink, or tiny creatures that grow. Spiders, ants, and wasps become frightening monsters if we lose our size advantage. If I were in such a situation, one of the animals I would least like to see would be a tiger beetle.

Tiger beetles belong to a relatively tiny corner of the beetle family tree. We have 109 species in the United States and Canada, far fewer than groups like the rove beetles, with over 4,000 described species in the same area. Yet there are at least five separate field guides devoted to tiger beetles, while most of the other, far more diverse groups of beetles are given only slivers of space in general guides to insects. What about the tiger beetles earns them so much attention from naturalists?

As with butterflies and dragonflies, two other groups of insects that receive much more attention than their species diversity would suggest, tiger beetles are what biologists call charismatic. Like those other two groups, they look flashy. Their exoskeletons are usually shiny, and several forms are metallic green, blue, red or purple. Even the many species that are brown or tan usually have handsome white marbling; the specific undulations of the white marks are often useful in distinguishing one species from another.

But tiger beetles are much more like dragonflies than butterflies in their lifestyle. They are fearsome predators, hence the “tiger” in their family’s name. Flies, ants, smaller beetles, even spiders are among their prey. Tiger beetles are quick runners that usually chase down their prey on foot, though they are capable flyers as well. Some species have a bite that can even get a human’s attention. They prowl areas of bare ground like beaches, sand dunes, trails, dirt roads and clay or mud banks.

Tiger beetle larvae are also predators, but lack the quickness, wings and long legs of their adult stage. Instead, they dig holes in the ground and ambush prey that walk past.

The larva’s head is often wide and flat and cryptically colored, such that it can block and conceal the entrance of its burrow. One unique feature of a tiger beetle larva is a hump on its back with a couple of short hooks that help anchor the beast in its burrow, giving it the traction to drag its prey underground, and to resist being pulled out by predators.

A sandy bank with a scattering of holes, especially holes that seem to disappear and reappear or that have critters playing peek-a-boo in them, is likely home to a brood of tiger beetle larvae. When a larva has fattened up enough — which can take two years — it seals itself inside its burrow and pupates underground.

One of the most common and widespread tiger beetles is also one of the most handsome and eye-catching: the six-spotted tiger beetle. The adult beetle is brilliant metallic green all over, except for six inconspicuous white spots (from which it gets its name). While some tigers are very picky about their habitats, the six-spot is happy on gravel roads, well-worn trails and other patches of bare dirt in the forest. This species is often the ambassador that draws naturalists into learning about the rest of the family.

The Pioneer Valley is home to another, far less common species. The Puritan tiger beetle was never really widespread, restricted to the banks of the Connecticut River in New England, and to a few counties in Maryland. It is now listed as a federally threatened species, and populations along the Massachusetts section of the Connecticut may have disappeared entirely. A picky species in its choice of habitat, it is found only in sandy areas near the water’s edge. Unfortunately, many such habitats are now underwater as a result of dam construction, or underground due to riverbank stabilization projects. More insidiously, dams have greatly reduced the action of ice floes washing downriver in the spring that used to scour the sandy areas clear of plants and debris, so much former habitat is now overgrown. It does not help that we humans share the beetle’s fondness for sandy areas near the water’s edge: we call them “beaches.” Rainbow Beach near Northampton is a popular human recreation area that is also habitat — at least it was until recently — for the Puritan tiger beetle.

These are two of about 20 species of tiger beetle that are found in New England. The festive, big sand, twelve-spotted and others also inhabit our area. Most prefer sandy soils (easier for their larvae to dig burrows), so beaches and places like the Montague Sand Plains are often productive places to look. Tiger beetles often fly at a human’s approach but, reluctant to give up their chosen territory, usually land just a few feet away, making them relatively easy to stalk with binoculars or a camera. There might even be a few more Puritans out there….

Joshua Rose is a naturalist who lives in Amherst. He is a Hitchcock Center member and a board member of the Hampshire Bird Club; he regularly leads programs for local nature-oriented groups.

Earth Matters, written by staff and associates of the Hitchcock Center for the Environment at 525 South Pleasant St., Amherst, appears every other week. For more information, call 413-256-6006, or write to us.

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