Friday, September 25, 2015

Black Blister Beetle

Black blister beetle on goldenrod - REK
The black blister beetles, Epicauta pennsylvanica are back for their annual attack on the goldenrod in our warm season grass field.  They are competing for space with the Pennsylvania leatherwings as well as a variety of small bees.  They generally emerge in August in Missouri, gathering on goldenrods and asters where they eat the leaves and especially the flowers and pollen.

Larva and Pupa
Females lay their eggs in the soil and the long legged larvae emerge to search soil cracks for grasshopper egg masses.  They gobble up the underground omelet and then go through seven larval stages (instars) each with diminishing legs before turning into an immobile legless grub for the winter.  They will pupate the following summer before emerging as an adult beetle.  Large populations usually means an abundant grasshopper population the previous year.

There are 7,500 species of blister beetles world wide.  The "blister" in their name comes from their ability to excrete cantharidin from their joints when attacked.  While discouraging some predators, there are some bugs, beetles and flies that are immune to the toxin and will eat the blister beetle, dead or alive, acquiring the toxin for their own protection.  They are called "cantharidinophilous insects," but only on rare occasions by the most serious of entomologists.

Fortunately it was on the flower head and not the hand. - REK
Cantharidin can cause blisters on human skin when a beetle is handled.  It is a potent toxin if ingested in small amounts, with only 10 mg considered a lethal dose in humans comparable to cyanide and strychnine.  Horses occasionally die from eating feed contaminated with the beetles.  Extracts of cantharidin are used by dermatologists in dilute solutions to treat warts.  In the past it had several other medical uses including drawing out "toxins," a treatment applied to the neck of George Washington as he lay dying of a respiratory infection.

The male blister beetle has high levels of cantharidin in its blood and passes some to the female which makes it into her eggs.  This "sperm gift" doesn't sound very romantic but it is apparently effective as we have lots of beetles this year.
Detail at

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