Recently, an eagle-eyed Master Gardener discovered an unusual beetle larva feeding on the leaf of a potato plant in the demonstration garden at the awesome Agricultural History Farm Park in Montgomery County Maryland. This little rascal first had me fooled as a larva of the Colorado potato beetle, a true nemesis to potato growers throughout the US. Upon closer examination, I realized that this tiny larva was instead the larva of the three-lined potato beetle, a somewhat infrequent visitor to potatoes.
I usually happen upon adults of this spectacular insect on the broad leaves of jimson weed or on leaves of the low spreading vine, bittersweet nightshade. Among the beautiful blossoms and deep green leaves, yellow and black striped beetles with bright orange thoraxes and equally colorful legs set about devouring tender leaves of these noxious plants. Members of the potato family, including bittersweet nightshade, rank high in the category of “beautiful but deadly” right along with their cousins belladonna and jimsonweed. All of these plants contain lethal chemicals known as tropane alkaloids in their roots, leaves, and fruit. If ingested, leaves or fruit of the plants can result in a host of nasty symptoms including light sensitivity, headache, delirium, hallucinations, convulsions, and sometimes death.
One famous account of tropane alkaloid poisoning comes from Robert Beverly in The History and Present State of Virginia (1705). It seems that a group of soldiers ate leaves of jimsonweed that were included in a meal. Beverly reported their antics as follows: "a very pleasant comedy, for they turned natural fools upon it for several days: one would blow up a feather in the air; another would dart straws at it with much fury; and another, stark naked, was sitting up in a corner like a monkey, grinning and making mows at them; a fourth would fondly kiss and paw his companions, and sneer in their faces with a countenance more antic than any in a Dutch droll. In this frantic condition they were confined, lest they should, in their folly, destroy themselves - though it was observed that all their actions were full of innocence and good nature. Indeed they were not very cleanly; for they would have wallowed in their own excrements, if they had not been prevented. A thousand such simple tricks they played, and after 11 days returned themselves again, not remembering anything that had passed."
Like other members of the leaf beetle clan, the three-lined potato beetle has evolved a physiological pathway to avoid or tolerate the effects of noxious chemicals. It now dines on several members of the potato and nightshade family with impunity. The striking colors of the adult beetles likely serve as a warning to would-be predators to avoid an attack lest they get a dose of distasteful chemicals. Larvae of the three-lined potato beetle also have a nasty habit of piling their excrement on their back. No doubt, this too serves to dissuade predators from an attack. Next time you happen upon a patch of potatoes, jimsonweed, nightshade, or other members of this interesting plant family, keep an eye out for the colorful three-lined potato beetle.
Larvae of the Colorado potato beetle have a more humped-back appearance and richer orange hue than their less attractive cousins.
Bug of the Week thanks Maryland’s industrious and observant Master Gardeners for inspiring this episode. Interesting accounts by R. Beverley, "Book II: Of the Natural Product and Conveniencies in Its Unimprov'd State, Before the English Went Thither. The History and Present State of Virginia, In Four Parts”, and by M. Kogan and R. D. Goeden, “The host-plant range of Lema trilineata daturaphila (Coleoptera: Chrysomelidae)”, were used to prepare this story.