3-lined potato beetles rely on the defenses of sacred datura

By Jeff Mitton

Three-lined potato beetles, Lema daturaphila, are native to North America, and they consume leaves from a variety of species, including tomato and potato.

But their specific name, daturaphila, indicates that they like Datura wrightii, or sacred datura.

Although the beetles feed on a variety of plants, all of their hosts are in the family Solanaceae, the nightshades. Feeding trials showed that 11 species of datura are in their 12 favorite host species.

Sacred datura has large, grayish-green, heart-shaped leaves that form a mound as large as 5 feet across and 4 feet tall. Its flowers are bright white funnels tipped with lavender, up to 8 inches long and 5 inches wide.

Sacred datura deters most herbivores with a variety of defensive tropane alkaloids including atropine and scopolamine, which are classified as deleriants, meaning that they produce hallucinations.

They are not hung up on hygiene and are loath to let those valuable compounds go. So they spool the sticky stench onto their backs, where it serves as a fecal shield."Some insects, including potato beetles, use plant defenses in their own defense. Monarch butterflies are known for their habit of taking defenses from milkweeds and sequestering defensive chemicals in fatty tissues to deter predators.

Although potato beetle larvae are unable to sequester chemicals, they employ an alternate method to use them.

Potato beetle larvae eat the leaves of sacred datura and defecate defensive alkaloids with the rest of their frass. But they are not hung up on hygiene and are loath to let those valuable compounds go. So they spool the sticky stench onto their backs, where it serves as a fecal shield.

Most insect predators are unwilling to dig through the nasty frass to get to the delicious larva, so they wander off in search of more appealing prey.

Sacred datura has yet another arsenal of defenses against herbivores, and these compounds are dispensed by trichomes, which are diverse structures on the surface of leaves, including hairs and glandular hairs.

Sacred datura is polymorphic for its trichomes, meaning that plants develop different types of trichomes. Some plants have simple hairs, and the leaves of these plants feel like velvet. Other plants have glandular hair trichomes, and these glands release fluids that make the leaves sticky.

A single gene determines the trichome polymorphism, with the allele for glandular trichomes dominant to the allele for the velvety hairs. In southern California, populations of sacred datura commonly have both phenotypes, with velvety plants most common in some populations and sticky plants most common in others.

Evolutionary forces controlling the frequencies of the two morphs are not fully understood.

The glandular hairs and the exudate that makes leaves sticky deter most of the insects that would consume sacred datura. The glands secrete glucose esters and aliphatic acids, which are generally irritating. These herbivores much prefer the velvety to the sticky datura.

But three-lined potato beetles seem indifferent to the glandular exudates, for they are seen as commonly on sticky plants as on velvety plants. So either the potato beetle larvae are immune to the glandular secretions, or there is good reason to tolerate the irritating goo.

When herbivores chew on the leaves of datura, the plants release volatile organic compounds that alert insect predators to the availability of herbivores — in effect, the plants send out a signal to recruit predators to eat the chewing herbivores.

Their fecal shields deter many predators of potato beetle larvae, but several, including a damsel bug, a big-eyed bug and a jumping spider, are able to ignore the shield. However, these three predators are effectively deterred by the glandular secretions that make the leaves sticky.

So we don't know if the stickiness is irritating to three-lined potato beetle larva, but it might be worth the irritation to forage in the tranquility and safety of a predator-free space.

The chemical ecology of plants, herbivores and predators can be astoundingly complex.

Jeff Mitton, mitton@colorado.edu, is a professor in the Department of Ecology and Evolutionary Biology at the University of Colorado. This column originally appeared in the Boulder Camera.

Feb. 16, 2016