Trilliums, Flies and Ants

For a few brief weeks in spring, Painted Trillium (Trillium undulatum) opens for business, the business of surviving as an individual plant and reproducing to ensure the continuation of the species.

Painted Trillium (Trillium undulatum)

Trilliums get their name from their structure.  Their leaves and flower parts are in threes or multiples of three.  Each plant has a whorl of three leaves below a single flower that has three petals, three sepals, six anthers, and a three-celled ovary.  ‘Undulatum’ refers to the wavy edges of the flower petals.

Painted Trillium’s three leaves act like solar panels to gather energy from the sun, enabling this plant to produce the carbohydrates it needs to grow and thrive, and to produce flowers that will enable reproduction if the flowers are pollinated.

Painted Trillium (Trillium undulatum)

Painted Trillium flowers have three bright white petals with paint-splatter-like dark red splotches at their base, bleeding into the petal’s veins.  This color contrast is an advertisement designed to attract potential pollinators to visit the flowers, implying that there is nectar and pollen available for a hungry insect to eat.  Painted Trillium offers these rewards because it needs help from a courier to move its pollen to another plant if it is to successfully reproduce through cross pollination.

Fly investigating Painted Trillium (Trillium undulatum), attracted by the dark red nectar guides

There is not a lot of documentation about the likely pollinators for Painted Trillium, but on more than one occasion, I have seen flies visiting the flowers.  The showy display works!  But the flies’ motivation is not to help with pollination.  They have their own needs to meet, mainly finding food.  They may visit the anthers to eat some nutritious pollen,

Fly harvesting pollen from Painted Trillium (Trillium undulatum)

and follow the colorful guides at the throat of the flower, diving deep to look for nectar.

Fly looking for nectar in a Painted Trillium (Trillium undulatum) flower

While foraging on the flower for food, the fly will likely rub its body or legs against the stigmas at the tips of the flower’s female reproductive parts, depositing pollen she may have brought on her body from a previous visit to a Painted Trillium flower.

Fly foraging on a Painted Trillium (Trillium undulatum) flower, with the underside of its abdomen brushing against the stigmas at the tips of the female reproductive parts of the flower. If this Painted Trillium is lucky, the fly will be depositing pollen picked up from another flower.

Fly foraging on a Painted Trillium (Trillium undulatum) flower, with its legs grasping the stigmas at the tips of the female reproductive parts of the flower.

It will also likely rub against the anthers of this flower, picking up pollen to take with it to deposit on the next Painted Trillium flower it visits.

Fly on a Painted Trillium (Trillium undulatum) flower, with its legs brushing against the anthers, from which pollen is dispensed.

Fly on a Painted Trillium (Trillium undulatum) flower, with the underside of its abdomen brushing against the anthers, hopefully picking up some pollen to take to another flower.

This inadvertent pollen transport is what Painted Trillium is counting on.  It has evolved to attract these unwitting pollination partners, and is willing to pay the energy price to reward them in exchange for their assistance with cross-pollination.

Several Trillium species with dark red flowers have evolved to attract flies as their pollinators.  The dark red flower color is often accompanied by a somewhat rank aroma. Together these features are meant to mimic dead rotting flesh (ok, carrion) or other decomposing matter.  Several fly species seek out this kind of material to lay their eggs.  When the larvae emerge from the eggs, they eat the decaying matter, breaking it down and add the result to the soil layer after it passes through the larva’s body.  These insects help crime scene investigators estimate time of death for a corpse, based on the stage and rate of development of the insect in the decaying body.  Many fly species are attracted to the flowers that use this strategy, only to be disappointed when there is no suitable place to lay their eggs.  At least the flies can console themselves with a pollen snack.

Trillium species that use this deceptive strategy include Red Trillium (Trillium erectum), and Toadshade (Trillium sessile, T. cuneatum).

Red Trillium (Trillium erectum)

Red Trillium has many aliases.  It is also known as Purple Trillium, Wake-robin and Stinking Benjamin, this last name a nod to the flowers’ scent, Wake-robin because it blooms at about the same time Robins returned from their wintering grounds, back when they used to migrate more.

Toadshade gets its name from stories of toads taking refuge under the umbrella-like leaves of the plant.

Toadshade (Trillium cuneatum, T.sessile)

In the photo below, a fly is investigating one of the anthers, the source of the flower’s pollen.

Toadshade with fly

If the flower is successfully pollinated, a fruit develops and ripens later in the summer.  The fruit is a berry that splits open when ripe, making its many seeds available for dispersal.  Each seed has an elaiosome attached, a nutritious food packet whose chemical content mimics that of an insect.  This mechanism has evolved to attract ants to disperse the seeds.  It works because ants are omnivorous; they eat some plant material and enjoy sweet treats such as nectar, but insect protein is an important part of their diet.  The ants are attracted to the seeds because of the elaiosome.  They take the seeds back to their homes, eat the elaiosome, and toss the seed on their compost heap, effectively planting the seed in a fertile, protected location.  This evolutionary strategy, known as myrmecochory, is shared by many spring blooming wildflowers.

If you need proof that other insects are important ant food, you’ll find some evidence in the photo below.  I watched while this ant worked tirelessly to drag to its home the moth it’s grasping, working its way across the trail, letting no obstacles like sticks, leaves or rocks deter it from its mission!

Insects are an important source of food for ants. This ant worked tirelessly to drag the moth back to its home.

The elaiosome strategy works in much the same way that fleshy fruits attract birds and other animals to eat their fruits and ‘disperse’ the seeds complete with fertilizer after the seeds pass through the animal’s digestive system, a trait used by many plants that bloom later in the season.

Trilliums typically grow in moist woods.  Painted Trillium is native from Ontario and Quebec south through northern New Jersey and much of Pennsylvania, from there south through the Appalachian mountains to northern Georgia and South Carolina.  It is also present in parts of Michigan.  Red Trillium’s range is a bit broader, from Ontario, Quebec and Michigan, south through northern Alabama, Georgia and South Carolina, with some presence in Indiana and Illinois.  Trillium sessile can be found from western New York state west to Illinois, Missouri and western Kansas, south from Oklahoma to north Carolina.  Trillium cuneatum is native in Pennsylvania, Maryland and Virginia, west to southern Illinois and south from Mississippi to Georgia.

These lovely Trilliums depend on flies and ants for their continued survival.

Painted Trillium (Trillium undulatum)

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A Carpet of Spring Beauty, Woven by … Ants!


Gracie, Carol.  Spring Wildflowers of the Northeast. 2012.

Eaton, Eric R.; Kauffman, Ken.  Kaufman Field Guide to Insects of North America.  2007.

Marshall, Stephen A.  Flies The Natural History and Diversity of Diptera.  2012.

Rhoads, Ann Fowler; Block, Timothy A.  The Plants of Pennsylvania.  2007

Michigan Natural Features Inventory, Trillium undulatum. Willdenow. 2004.

USDA NRCS Plants Database

USDA Forest Service Plant of the Week, Painted Trillium (Trillium undulatum), Larry Stritch