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Thursday, April 17, 2014

Ghosts in the Graveyard

Twistleaf Yucca, Yucca rupicola

Commonly found growing in rural areas including graveyards, Twistleaf Yucca (Yucca rupicola) is sometimes called ‘ghosts in the graveyard’, for when in bloom the clusters of white flowers on thin stalks appear as floating apparitions.  But this common plant, which is widespread in our area, has a much more uncommon, mysterious association with a rarely seen butterfly called the Yucca Giant-Skipper (Megathymus yuccae). 

Flying in the spring, the Yucca Giant-Skipper is a medium-sized, robust-bodied butterfly with a fast, powerful flight.  Above, its dark forewings are elongated with a variable pale yellow outer band, and a yellow marginal border can be seen on the hindwings.  Below, the hindwings are a dark blackish brown with a violet-white frosting and prominent triangular white spot along the leading margin. Males and females are similar, but females are generally larger and males have wider, more rounded forewings.  Although fairly common, this species is like a ghost in the butterfly world, as sightings of adults are rare.

Tent formed by larva of Yucca Giant-Skipper

The most fascinating aspect of these butterflies is how they depend on the Twistleaf Yucca to carry out their life cycle.  Males perch on low vegetation or on the ground near twistleaf yuccas, awaiting passing females.  Producing only one generation per year from February to May depending on location, the cycle begins when the female butterfly lays a single egg on the leaf of a Twistleaf Yucca.  The young larvae (or caterpillar) feeds on the leaves of this plant and constructs small, individual silken shelters to protect itself as it grows.

Once larger, older larvae bore deep into the plant crown and feed within the root, constructing a prominent silk tent at the opening of the burrow.  Active tents can be discovered by looking for larvae excrement (called ‘frass’) that is pushed out of the tent opening before the larvae pupates.  In the spring, the adult butterfly emerges from the tent opening and allows its wings to dry before it takes flight and begins the search for a mate.  Adult yucca giant-skippers do not visit flowers to feed, and it is unknown how long they live, or if they utilize other food sources.

Yucca Giant-Skipper, Megathymus yuccae
Look closely and take care when performing your winter landscape cleanup, so as not to unwittingly destroy the tents that may be present on your Twistleaf Yucca plants.  These ghosts of the graveyard may just be harboring ghosts of their own!