Gall Moths

During the winter, you may often see the remainder of wildflowers from last summer standing like lone sentinels above the frozen surface. One of these flowers, the goldenrod is the host to several fascinating insects which spend a large part of their life buried within the tissues of this common plant.

A small moth with the attractive name of Gnorimoschema gallaesolidaginus and no simple common name seems to be attracted to this flower. A literal translation of the name breaks down Gnorimoschema into "known to form" and gallaesolidaginus into "galls belonging to goldenrods". So if we put it together it means "known to form galls belonging to goldenrods". This is a classic case of a scientific name telling us more about a species than most common names. This "gall moth" as we'll call it here is quite unusual among moths as very few moths are involved in the formation of galls. What is a gall? It's simply a thickening of the plants tissues caused by an irritation from the insect. Look for an elliptical swelling on the stem of the goldenrod. A round swell is the sign of the spotted-winged fly, another insect with a similar affinity to goldenrod.

In fall, the adult moth lays her eggs near the base of the goldenrod and the insect overwinters in this stage. As a result, you generally won't find this insect in the gall during the winter months. In the spring, the insect hatches and the larvae find a new flower and crawl upwards to the new buds where they burrow in. The plant will then begin to produce a gall around this tiny irritant until the usual elliptical bulge in the stem has formed. The plants reaction to the insect is ideal to the little parasite who spends his early months feeding contentedly within this fleshy bulge. Late in July it chews a tiny entrance whole to allow it easy exit when it is time to emerge as an adult moth. It will seal this whole with some silk and plant matter before forming it's tiny cocoon. In early autumn it emerges as an adult, mates and lays the eggs necessary for the next years generation.

Although this may sound like the ideal life for a tiny insect--protected from predators by a large gall--it isn't the perfect defense. Even the gall moths aren't safe from danger. There are many predators well designed to take advantage of the insects lack of mobility within the gall. Ichneumon wasps have a very long ovipositor and they are able to insert this egg depositor through the gall and thus lay an egg directly on the larval moth. Upon hatching, the larval wasp will slowly eat its unwitting host--a parasite upon a parasite. Another wasp lays its eggs on the moths eggs and they allow the moth to live normally until it makes its cocoon and then dies. The wasp will then hatch, develop and form its own cocoon within the dead moth larva--and you thought the movie "Alien" was gruesome. Sometimes real life can be stranger than fiction.