Can apples grow on oak trees? Will
pine cones form on willows? Not true ones, but there are oddities of nature
that might make us think
they can - "galls". These unusually-shaped growths and swellings are a plant's
reaction to an irritation. The irritation can be caused physically or
chemically. What causes it? Primarily insects.
Over 1500 species of
insects in North America cause galls, and several hundred more are the result of
fungi and mites (spider relatives). That many gall-makers results in a vast
variety of shapes, sizes and colors of galls. These deformities are common
occurrences. Over one-half of all plant families are attacked by gall-making
But why do insects do
it? The answer lies in the food and protection that the plant tissues provide
for the developing insect.
Galls are more easily
identified than the gall makers. Each species of insects makes only one type of
gall, but you may find several species using the same plant. All plant parts may
exhibit galls - leaves,
stems, buds, flowers, twigs, and even roots.
Oaks are the most
commonly attacked plants. Over one-half of all galls produced can be found on
oaks. The insect you find inside the gall however may not be the one that made
it! One investigator found 75 different insects utilizing oak apple galls, some
as parasites to the make, others as a convenient place to call home.
The life history of each
species of gall maker can be quite different, ranging from the simple to the
complex. As a result, galls have received a great deal of scientific interest.
A field of goldenrod
will yield the easiest to find of all galls, in quantity and variety. The
spotted-winged-fly (Eurosta solidaginis) causes the ball-shaped swellings found
on stems. Another fly (Rhopalmivia solidaginis) produces a flowery-appearing
structure of stunted leaf growth near the top of the plant.
gall is caused by the larva of a moth, an unusual event, since few moths cause
galls. In the fall of the year, the adult moth lays eggs singly on the lower
leaves and plant stem. The larva hatches in spring and crawls to the new plant
growth, burrows into a bud and down the stem. The plant then forms the gall
around the larva at this point. The immature insect feeds off the tissues and
tunnels to the edge of the gall. It then plugs the tunnel with silk and plant
tissue and returns to the center of the swelling. It pupates and emerges via the
tunnel as an adult moth in late summer.
Willow pine cone gall forms
at the branch tips of willows. The swelling appear as gray-colored cones,
complete with overlapping scales. A small gnat (Rhavdophaga stobiloides) is the
maker, which over-winters as a larva in the gall, emerging as an adult in
spring. Numerous other insects use this gall for their needs. In one study, two
dozen galls produced 564 insects! Only 15 were the original gnat hosts. Others
included wasps, other species of gnats and grasshopper eggs.
The majority of galls
have little effect upon their host plants other than changing their outward
appearance. Besides stimulating human curiosity with their odd appearance, galls
have generated other interest and use. The oddities are studied in cancer
research, as they are believed to be examples of tumorous growth. Tissues of
many galls are high in protein and have been utilized for livestock feed. Tannic
acid in oak galls has found uses in tanning leather, dyes, inks and medicines.
Written by Nick
for this article from The Pennsylvania Deptartment of Conservation and