Goldenrod Galls
Objectives:
 Apply the scientific method to evaluate the interactions of various aspects of
gall phenotypes (e.g. height on stem & gall diameter)
 Develop testable hypotheses and predictions
 Present your study
Introduction:
The local goldenrod species of the genus Solidago are extremely common in
the United States, and often grow in dense fields. Shoots develop in the spring from
over-wintering rhizomes, and flower throughout much of the summer, eventually
producing large numbers of seeds. True to its name, goldenrod has dense clusters of
yellow flowers. In this lab we will examine galls formed by insect larvae on
Goldenrod plants.
Gall forming insects (Parasites of goldenrod)
Insect larvae are able to stimulate abnormal plant growth in its host leading to
structures that are referred to as galls. Galls provide food, protection from the
elements and predators to the insect larva. Goldenrod plants are subject to attacks
by three different types of gall-forming insects:
1. Common round galls are caused by the larva of the gallmaker fly, Eurosta
solidaginis. Adult flies emerge in May from galls and lay eggs in the terminal
buds of the goldenrod. The larva hatches and bores into meristematic stem
tissue and a gall forms around it. The alrva is full grown by mid-September,
but remains in the gall until spring when it pupates.
2. Elliptical galls are caused by the larva of the moth, Gnorimoschema
galloesolidaginus. This elliptical-shaped gall is usually found lower down the
stem of the goldenrod than a gall ball, indicating earlier oviposition by the
moth.
3. Rosette galls are caused by the larva of the midge, Rhopalomyia solidaginis.
This midge causes proliferation of leaves at the tip of the growing stem
forming a dense rosette of leaves.
Parasitoids of gall forming insect larvae
A number of parasitoids are able to exploit the larvae as a food source. A large
component of mortality for the larvae is caused by the larvae of two parasitoid
wasps Eurytoma obtusiventris and Eurytoma gigantea.
E. obtusiventris larva causes the fly larva to pupate (stage of development when it
changes from a larval body-form to an adult form) in mid-August, consumes the
larva, and remains inside the puparium until spring.
E. gigantea larva consumes the fly larva and also eats some of the gall but does not
pupate until the spring. The success of E. gigantea is dependent upon the wasp’s
ability to oviposit through the toughened walls of the gall, and not surprisingly this
species is larger than the closely related E. obtusiventris.
Predators of gall forming insect larvae
Larvae mortality due to predation can be as high as 96%.
A beetle larva, Mordellistena unicolor, bores into the gall and usually, but not
always, eats the fly larva.
Bird predators (e.g. downy woodpecker and black-capped chickadee) often attack
the gall and eat the larva during the winter months.
Note: Small larva of different fly species may live in gall walls without harming the
larva inside the galls.
Vocabulary
Symbiosis – An ecological relationship between organisms of two different species
that live together in direct contact.
Mutualism – A symbiotic relationship in which both, the host and the symbiont
benefit.
Parasitism – A symbiotic relationship in which the symbiont (parasite) benefits at
the expense of the host.
Commensalism – A symbiotic relationship in which the symbiont benefits but the
host is neither helped nor harmed.
Ovipositioning – The laying of eggs, espcieally by means of an ovipositor (special
organ in female insects).
Parasitoid – A parasite that ultimately destroys its host , as any wasp larva, which
feed progressively on the tissue of an immature stage of a host species. Parasitoids
spend weeks feeding on a single larva of the gall forming parasite inside th gall.
Parasitoids only eat one parasite in their lifespan.
Predator – predators rapidly eat the gall forming parasite inside the gall and have no
long term interactions with this parasite. Predators can eat more than one parasite
in their lifetime.
ACTIVITY
I. Brainstorming session:
1.
Formulate one question your group is interested in based on the
information given and introduction of this laboratory exercise. You may
want to think about how gall size, shape, or parasite species may
influence the mortality of the gall’s parasite. Is there any reason why you
would expect one predator to be more prevalent in the goldenrod
population around here? Be creative!
2.
State one hypothesis that may explain the question you are interested
in.
3.
Describe each of your predictions for this hypothesis in one sentence
and create graphs that depict the data you expect to collect in support of
your hypothesis. What are your dependent and independent variables?
4.
Write on the chalkboard which measure (= dependent variable) you
need to gather from each goldenrod specimen in order to test the
predictions of your hypothesis. Possible measures may include: gall type,
gall diameter, gall wall thickness, gall height, gall content such as empty
or presence of pupae or larvae, damage of gall.
II. Data Collection:
1.
Each group will collect data from a bunch of goldenrod stalks with galls
that have been collected for you. To increase the sample size, all the
groups will share the data they gather from their set of goldenrod plants
with the rest of the class. Therefore, it is crucial that ALL groups measure
ALL the variables listed on the board to ensure that everyone can benefit
from a large data set.
2.
Record all external features before you start dissecting the galls.
3.
Carefully dissect each gall, record all parasitoids, predators, commensal
species or unspecified mortality. On the following page is a list of
descriptions that should help you with this task. Don’t forget that gall
type can indicate parasite too.
Terminology: remember that any goldenrod that has a gall was therefore
parasitized. The larva that makes the gall is thus the parasite of the goldenrod.
This larvae, which makes the gall, can itself become the host to other larvae that are
called parasitoids. Those parasitoids feed on the host (which is the parasite of the
goldenrod and the maker of the gall).
Parasite
a) Eurosta solidaginis larva: yellowish-white, football-shaped, and fleshy with
black
mouthparts and no legs. This is the fly larvae, i.e. the maker of the round gall,
i.e. the host to potential parasitoids.
Parasitoids
b) Eurytoma obtusiventris larva: inside small brown Eurosta puparium (~2.2 x
7.4 mm), resulting from forced early pupation. Host has been consumed. This
is a parasitoid: feeds on the parasite (host/fly larvae that made the gall).
c) Eurytoma gigantea larva: white, teardrop shaped, often surrounded by large
black frass pellets. Host has been consumed by the end of August. This is also
a parasitoid: feed s on the parasite (host/fly larvae that made the gall).
Predators
d) Beetle larva, Mordellistena unicolor: a slender, elongated, white larva with
small appendages. May or may not consume host pupa. This is a predator:
rapidly feeds on the parasite (host/fly larvae that made the gall).
e) Birds, break through gall wall and consume host pupa. This is a predator:
rapidly feeds on the prasite (host/fly larvae that made the gall)
Downy woodpecker creates a narrow hole.
Black-capped chickadee creates a cruder, conical shaped hole.
Natural Mortality
f) Includes all larvae found dead for reasons such as improper gall
development, or imprisonment of larva in resins.
III. Data Analysis and presentation:
Share your data with the other groups after you have completed your data
collection.
Plot your data and prepare your presentation including your question, hypothesis,
and its predictions, a graph representing your predictions, a graph presenting the
data, and your conclusions
Data Sheet for Goldenrod Laboratory
Question:
Hypothesis:
Prediction(s):
Variables(s):
Prediction graph(s):
Names:
Data & Results: