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.
  1. 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.
  1. 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 Names:

Question:

Hypothesis:

Prediction(s):

Variables(s):

Prediction graph(s):

Data & Results: