Lab. 1. Evolutionary ecology of Eurosta on Golden-rod (Solidago spp.)
Overview
In general, studies involving interactions between insect gallmakers and their plant hosts provide an excellent introduction to many of the concepts we will focus on throughout the term (e.g. coevolution, specialization, conditional outcomes, and geographic mosaic).
The problems the gall-formers face (getting into a host, staying in, and reproduction) are very similar to those faced by parasites and other symbionts. Moreover, there are many examples of gall-forming arthropods in the Lethbridge area, they are obvious to the naked eye, and they are straightforward to sample. The system we will be studying in detail,
Eurosta flies in goldenrod, is one of the best-studied natural insect/gall systems, although it is virtually unstudied in western North America. As such, it represents one of the most thorough studies to date of a single system of interacting, specialized species. The purpose of this lab is to examine several biological features of the interaction involving gall-makers on common Goldenrod. We will collect the data as a class, but each of you will be responsible for completing a written report.
The host
The common Goldenrod, Solidago altissima, is a familiar species, common throughout
North America. The taxonomic status of this group is complicated and unresolved. This is despite the fact that there are over 100 species in the genus. They are insect pollinated
(honeybees and bumblebees are most common) and self-incompatible.
The species that frequently host the goldenrod ball gallmaker in our region are members of the widespread S. canadensis complex. There are three common species within the complex; two are common around Lethbridge, S. canadensis and S. altissima. We will be concentrating our efforts on S. altissima. This plant has been widely introduced throughout North America. However, it is thought to be native to this area, particularly along disturbed riparian floodplains. Its’ presence on farms and roadsides is probably anthropogenic.
Solidago altissima is strongly clonal, frequently forming extensive, nearly pure clusters of ramets. Its’ clonal growth habit means that hosts occur as a clumped resource for ovipositing flies. The plant is a perennial, with shoots emerging each spring from overwintering rhizomes. Some clones of S. altissima have been estimated to live for up to 100 years.
The parasite, and gall formation
Solidago spp. are known to host up to 100 different parasitic arthropods, many of them absolute specialists. There at least 12 species of Eurosta in North America. This genus of flies is closely related to the familiar fruit flies (Drosophila spp.). Most of these species are gall formers, but surprizingly all but E. solidaginis form galls on the roots, not the stems.

The flies are about 8 mm long and 2mm wide (similar to typical fruit flies). After emergence from the gall in spring, males position themselves on the tip of growing goldenrods. Mating usually occurs on the day they emerge and usually right on a plant from which they emerged. After mating, females spend the majority of their time walking over a neighbouring plant, searching for an oviposition site. Oviposition behaviour is remarkable. She will usually climb to the tip of the growing stem and rapidly rub it with her forelegs, occasionally pulling it into her mouth. This behaviour apparently tells her something about host quality and she will either oviposit, or leave to find another plant. To oviposit, she pierces the plant and deposits one egg on the tip of the growing meristem. If she leaves the plant, she will only fly up to 1 m to find a different one. Females have up to 200 eggs in utero , but oviposit no more than 20 eggs
(i.e. 20 plants).
Eggs hatch in about 6 days. The larvae bore into the base of the bud meristem, where it creates a small feeding chamber. Many larvae die at this stage, due to a hypersensitive host response. Surviving larvae induce the formation of the gall. The gall reaches its maximum size about 6 weeks later. The insect continues to feed on tissues within the gall until about September. Just prior to overwintering, the larvae excavate an exit tunnel. Then it returns to the centre of the gall, to emerge along the tunnel in spring. The ability for these larvae to survive winter temperatures is one of the classic examples of the use of cryoprotectants to prevent damage to the cells due to freezing and thawing.
The mechanism of gall formation is not well known for any gall former. Studies with
Eurosta seem to indicate that both the mechanical action of chewing, and the release of specific growth-inducing chemicals (auxins and gibberelins), are important. If the gallformer is removed from the gall, or killed, gall growth ceases. Molecular mimicry between plant growth substances and substances found in the saliva of gall-formers is often cited as an example of tight coevolution. However, many details are unknown and no examples exist of the artificial production of a ‘natural’ gall. This suggests that the mechanisms behind gall formation are exceedingly complex and involve many factors of both host and fly origin.
Gall predators and parasites
Gallmaking insects always support complex associations of parasitoids and other enemies, and Eurosta is no exception. There are two insects (a wasp and a beetle) and two avian carnivores (Downy woodpeckers and chickadees) that are especially important in the Lethbridge area. In some populations, mortality due to these enemies can be as high as 96%. The parasitoid wasp, Eurytoma is specific to Eurosta flies. Female wasps use host plant cues to search for goldenrods, then oviposit directly into galls. When the wasp egg hatches, the larva consumes Eurosta and then continues to feed on the gall tissue. Most wasp larvae overwinter in the gall and emerge in Spring. Wasps tend to be constrained to ovipositing on small galls. More specifically, the female’s ovipositor can only penetrate a certain distance through the outer gall tissue.
The beetle Mordellistena is known as an inquiline (rather than a parasitoid). Female beetles deposit eggs on the gall itself and the larvae burrow into the gall in search of

Eurosta. During winter, it searchs for Eurosta and eats it. Studies have shown that these beetles will attack Eurosta galls of any size.
Downy woodpeckers and chickadees also play a role in this interaction. The birds feed almost exclusively on the largest galls, leaving tell-tale signs of their presence. I will show you examples of scars left by Downy Woodpeckers during the field trip. The woodpeckers themselves provide a good example of behavioral specialization. You will observe from the scars that the birds have the ability to tap the gall and only penetrate the gall through the emergence tunnel. This is almost certainly adaptive because the tunnel provides unambiguous information that the gall contains Eurosta and not one of its toxic parasites.
Objectives
We can only scratch the surface of this symbiosis. Although I expect you to understand the general natural history of this tri-trophic interaction, our study will be focused on two general questions. First, what is the extent of inter-population (which in this case can also be considered inter-clone) variation in gall prevalence and parasite prevalence ?
This is a common question in symbiosis, especially in parasite/host systems (e.g. why are hosts in one village more heavily infected than in a neighbouring village?). Second, what is the effect of infection on the host ? This is probably the most typical question asked in any antagonistic symbiotic interaction. To do this, we will simply compare the size and reproduction of infected vs. uninfected ramets, collected from 2-3 populations.
Methods
Effects of Eurosta on the host
We will compare general infection characteristics of Solidago ramets from 2-3 populations. Each population probably represents a unique host genotype. I tried as much as possible to sample every plant within the clone. I did it to avoid any bias in sampling (ideally, the plants should have been sampled at random). Thus, within each clone, we will have infected and uninfected ramets. The class will be split into groups, each responsible for quantifying 4 response variables: the length (in mm) of each plant, florets present or not, width of each gall (in mm), occupant of each gall (I will help with the identifications). Each group should fill in the following table; you will hand-in the data to me, I will collate it, and then provide you with the class data.
Population Name
Ramet no. shoot length
(mm) gall width
(mm) occupant # galls florets ? comments
1
2
3 n

The summary data should look something like this:
#1 #2 #3 total number of plants number of infected plants prevalence of infection (%) number of multiple infections prevalence of parasites (%) prevalence of beetle (%) mean (±sd) gall diamater (mm)
From this data we will evaluate 1) spatial variation in gall-induced effects on Solidago growth and reproduction, 2) spatial variation in rates of parasitism by wasps, and rates of predation by birds.
Natural selection on gall size
The data collected above will also allow us to test the prediction that birds predate large galls, while parasitoids prefer small galls. If so, then we can have confidence in the prediction that natural selection optomizes mean gall size in relation to the local abundance of bird vs. parasitoid enemies. We will discuss the concepts of local adaptation and geographic mosaic in class.
Assignment
Please follow the instructions regarding format very closely. As this is the first report, I will be placing equal weighting on presentation and content (the second report will emphasize content). We will discuss the results in class and decide from there on the specific approach to take.
References
There are 100’s of references on this complex interaction. You should start by visiting the outstanding website from the Abrahamson lab. The references below provide an appropriate background, but you should consider them as places to start only.
Hartnett, DC and WG Abrahamson. 1979. The effects of stem gall insects on life history patterns in Solidago canadensis. Ecology 60: 910-917.
McRea, KD, WG Abrahamson and AE Weis. 1985. Goldenrod ball gall effects on
Solidago altissima: C-14 translocation and growth. Ecology 66: 1902-1907.

Weis, AE, WG Abrahamson and KD McCrea. 1992. Variable selection on Eurosta’s gall size. I. The extent and nature of variation in phentypic selection. Evolution 46:
1674-1697.