Kenneth D. McCrea

Goldenrod Ball Gall Effects on Solidago altissima: 14C Translocation and Growth

The purpose of this investigation was to determine whether the effects of the goldenrod ball gall on carbon translocation and growth in ramets of Solidago altissima can account for changes in biomass allocation and total biomass observed in galled ramets. To do this, individual leaves of S. altissima were labeled with carbon—14 introduced as CO2. The 14C was introduced into ramets that had ball galls caused by the fly Eurosta solidaginis and into ungalled control ramets; gall size (large vs. small) and point of introduction of the label (above vs. below the gall) were experimental factors. After 5 d the ramets were harvested and their component organs were assayed for 14C using liquid scintillation. In addition, a field cohort of 359 galled and ungalled was followed during the period of gall growth to determine the effect of the gal on stem height growth. Gall size and labeling position had no effect on the percent of 14C translocated out of the labeled leaf but did affect the distribution of translocated 14C. Translocation to underground organs was reduced when the label was introduced above the gall, the reduction being related to gall size. Large galls reduced translocation to the apical bud when the label was introduced below the gall, but small galls did not. Translocation to underground organs was not affected by the gall when the label was introduced below the gall and translocation to the apical bud was not affected by the gall when the label was introduced above the gall; these results indicate that the goldenrod ball gall is a nonmobilizing gall. The presence of a gall did not significantly affect final stem height but did slow the growth of ramets during the period of most rapid gall growth. The observed effects of the gall probably explain changes in resource allocation shown by other studies but do not account for the overall decrease in biomass of galled ramets.

Host Genotype Choice by the Ball Gallmaker Eurosta Solidaginis (Diptera: Tephritidae)

We examined Eurosta solidaginis host choice among 38 naturally occurring clones of tall goldenrod, Solidago altissima. Significant variation was found in the per- centages of (1) ramets ovipunctured, (2) punctured ramets with eggs, (3) ramets galled, and (4) punctured ramets that formed galls. These findings indicate that all genotypes of S. altissima are not equally suitable (capable of supporting larval development) and/or ac- ceptable (having the proper cues to stimulate oviposition). The two most resistant and two of the most susceptible of 38 clones were monitored for ovipuncturing the following season. A total of 244 apical buds on ramets were dissected at intervals of 1, 2, and 3 wk after oviposition. Significantly more larval mortality occurred in the meristems of resistant clones than in the meristems of susceptible clones. Further, 73% of the dead Eurosta larvae were surrounded by necrotic plant tissue, apparently due to a hypersensitive response by the plant to the gallmakers stimulus. Finally, a common garden experiment using replicates of 30 clones of known resistance showed that susceptible clones were growing faster at the time of oviposition.

Host gall size and oviposition success by the parasitoid Eurytoma gigantea

Abstract. 1. Eurytoma gigantea Walsh is a specialist parasitoid of the tephritid gallmaker Eurosta solidaginis (Fitch).

Nutrient concentration-stream discharge relationships during storm events in a first-order stream

The relationship between nutrient element concentration and stream discharge during storm events was studied in a nutrient-rich first order stream. Stream concentrations of NO3-N, phosphate-P, Ca, and Mg were determined during the course of and following thunderstorms. Nutrient element trajectories were constructed for the elements monitored and some recognizable and reproducible patterns in nutrient concentrations emerged. NO3-N and phosphate-P generally increased in concentration during the early stages of increasing stream discharge resulting in a general clockwise trajectory. Ca and Mg consistently showed decreases in concentration during rising water, but the pattern of the trajectories was less constant. The patterns seen for NO3-N and phosphate-P suggest surface run-off as their origin while the patterns for Ca and Mg reflect their primary origin in groundwater. The ability to detect these differences from the analysis of nutrient trajectories suggests the use of this technique for determining the source of other elements in streams.

Nutrient and biomass allocation in Solidago altissima: effects of two stem gallmakers, fertilization, and ramet isolation

SummaryThe allocations of biomass, N, P, and K were determined by standard methods in goldenrod ramets (1) parasitized by dipteran and lepidopterous gallmakers, (2) from fertilized and unfertilized plots, and (3) whose rhizome connections to their parental clone were severed. The presence of ball galls and their larvae increased allocation to stem but decreased allocation to leaves and seed production, and reduced the number of new rhizomes. There was a marked magnification of N and P concentrations going up the food chains; from goldenrods to gallmakers to the gallmakers parasitoid/inquiline guild. Nutrient budgets expressed as flow diagrams indicated that N and P costs of gall presence were similar to energy costs under conditions where nutrients did not limit plant growth. Our data do not indicate that the growth of the galls of these gallmakers is limited by either N or P. Ramets from fertilized plots contained more N and P than controls but decreased the percentage of biomass allocated to leaves and inflorescences; ramets isolated by rhizome-cutting compensated their loss by increased allocation to roots, current rhizomes, and new rhizomes but at a cost of lower allocation to seed production and leaves. Gallmakers have a negative impact on host plant fitness characteristics. This may be especially important to establishing perennial hosts, given that herbivore effects would reduce clonal expansion and hence the ultimate clone size, thereby decreasing lifetime plant fitness.

Variation in Herbivore Infestation: Historical vs. Genetic Factors

Variation in insect infestation levels among plants is a result of both genetic variation in susceptibility and variation in previous insect attack. A plants history of insect attack can either increase or decrease susceptibility and can also influence the number of potential colonizers. Experiments were conducted to determine whether infestation levels of Eurosta solidaginis, the goldenrod ball gallmaker, on Solidago altissima were a result of genetic variation among plant clones (i.e., resistance) or previous attack levels. Results indicated that current infestation levels are primarily due to genetic variation in the plants resistance. Plant clones were found to differ in the percentage oframets in which oviposition was attempted by Eurosta, the percentage of ramets galled, and in the percentage of ramets with Eurosta oviposition punctures that were galled. Field resistance or susceptibility was maintained in plant clones grown in a common garden.

Variation in selection pressures on the goldenrod gall fly and the competitive interactions of its natural enemies

SummaryLarvae of the tephritid fly Eurosta solidaginis induce ball-shaped galls on the stem of tall goldenrod, Solidago altissima. Survival probability depends on gall size; in small galls the larva is vulnerable to parasitoid oviposition, whereas larvae in large galls are more frequently eaten by avian predators. Fly populations from 20 natural old fields in central Pennsylvania were monitored in 1983 and 1984 to examine the distribution of the selection intensity imposed by natural enemies, the parasitoids Eurytoma gigantea and E. obtusiventris, the inquiline Mordellistena unicolor, and the predatory birds Dendrocopus pubescens and Parus atricapillus. Mordellistena and E. obtusiventris are able to attack galls of all diameters while E. gigantea and the predatory birds preferentially assaulted small and large diameter galls, respectively. Eurosta in intermediate sized galls had the highest survivorship, hence selection had a stabilizing component. However, parasitoid attack was more frequent than bird attack, and the two did not exactly balance, thus there was also a directional component. The mean directional selection intensity on gall size was 0.21 standard deviations of the mean, indicating that larger gall size was favored. Interactions among the insect members of the Eurosta natural enemy guild are complex and frequent.

The role of phenolics in goldenrod ball gall resistance and formation

Some genotypes of Solidago altissima have been shown to exhibit a hypersensitive response to the tephritid ball gallmaker Eurosta solidaginis. When the gallmaker attempts to stimulate gall formation in these genotypes, necrotic tissue surrounds the larva and apparently causes larval death [Anderson, S. S., et al. (1989) Ecology 70, 1048]. Other genotypes have varying degrees of resistance or susceptibility [McCrea, K. D. and Abrahamson, W. G. (1987) Ecology 68, 822]. Studies of other host plants have shown that phenolic compounds can play an important role in this defensive reaction. To determine the role of phenolics in the resistance of S. altissima to ball gallmakers, weekly tissue samples were collected from two resistant and two susceptible clones, both with and without Eurosta attack. These samples were analysed for total phenolics using the Folin-Ciocalteau procedure. Phenolic content increased over time in all clones. In unattacked ramets, phenolic levels were higher in susceptible clones than in resistant clones. No evidence of an increase in phenolics was seen with a hypersensitive response, but a significantly higher concentration was found in plants that had initiated normal gall development. A second study of rapidly growing galls in three additional susceptible clones confirmed this increase in phenolics, demontrated that the higher phenolic concentrations were restricted to gall tissue, and showed that phenolic levels increase as much as five-fold in galls near their peak growth period. Increases in total phenolic levels were clearly not responsible for the hypersensitive reaction for resistance, but phenols could potentially play a role in gall formation by influencing the hormonal control of growth and/or in protection of the gallmaker from its natural enemies.

Evolutionary impacts of the goldenrod ball gallmaker on Solidago altissima clones

SummaryThree ramet clones of Solidago altissima were grown under greenhouse conditions to determine the effects of varying levels of attack by the goldenrod ball gallmaker (Eurosta solidaginis) on biomass allocation, leaf senescence rate and rhizome connections among ramets. The results, examined at both the individual ramet level and the clone level, showed that galled ramets became isolated from their clone through deterioration of rhizome connections. Gall effects were only observed at the ramet level although rhizome connection effects were detected at both the ramet and clone levels. The goldenrod ball gallmaker may therefore have little evolutionary impact on large clones but could appreciably affect newly established clones.

Host preference and recognition by the goldenrod ball gallmaker Eurosta solidaginis (Diptera: Tephritidae)

-Goldenrod ball galls produced by Eurosta solidaginis are almost invariably found on Solidago altissima in central Pennsylvania even though they occur on several other Solidago species elsewhere within the range of E. solidaginis. In two no-choice experiments, S. altissima was strongly preferred by Pennsylvania Eurosta relative to the sympatric species S. gigantea, S. canadensis and two ploidy levels of S. rugosa. A third experiment was conducted to determine if chemical and/or tactile stimuli sensed by the female flys ovipositor serve as cues for host recognition. Buds of S. altissima and S. rugosa were wrapped with bud leaves from S. altissima and presented to mated female flies. The flies punctured the wrapping leaves with their ovipositor on both plant species with equal frequency, but only the wrapped buds of S. altissima were punctured and used for oviposition. The wrapped S. rugosa buds were not punctured indicating that the E. solidaginis females can sense physical and/or chemical features of potential host plant buds with their ovipositor. Our data suggest that host affiliation patterns in Eurosta are the result of herbivore behavioral preferences that are selected for by host suitability.