This document summarizes a study examining latitudinal variation in herbivory and higher trophic interactions involving native and invasive genotypes of common reed (Phragmites australis). The study found that gall infestation by introduced fly species (Lipara spp.) increased with latitude and was higher on native and invasive P. australis genotypes in North America compared to Europe, suggesting enemy release contributes to invasion success. Higher trophic levels like parasitoids and predators that influence Lipara spp. in Europe were largely absent in North America. Overall, the results show latitudinal gradients in species interactions and support the idea that a lack of natural enemies facilitates P. australis invasion.

1. Latitudinal variation in tritrophic interactions
associated with native and exotic genotypes of
Phragmites australis
Warwick Allen, Randee Young, Ganesh Bhattarai, Jordan Croy, and
James Cronin
Louisiana State University
wallen7@lsu.edu
Laura Meyerson
University of Rhode Island

2. Invasive Species
• One of the world’s worst environmental problems
• Enemy-release hypothesis (Elton 1958) is a wellsupported mechanism explaining invasion success
1) Decreased herbivory in introduced range relative to native
range
2) Greater impact of herbivores on natives than exotics in
introduced range
• Influence of higher trophic
levels has been ignored
(Harvey et al. 2010)
Hyalopterus pruni and predator

3. Latitudinal Gradients
• Theory predicts that species
interactions increase in strength
towards the tropics
Interaction Strength
• Little is known about how mechanisms of invasion
success may change over broad spatial scales
Latitude

4. Latitudinal Gradients
• Little is known about how mechanisms of invasion
success may change over broad spatial scales
• Theory predicts that species
interactions increase in strength
towards the tropics
Herbivory
Chemical defense
• Mixed results in empirical studies
(Moles et al. 2011 meta-analysis)
• Latitudinal gradients in species
interactions may alter invasion
success
Physical defense

5. Phragmites australis (common reed)
• Tall perennial grass
• Virtually worldwide distribution
• Found in brackish and freshwater
marshes, pond edges, ditches, and
other disturbed wet areas

6. Phragmites australis (common reed)
• Multiple native and invasive
genotypes in North America
• Exotic genotype M ( ) from
Europe (mid 1800s) forms
extensive monocultures
– Known invasion history
• Gulf genotype I ( ) prevalent 25 yrs. a
in the south
• Native genotypes (all others)
widespread but declining
100 yrs.
50 yrs.
Exotic
Gulf
All others native
Modified from Saltonstall (2002)

7. Gallers: Lipara spp. (Chloropidae)
• Introduced from Europe
L. rufitarsis
• Specialists on P. australis
– L. rufitarsis
– L. pullitarsis
– L. similis
L. similis
• Inquilines (commensals) and
natural enemies
Calamoncosis sp.
Anthomyza sp.
L. pullitarsis
• Galls inhibit flowering 100%
of time
– Damage quantified as
proportion of stems infested
– Biological control potential?

8. Research Objectives
1. Examine distribution of Lipara spp. in North America
2. Test for the presence and direction of latitudinal
gradients in herbivory, parasitism, predation, and
commensalism
3. Compare patterns of species interactions between P.
australis genotypes to investigate implications for
invasion success
4. Explore the potential of Lipara spp. as biological control
agents of invasive P. australis

9. Methods: Field Survey
• 120 sites sampled in NA (M, I, and native genotypes)
• 21 sites in Europe
Exotic (M)
Gulf (I)
Native

10. Lipara spp. and Associates Survey
• 26 sites with Lipara spp. present in NA
• Data collected:
– Infestation rate (Europe also)
– Flowering frequency
– Stems reared or dissected (Lipara species,
predation/parasitism rate, inquilines)
– Lipara and associates performance (body
mass)
L. rufitarsis and inquiline

11. Lipara spp. Distribution
Lipara present
Exotic (M)
Gulf (I)
Native
Lipara absent
Exotic (M)
Gulf (I)
Native

12. Lipara spp. Distribution
Lipara present
Exotic (M)
Gulf (I)
Native
Lipara absent
Exotic (M)
Gulf (I)
Native

13. Lipara spp. Distribution
Lipara present
Exotic (M)
Gulf (I)
Native
Lipara absent
Exotic (M)
Gulf (I)
Native

14. Lipara spp. Latitudinal Gradients
Proportion of stems galled by Lipara spp.
• Infestation rate increases with latitude
Native
Exotic (M)
0.6
Genotype – F1, 24 = 20.813, P < 0.001
Latitude – F1, 24 = 8.509, P = 0.008
0.5
0.4
R² = 0.261
0.3
Enemy
release
0.2
R² = 0.276
0.1
0
36
37
38
39
40
41
Site Latitude
42
43
44
45

15. Lipara spp. Infestation Rate
• Higher in NA native and invasive genotypes than Europe
Proportion of stems galled by Lipara spp.
0.4
a
F2, 44 = 31.483, P < 0.001
Error bars = 95% CI
0.35
0.3
0.25
0.2
b
0.15
0.1
0.05
c
0
North America Native
North America Invasive (M)
Europe (M)
Phragmites australis genotype/location

16. Lipara rufitarsis body mass
• Body mass marginally higher in L. rufitarsis developing on
native P. australis genotypes
• Body mass decreases marginally with latitude
F1, 17 = 3.509, P = 0.078
Error bars = 95% CI
2.5
2
1.5
1
0.5
0
Native
Invasive (M)
Phragmites australis genotype
Native
Invasive (M)
2.9
Mean weight per individual (mg)
Mean weight per individual (mg)
3
2.7
F1, 17 = 3.720, P = 0.071
2.5
2.3
R² = 0.082
2.1
R² = 0.405
1.9
1.7
1.5
36
38
40
Latitude
42
44

17. Lipara spp. Parasitism/Predation
• Parasitism/predation by arthropods
– 0% in NA
– 26% on L. rufitarsis in UK
(Reader 2003)
– 19% on L. rufitarsis and 22%
on L. similis in Germany
(Tscharntke 1994)
– Europe species rich in parasitoids (Nartshuk 2006)
*

18. Proportion of Lipara spp.
galls containing inquilines
Lipara spp. Inquilines
• Inquiline frequency of
occurrence
Calamoncosis sp.
Anthomyza sp.
0.6
F1, 18 = 0.427, P = 0.552
Error bars = 95% CI
0.5
0.4
0.3
0.2
0.1
0
Native
Invasive (M)
Phragmites australis genotype
• Follows pattern of
infestation rate of Lipara
spp.
Proportion of Lipara spp.
galls containing inquilines
• Latitudinal pattern but no
difference between
genotypes
0.9
F1, 18 = 48.982, P < 0.001
0.8
0.7
R² = 0.796
Native
Invasive (M)
0.6
0.5
R² = 0.679
0.4
0.3
0.2
0.1
0
36
38
40
42
Latitude
44
46

19. Conclusions
• Lipara species currently restricted to eastern US
– L. rufitarsis common from NC-ME, L. similis abundant in
Northeast, L. pullitarsis only in Mid-Atlantic
• Latitudinal gradients present in herbivory, inquiline
frequency, and possibly performance
• Enemy release from Lipara spp. may contribute to P.
australis invasion success in NA
– Biological control using Lipara spp. unlikely to succeed
• Higher trophic levels not influencing invasion success
– Lack of natural enemies may explain higher infestation rates
relative to Europe

20. Future Directions
• Common garden experiment
– Test if patterns found in field survey have a genetic basis
– Examine traits underlying oviposition preference
– URI common garden (>200 P. australis populations from NA)
• L. rufitarsis galls
collected from nearby
field site and spread
throughout garden

21. • Cronin Lab
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Ganesh Bhattarai
Anthony Chow
Forrest Dillemuth
Heidi Stevens
• Undergraduates
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Acknowledgements
Randee Young
Jordan Croy
Ray Andrews
Allison Hunt
April Simmons
Patrick Mooney
• Land Owners
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Mackay Island National Wildlife Refuge
Palm Beach County Parks Department
Rockefeller Wildlife Refuge
Alice Welford
Pettipaug Yacht Club
Rachel Carson National Wildlife Refuge
The Nature Conservancy
Estell Manor State Park
• Committee/Advisors
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Jim Cronin
Kyle Harms
Mike Stout
James Geaghan
Laura Meyerson
Bret Elderd
• Funding Sources
– NSF (DEB 1050084)
– LEEC
– LSU BioGrads

22. Questions?