Predation of Introduced Mosquito Larvae by the Midge
Metriocnemus knabi in the Phytotelma of the Pitcher Plant
Sarracenia purpurea and Colonization Following Dry
Conditions
Gary Joseph Torrisi, W. Wyatt Hoback, John E. Foster, Elvis A. Heinrichs,
and Leon G. Higley
Northeastern Naturalist, Volume 22, Issue 3 (2015): 513–520
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G.J. Torrisi, W.W. Hoback, J.E. Foster, E.A. Heinrichs, and L.G. Higley
2015
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2015 NORTHEASTERN NATURALIST 22(3):513–520
Predation of Introduced Mosquito Larvae by the Midge
Metriocnemus knabi in the Phytotelma of the Pitcher Plant
Sarracenia purpurea and Colonization Following Dry
Conditions
Gary Joseph Torrisi1,*, W. Wyatt Hoback2, John E. Foster3, Elvis A. Heinrichs3,
and Leon G. Higley4
Abstract - The leaves of the carnivorous Sarracenia purpurea (Purple Pitcher Plant)
provide habitat for obligate insects. Within the pitchers of this plant, Metriocnemus knabi
(Pitcher Plant Midge) larvae coexist with Wyeomyia smithii (Pitcher Plant Mosquito)
larvae. No other mosquito species has been reported to utilize this habitat in the presence
of the midge. We tested whether the midge larvae were responsible for the elimination of
other mosquito species. We introduced 1 Aedes triseriatus (Eastern Treehole Mosquito)
larva, into each of 90 different pitchers. After 45–75 minutes, we extracted the fluid from
the Purple Pitcher Plant and counted mosquito and midge larvae. Although 98% of pitchers
contained Pitcher Plant Mosquito larvae, we did not detect 61 of the Eastern Treehole
Mosquito larvae. Of the 29 surviving introduced larvae, we found 13 (45%) in pitchers
that had no midge larvae. Drier than normal conditions in 2012 provided the opportunity
to investigate Purple Pitcher Plant leaves devoid of water and obligate insect larvae, and
the potential for foreign mosquito larvae to colonize unoccupied pitchers. We found Pitcher
Plant Midge or Pitcher Plant Mosquito larvae within 13 days following the addition
of water. We observed no foreign mosquito larvae. The inquiline larvae did not develop
when we added water to dry pitchers in the laboratory, suggesting that oviposition by
Pitcher Plant Midge and Pither Plant Mosquito adults occurred after a rainfall event. During
dry conditions, shaded Purple Pitcher Plants retained some fluids, and adults likely
completed their life cycle in these plants. However, severe, prolonged drought may eliminate
Purple Pitcher Plant inquilines and potentially make the pitchers available for exotic
mosquito larvae.
Introduction
Ombrotrophic bogs are precipitation-filled wetlands (Small 1972) that contain
uniquely adapted plant and animal communities. The Brighton Bog in upstate New
York has a large population of the Sarracenia purpurea purpurea Wherry (Purple
Pitcher Plant). This pitcher plant retains fluid, called phytotelma, in specialized
leaves. Larvae of 3 obligate dipterans can occupy this liquid. Wyeomyia smithii
Colquillett (Diptera: Culicidae, Pitcher Plant Mosquito) and Metriocnemus knabi
Coquillett (Diptera: Chironomidae, Pitcher Plant Midge) are common, whereas
16 Jennifer Court, Saratoga Springs, NY 12866. 2Department of Entomology and Plant
Pathology, Oklahoma State University, Stillwater, OK 74078. 3Department of Entomology,
University of Nebraska, Lincoln, NE 68583. 4School of Natural Resources, University of
Nebraska, Lincoln, NE 68583. *Corresponding author - gjtorrisi@aol.com.
Manuscript Editor: Christopher M. Heckscher
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Fletcherimyia fletcheri (Aldrich) (Diptera: Sarcophagidae, Pitcher Plant Flesh Fly)
is occasionally found (Krawchuk and Taylor 2003, G.J. Torrisi, pers. observ.).
In the laboratory, Petersen et al. (2000) found that Pitcher Plant Midge
eliminated introduced mosquito larvae from simulated Purple Pitcher Plant
phytotelmata and suggested that the longer setae on Pitcher Plant Mosquito larvae
are an adaptation to prevent predation by midge larvae. To our knowledge, these
laboratory results have not been tested under natural conditions, which can often
produce different results (e.g., Diamond 1986) and overestimate the effects of
predation (Cooper et al. 1990). Therefore, our objective was to test predation by
Pitcher Plant Midge in the field on larvae of Aedes triseriatus Say (Eastern Treehole
Mosquito). Drier than normal conditions in the second year of our study also
allowed us to investigate colonization of dry pitchers after water was artificially
or naturally added.
Materials and Methods
Study site
We conducted this study at the Brighton Bog, in the town of Brighton, Franklin
County, NY (44°26'50"N, 74°12'52"W), within the 2.43-million ha Adirondack Forest
Preserve high-peaks region. The habitat investigated is an ombrotrophic wetland
approximately 2.5 ha in size with a large open-water flark pool. The surrounding
temperate forest is dominated by Pinus strobus L. (White Pine), Tsuga canadensis
(L.) Carr. (Estern Hemlock), Picea spp. (spruces), Larix laricina (Du Roi) K.
Koch (Tamarack), and Quercus spp. (oaks). The bog is covered in Sphagnum spp.
(peat moss) and stunted Picea mariana (Mill.) Britton, Sterns & Poggenb. (Black
Spruce). In addition, Chamaedaphne calyculata L. (Leather Leaf) and Eriophorum
sp. (cotton-grass) grow around the perimeter ~20–30 m inward from the lagg. Johnson
(1985) has characterized peatlands like Brighton Bog, which occurs at ~600 m
elevation, as cold-climate habitats.
An abundance of Purple Pitcher Plants occurs within the shaded area of Brighton
Bog. These plants contain an inquiline community dominated by the Pitcher Plant
Midge, and the Pitcher Plant Mosquito. Our research in the bog began in 2010 and
we conducted field experiments in 2011 and 2012. During these surveys, we found
that Pitcher Plant Midge and Pitcher Plant Mosquito larvae occupied 98% of sampled
Purple Pitcher Plant leaves with phytotelma (Torrisi 2013). In 2012, Brighton
Bog received much less than average rainfall (Weather.org 2015). Some pitchers
were either void of fluid or contained very little fluid, and in several instances, one
or both obligate insects were absent from the leaves. By mid-July, many of the new
pitcher plant leaves remained unopened, appeared stunted, and in some cases were
senescing. The older red or purple leaves were also dying or dead. Approximately
half of all the open pitchers in the bog were void of liquid.
Study organisms
We employed natural Pitcher Plant Mosquito and Pitcher Plant Midge communities
for the field studies. We chose Eastern Treehole Mosquito as the
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foreign mosquito for our study because they proved to be the most abundant
species colonizing our artificial containers. They were field collected and reared
in containers.
Field introduction of larvae
In 2011, we collected Eastern Treehole Mosquito larvae from naturally colonized
2-L containers near Saratoga Springs, NY. We transported first-instar larvae
to the bog, siphoned them from the transport container, and placed 1 larva into each
of 40 large (15–20 cm tall) Purple Pitcher Plant pitchers. We marked selected pitchers
by placing a color-coded wooden dowel next to the leaf. After a 45-min waiting
period, we quickly removed the phytotelma of the pitchers using a modified culinary
turkey baster fitted with clear, flexible, 7-mm diameter tubing. We squeezed
the baster bulb, inserted the tubing to the bottom of the pitcher, and siphoned the
fluid by allowing the bulb to expand. We examined the contents, removed any introduced
larvae, discarded any surviving Eastern Treehole Mosquito larvae, and
returned the fluid to the pitcher. We recorded the presence or absence of the foreign
larva, predatory midge larvae, and Pitcher Plant Mosquito larvae. We repeated the
experiment in 2012, introducing one Eastern Treehole Mosquito larva into leaves
of 50 different Purple Pitcher Plants.
Dry-conditions investigation
In 2012, when near-drought conditions existed, we investigated 3 environmental
conditions: pitcher leaves naturally retaining fluid, open pitcher leaves devoid of fluids,
and dry pitcher leaves to which we added water. On 20 July 2012, we selected 50
pitchers for each treatment and marked each with a color-coded dowel. Purple Pitcher
Plants were located in the shaded area of the bog perimeter, the edge boundary
between shade and open bog, and the open, full sun-exposed center region of the bog.
We selected 50 leaves that naturally contained fluid at the time of the investigation as
controls, 50 leaves without fluid as dry controls, and 50 leaves of approximately the
same size but without fluid for the treatment. We added 25 ml of untreated well water
to each leaf in the latter group. On the fourth day of this study, a single rainstorm deposited
0.94 cm of precipitation which added water to all leaves.
On 3 August 2012, we examined each pitcher leaf for the presence or absence of
insect larvae. We also recorded the size in cm above the moss bed, color scheme (either
all green, all red, or mixed coloration), and venation patterns (slight, moderate,
or heavily veined). Color and veination were qualified and not quantified descriptors
of the pitcher leaves. We noted the number of damaged leaves, the larval stage
of the insects found in the phytotelma, as well as leaves with reduced or increased
amounts of phytotelma, which we measured (in ml) by removing the fluid and placing
it into a graduated cylinder.
Results
Field introduction of larvae
Sampling from 40 pitcher leaves in 2011 revealed that all leaves contained
Pitcher Plant Midge larvae and Pitcher Plant Mosquito Larvae. Approximately 45
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min after we introduced the Eastern Treehole Mosquito larvae, they were no longer
present in 31 of the 40 leaves (77.5%) (Table 1). We observed all 3 species in the
remaining 9 leaves (22.5%).
In 2012, 30 of the 50 Eastern Treehole Mosquito larvae (60%) were absent from
the pitchers during resampling. All samples with missing foreign mosquito larvae
contained Pitcher Plant Midge larvae. In 7 of the leaves (14%), both Pitcher Plant
Midge larvae and Eastern Treehole Mosquito larvae were present. The remaining
13 leaves (26%) contained the introduced Eastern Treehole Mosquito larva but no
Pitcher Plant Midge larvae. During 1 resampling event, we observed a Pitcher Plant
Midge larva clinging to the abdomen of the Eastern Treehole Mosquito larva but it
released its hold upon transfer to the sampling container. We did not observe other
interactions during the procedures. Across years, of the 90 total Purple Pitcher Plant
leaves tested, 29 (32%) contained surviving Eastern Treehole Mosquito larvae, 88
(98%) had Pitcher Plant Mosquito larvae, and 77 (86%) of the leaves contained at
least 1 Pitcher Plant Midge larva (Table 1). Elimination of foreign mosquito larva
was significantly greater when the Pitcher Plant Midge was present than when it
was absent (Mann-Whitney rank-sum test; n = 90, t = 988, P < 0.001).
Dry-conditions investigation
A total of 46 of the 50 leaves that contained pitcher phytotelma from the onset
of our investigation maintained fluid throughout the investigation. Between visits, 3
pitchers were lost to leaf damage and we could not relocate 1 leaf during our second
visit. We documented insect larvae in 43 of the 46 pitchers. Both midge and mosquito
larvae were present in 26 of the leaves, Pitcher Plant Mosquito larvae alone
were present in 13 leaves, and only Pitcher Plant Midge larvae were present in 4
leaves (Table 2).
Table 2. Number of Purple Pitcher Plant leaves (n = 50)—categorized according to whether they had
phytotelma, or were dry and had water added by investigators, or were dry and then filled only by
a precipitation event—that contained inquiline larvae after 14 days at Brighton Bog, NY, during the
summer of 2012.
Larval type Fluid present Water added Precipitation filled
Midge only 4 7 4
Mosquito only 13 6 6
Both larvae present 26 12 6
No larvae present 6 25 34
Table 1. Recovery of the introduced larvae of Aedes triseriatus (Eastern Treehole Mosquito) and 1
or more Metriocnemus knabi (Pitcher Plant Midge) larvae from phytotelma of Purple Pitcher Plants
at Brighton Bog, NY.
Number of Foreign mosquito Foreign mosquito present
Year pitcher leaves absent with midge present With midge present With midge absent
2011 40 31 9 0
2012 50 30 7 13
Totals 90 61 16 13
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Twenty-five of the leaves to which we added water contained insect larvae
during our second visit and 8 additional leaves contained fluid but no larvae. We
found Pitcher Plant Midge larvae in 7 leaves, Pitcher Plant Mosquito larvae in
6 leaves, and larvae of both insects in 12 of the leaves where water was added
(Table 2). The remaining 17 leaves either dried out or were damaged during the
investigation period.
Sampling of the dry leaves that did not receive additional water except from
the rain event revealed 28 leaves that either remained dry or were damaged during
our investigation. Of the remaining leaves that held fluid, 4 leaves contained only
Pitcher Plant Midge larvae, 6 contained only Pitcher Plant Mosquito larvae, and
6 leaves contained both species. Only first-stage larvae were present in any of the
leaves to which water had been added, either by the investigators or precipitation
(Table 2). We did not detect Eastern Treehole Mosquito larvae in any of the pitchers
with or without the midge.
We conducted an experiment to test if dry leaves were colonized or if desiccation-
resistant eggs hatched when water was added. We brought 14 dry leaves back
to the laboratory, added well water to each leaf, and checked the leaves every 2
days for the presence of insect larvae. No larvae were found after a 2-week period
in the laboratory; thus, it appears that larvae found in the field were the result of
oviposition by adults rather than by hatching from desiccation-resistant eggs.
Discussion
Purple Pitcher Plant leaves provide a microhabitat for a number of micro- and
macro-invertebrates. Of these associates, the Pitcher Plant Mosquito and the
Pitcher Plant Midge are obligate inhabitants (Fish and Hall 1978, Hamilton and
Duffield 2002, Heard 1994, Miner and Taylor 2002, Nastase et al. 1995, Petersen
et al. 2000). These insects live within the phytotelma and spend their egg, larval,
and pupal periods in a single leaf.
This plant–insect ecosystem has served as the basis for much research extending
back several decades (Bradshaw and Hozapfel 1983). Petersen et al. (2000)
utilized a laboratory study to address the question of why no mosquito species
other than Pitcher Plant Mosquito have been collected from the leaves of Purple
Pitcher Plants. However, Milne et al. (2008) reported colonization of pitcher plant
phytotelma by Aedes albopictus (Skuse) (Asian Tiger Mosquito), in the absence
of Pitcher Plant Mosquito and Pitcher Plant Midge. Our results suggest that Pitcher
Plant Midge larvae prey upon introduced Aedes (mosquito) larvae. Although
we only directly observed one instance of this behavior, midge predation is the
most likely explanation for our results. In 2012, foreign mosquitoes were always
recovered from pitchers that did not contain Pitcher Plant Midge larvae (Table 1),
which supports the conclusion that our use of the siphon was sufficient to capture
introduced mosquito larvae. The disappearance of mosquito larvae was strongly
correlated with the presence of the Pitcher Plant Midge with only 16 of 77 pitchers
containing both the midge and the introduced larvae in 2012 (Table 1). These
results support the conclusions of Petersen et al. (2000) that predation by the
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midge larva likely reduces interspecific interactions with other mosquito larvae
for Pitcher Plant Mosquito larvae in pitcher phytotelma.
Successful colonization of an occupied niche by a new species requires displacement
of residents, use of available resources, the availability of additional
resources, or the absence of predators (Miller et al. 2002). In our investigation,
introduced Eastern Treehole Mosquito larvae disappeared from the Purple Pitcher
Plant leaves within 1 h in the presence of the Pitcher Plant Midge in ~80% of the
tests. The midge appears to be an effective predator that is able to take advantage of
a small habitat and is capable of capturing active prey (Petersen et al. 2000). These
results suggest that the Pitcher Plant Midge may provide a form of guardianship
for the Pitcher Plant Mosquito larvae by eliminating foreign mosquito larvae that
hatch in the leaves of the pitcher plant and lack the long setae that seem to protect
the former species from midge predation.
In our experiments, we allowed ~45 minutes for interactions and we always
introduced a single first-instar Eastern Treehole Mosquito larva. It is likely that a
longer time interval for interaction would have resulted in higher predation rates
by the Pitcher Plant Midge larvae, though Petersen et al. (2000) reported that all
introduced mosquito larvae placed in an observation chamber were killed by Pitcher
Plant Midges within 30 minutes.
In 2012, when the bog experienced very dry conditions, many Purple Pitcher
Plant leaves were either dry or nearly dry. Our observations of the bog during the
predation experiments led us to question whether midge and mosquito larvae would
colonize the pitcher plants if water was added to the leaves. Two questions arose:
first, would the insects colonize the leaves late in the season and, second, would
mosquitoes other than the Pitcher Plant Mosquito colonize the leaves in the absence
of the predatory midge? In no more than 13 days after the addition of water, most
leaves contained either the Pitcher Plant Mosquito, the Pitcher Plant Midge or both
obligate species. On 24 July, 4 days after we experimentally added water to a set of
pitchers, a rainstorm added additional water and provided water to the leaves that
were dry. All insect larvae that we recovered from pitchers that had been dry were
early instars. We found that pitchers containing water at the onset of the investigation
contained both mosquito larval instars and some pupae; however, it seems that
the dry conditions delayed the maturation of these larvae. We counted as many as
28 early-instar mosquito larvae from 3 pitchers that retained phytotelmata. Pitcher
Plant Midge larvae were also in early stages but in much lower numbers, with 2–7
larvae observed.
After we added water to the 100 pitchers that were void of water, there was
a mixed response among pitchers. Some had Pitcher Plant Midge larvae only,
some had Pitcher Plant Mosquito larvae only, and in most cases, both species
were present. At no time was a foreign mosquito larva found. It would seem that
an unoccupied habitat, suddenly made available in a time when other oviposition
sites were likely rare, would attract females of mosquito species other than Pitcher
Plant Mosquito. Although treeholes hold more water than pitchers and are more
protected from desiccation, the dry conditions were of such duration that even
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natural treeholes may have been dry or very nearly so, limiting available sites
and likely reducing numbers of adult Eastern Treehole Mosquitoes. In a previous
study, Eastern Treehole Mosquito females preferred larger and darker containers
(Torrisi and Hoback 2013), and females that deposited eggs in pitchers may have
differentiated among leaf ages. In the absence of the midge, it would seem possible
that foreign mosquitoes could compete with Pitcher Plant Mosquito larvae. However,
this assumption has not yet been tested. From our results, it appears that adult
Pitcher Plant Mosquitos are able to persist in the environment and colonize newly
available pitchers more readily than other mosquito species, which may survive dry
conditions with aestivation of eggs.
The insect inquilines of the Purple Pitcher Plant require a phytotelma association
in order to develop prior to adult emergence (Mouquet et al. 2008). It appears that
the temporal delay in adult oviposition during drier conditions allows the adults to
colonize newly grown or dried pitchers once precipitation fills the leaves. Climatic
factors, primarily temperature and precipitation, strongly influence the development
of these inquiline inhabitants (Tauber and Tauber 1976). Insect fauna within
the leaves of the Purple Pitcher Plant are influenced by physical, chemical, and
biological factors that modify their growth and affect their phenology (Juliano and
Stoffregen 1994, Williams 1996). Our study emphasizes the importance of conducting
field studies to confirm interactions observed in the laboratory and to determine
the effects of variable or changing weather patterns on highly co-evolved systems.
Acknowledgments
We thank Apple Pools, Inc., for their financial support and for providing assistance in
collecting field data over the past 2 years. We also extend our gratitude to P. Torrisi, B.
Noden, and H. Malolk for helpful comments on an earlier version of this manuscript.
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