Vol.3, No.4, 311-313 (2013) Open Journal of Animal Sciences
Habitat abundances of a cricket-parasitizing wasp
Rhopalosoma nearcticum (Hymenoptera:
Rhopalosomatidae) in a United States
mid-Atlantic park
Edward M. Barrows
Laboratory of Entomology and Biodiversity and Georgetown University Center for the Environment, Department of Biology, George-
town University, Washington DC, USA; barrowse@georgetown.edu
Received 25 August 2013; revised 28 September 2013; accepted 14 October 2013
Copyright © 2013 Edward M. Barrows. This is an open access article distributed under the Creative Commons Attribution License,
which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Adults of the nocturnal, cricket-parasitizing wasp
Rhopalosoma nearcticum flew in Dyke Marsh
Wildlife Preserve (DMWP) from late June through
late September, based on a 2-year, Malaise-trap
sample of 617 individuals from three habitats.
These wasps were significantly more abundant
in the floodplain forest compared to the tidal,
freshwater marsh and forest-marsh ecotone.
Females were more likely to be in the ecotone
and marsh than males. The pooled sample from
all three habitats was significantly male biased.
This study provides baseline information on R.
nearcticum that can be used in assessing the
health of the DMWP entomofauna in view of glo-
bal change, accelerating DMWP erosion and
marsh loss, invasive species, and other threats
to this fragile preserve.
Keywords: Deciduous Forest; Freshwater Tidal
Marsh; Malaise-Trap Sampling; Parasiti c Wasp;
Phenology; Sex Ratio
The nocturnal, parasitic wasp Rhopalosoma nearcti-
cum Brues is native to Maryland south through Florida
and west through Texas, and Malaise-trap samples indi-
cate that this wasp can be common in some parts of its
range such as Florida [1,2]. This wasp is a larval parasite
of Hapithus and Orotharis crickets [3-5]. Fifth-instar R.
nearcticum larvae leave their hosts, spin cocoons and
pupate in soil, and emerge as adults from their cocoons
in summer.
This report regards the flight periods, abundances, and
sex ratios of adult R. nearcticum in the floodplain forest,
tidal freshwater marsh, and their ecotone based on Mal-
aise-trap samples from Dyke Marsh Wildlife Preserve
(DMWP) part of the George Washington Memorial Park-
way (GWMP), a park in the US Mid-Atlantic Region
administered by the National Park Service. This study,
which is part of the DMWP Arthropod Survey, is the first
quantitative investigation of R. nearcticum abundances in
different habitats, including a rare freshwater marsh.
Using six Townes-style Malaise traps [6], my lab col-
lected arthropod samples from April 1998 through De-
cember 1999 in Dyke Marsh Wildlife Preserve, described
by Johnston [7]. The Preserve includes 154 ha of land on
the west side of the Potomac River and part of the River
in Fairfax County, is 0 - 3.25 m above sea level (B.
Helwig, pers. comm.), and contains the largest remaining
freshwater tidal marsh in the Washington, DC, Area. This
fragile park has experienced marked degradation in re-
cent decades due to air pollution, alien invasive organ-
isms, accelerating erosion and marsh loss, and water pol-
lution [7,8, pers. obs.].
Two traps were run in each of three habitats—low
forest, freshwater tidal marsh, and the ecotone between
them as described by Barrows et al. [9]. The six traps
were in a broad transect that ran in an east-west direction.
The ecotone (defined as 10 m on each side of the forest-
marsh edge) ran about 200 m in north-northeast and
south-southwest direction in the sampling area. I oriented
each trap so that its longitudinal axis ran east-west, and
its collecting head faced due east. The forest traps were
about 50 m west of the ecotone, and the marsh traps av-
Copyright © 2013 SciRes. OPEN ACCESS
E. M. Barrows / Open Journal of Animal Sciences 3 (20 13) 311-313
eraged about 60 m east of the ecotone. The mid-point
location of the forest traps is 38.77194˚N, 77.05083˚W;
ecotone traps, 38.77139˚N, 77.05056˚W; and marsh traps,
38.77172˚N, 77.04990˚W.
Barrows et al. [9] described the Malaise traps in detail.
In summary, each trap was 1.2 m wide, 1.7 m long, 1.0 m
high at its back and 2.0 m high at its front and mounted
on a floating platform that could rise and fall with the
tide which can be as high as about 1 m during nonflood
periods. The wasps flew or crawled into a trap’s collect-
ing head where they were preserved in 95% ethanol.
Forest and ecotone traps ran during the entire 21-mo
sampling period. My lab removed marsh traps from late
December 1998 through late March 1999, because possi-
ble flooding during that time could have destroyed them.
The lab emptied traps every 3 - 24 days, collecting sam-
ples less frequently during November through March
when daily arthropod captures were low.
To identify R. nearcticum, I used the insect collection
at the National Museum of Natural History, Washington,
D. C., and the key in Stange [2]. To test for possible dif-
ferences in wasp numbers among habitats, I used re-
peated-measures analysis of variance (rmANOVA) and
the Scheffé test [10] on data transformed into cube-root
values to meet the assumption of homoscedasticity. To
test whether sample sex ratios were different than hypo-
thetical 50:50 sex ratios, I used an online Fisher’s Exact
Probability Test program [11]. Voucher specimens are in
the Georgetown University Arthropod Collection.
The traps captured 617 adult R. nearcticum during the
2-yr trapping period. This was the only rhopalosomatid
species found in the samples. More R. nearcticum oc-
curred in 1998 samples than in the 1999 samples (Table
1), possibly as a result of this species’ natural popula-
tion-size fluctuations. Adults flew from late June through
late September and had a longer flight season in 1998
compared to 1999 (Table 1). Peak abundances were in
early August 1998 and mid-August 1999. Farther south
in Florida, US, R. nearcticum flew from May through
October [2].
In 1999 and in both years combined, R. nearcticum
was significantly more common in the forest than in the
other two habitats (Table 1). This may be due to a
greater abundance of hosts, other resources, or both in
the forest than in the other habitats, subjects not investi-
gated in this survey. Overall in DMWP, arthropod abun-
dances among the three habitats vary among taxa. For
example, two lampyrid species and a group of five me-
copteran species were also more common in the forest
than the other habitats [12,13]. In contrast, a sialid spe-
cies was more common in the ecotone [9].
Sample sex ratios of R. nearcticum were male biased
in 1999 and both years combined (Ta bl e 1 ). If this spe-
cies has an actual adult sex ratio of 1:1, male-biased trap
samples do not indicate this species’ true adult sex ratio.
Both females and males were common in the forest, but
females were more likely to be in the ecotone and marsh
than males. Females may have been searching outside of
the forest for prey and possibly other resources, and
males may have been searching primarily in the forest
for females. Sample sex ratios from other DMWP sam-
ples varied with taxon. For example, one sialid species
and one lampyrid species and genus had statistically
male-biased sex ratios, but one panorpid species had a
female-biased sex ratio [9,12,13]. Many aspects of R.
nearcticum’s biology await study including foraging and
mating behavior and population-size responses to envi-
ronmental factors.
In conclusion, this investigation found that adult R.
nearcticum occurred in summer and early autumn, pri-
marily in the flood-plain forest, providing baseline data
that can inform future monitoring and managing arthro-
pods in DMWP. Since this is a time of rapid global
change, it is worthwhile to resample DMWP for R.
nearcticum and other arthropods on a regular basis. The
preserve is near sea level and threatened by higher tides
due to sea-level rise that could eliminate much of DMWP
in this century.
Table 1. Abundance in three habitats, % females, and flight periods of Rhopalosoma nearcticum based on Malaise-trap samples from
Dyke Marsh Wildlife Preserve, Virginia.
Number of wasps1 % female (sample size)
Forest Ecotone Marsh Total Forest EcotoneMarsh Total
Flight Period
1998 117.5 ± 70.0
(68 - 167, 235) a
11.5 ± 9.2
(5 - 18, 23) a
9.5 ± 3.5
(7 - 12, 19) a277 39* (235)91** (23)100*** (19) 47 (277) 24 June - 28
1999 164 ± 46.7
(131 - 197, 328) a
3.0 ± 2.8
(1 - 5, 6) b
3.0 ± 1.4
(2 - 4, 6) b 340 30*** (328)832 (6) 1002 (6) 32*** (340) 2 July - 26
1998-1999 281.5 ± 116.7
(199 - 364, 563) a
14.5 ± 6.4
(10 - 19, 29) b
12.5 ± 2.1
(11 - 14, 25) b617 33*** (563)90*** (29)100*** (25) 30*** (617) 24 June - 26
1Within year, trap site totals (N = 2 traps) followed by different letters indicate that their respective sites have significantly different abundances from one other
(P 0.05, ANOVA, Scheffé Test, using cube-root transformed data); 2This sample is too small for analysis with Fisher’s Exact Probability Test; *P 0.05; **P
0.01; ***P 0.001 (Fisher’s Exact Probability Test).
Copyright © 2013 SciRes. OPEN ACCESS
E. M. Barrows / Open Journal of Animal Sciences 3 (20 13) 311-313 313
Many people have helped with the Dyke Marsh Wildlife Preserve
Arthropod Study (DMWPAS), including R. O’Hanlon (building and
maintaining traps), K. Differding, L. Gardner de Beville, H. Goldfrank,
D. L, Mead, and S.-M. K. Wise (processing specimens). Aaron F. How-
ard and H. A. Bookstein critically read a preliminary draft of the manu-
script of this paper. Friends of Dyke Marsh, the National Park Service,
and the Washington Biologists’ Field Club provided financial support
for the DMWPAS.
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