Habitat abundances of a cricket-parasitizing wasp <i>Rhopalosoma nearcticum</i> (Hymenoptera: Rhopalosomatidae) in a United States mid-Atlantic park

doi:10.4236/ojas.2013.34046

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Vol.3, No.4, 311-313 (2013) Open Journal of Animal Sciences

http://dx.doi.org/10.4236/ojas.2013.34046

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; [email protected]

Received 25 August 2013; revised 28 September 2013; accepted 14 October 2013

which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

ABSTRACT

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

1. INTRODUCTION

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.

2. MATERIALS AND METHODS

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-

E. M. Barrows / Open Journal of Animal Sciences 3 (20 13) 311-313

312

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.

3. RESULTS AND DISCUSSION

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)

Year

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

August

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

September

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

September

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).

E. M. Barrows / Open Journal of Animal Sciences 3 (20 13) 311-313 313

4. ACKNOWLEDGMENTS

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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