Scientific Notes
529
NEW HOST PLANT RECORD FOR THE POISON IVY SAWFLY, ARGE
HUMERALIS (HYMENOPTERA: ARGIDAE), AND ITS PERFORMANCE ON
TWO HOST PLANT SPECIES
NICOLE BENDA1,*, JENNIFER POSSLEY2, DEVON POWELL2, CATHERINE BUCHANAN-MCGRATH3 AND JAMES CUDA1
1
Department of Entomology and Nematology, University of Florida, Gainesville, FL 32608
2
Center for Tropical Plant Conservation, Fairchild Tropical Botanic Garden, Coral Gables, FL 33156
3
Dept of Medicine, University of Florida, Gainesville, FL 32608
*Corresponding author; E-mail: nbenda@gmail.com
ABSTRACT
The poison ivy sawfly, Arge humeralis Beauvois, was previously known to feed only on poison ivy, Toxicodendron radicans (L.) Kuntze. However, in 2009, larvae were discovered in
southern Florida feeding on poisonwood, Metopium toxiferum (L.) Krug and Urban. To better
characterize the host range of A. humeralis, we compared larval performance on T. radicans
and M. toxiferum. Adults oviposited on both plant species, and days to pupation and survival
to adulthood were similar. With the exception of longer 1st and 4th stadia on T. radicans,
stadia were also similar. Most mortality occurred during pupation. The use of M. toxiferum
by A. humeralis indicates a wider range of hosts and of ecosystems than previously known
for this insect.
Key Words: Toxicodendron radicans, Poisonwood, Metopium toxiferum, Anacardiaceae, host
range, larval performance
RESUMEN
La unica hospedera previamente conocida de la avispa de sierra de la hiedra venenosa,
Arge humeralis Beauvois, era la hiedra venenosa, Toxicodendron radicans (L.) Kuntze. Sin
embargo, en 2009, las larvas fueron encontradas en el sur de Florida alimentándose en
poisonwood, Toxiferum metopium (L.) Krug y Urban. Para caracterizar mejor el rango de
los hospederos de A. humeralis, comparamos el rendimiento larval en T. radicans y M. toxiferum. Los adultos ovipositaron en las dos especies, y el tiempo para llegar a la etapa de
pupa y la sobrevivencia a la etapa adulta fueron similares. La mayoría de la mortalidad
ocurrió durante la pupación. Con la excepción de los estadios primero y cuarto mas largo en
T. radicans, los estadios tambien fueran similares. El uso de M. toxiferum por A. humeralis
representa un rango más amplio de hospederos y de ecosistemas que los reportados previamente para este insecto.
Translation provided by the authors.
The poison ivy sawfly, Arge humeralis (Beauvois) (Hymenoptera: Argidae) was first described
in 1809 and ranges throughout eastern North
America (Smith 1989; Palisot de Beauvois 1809).
Two plant species and one subspecies, all in the
genus Toxicodendron (Sapindales: Anacardiaceae), have been recorded as hosts: T. radicans (L.)
Kuntze (poison ivy), T. radicans (L.) Kuntze ssp.
eximium (Greene) Gillis, and T. vernix (L.) (poison sumac) (Smith 1989). Arge humeralis is the
only sawfly in the family Argidae known to feed
on Anacardiaceae. This sawfly has been considered for use in biological control of T. radicans in
Bermuda (Regas-Williams 1979), making its host
range of particular interest.
Although T. radicans occurs in a wide range of
disturbed habitats, A. humeralis has previously
been most common in open, disturbed cypress
swamps (Smith 1989). Eggs are inserted into the
leaf tissue along the leaf margin. Early instars feed
gregariously, whereas later instars are usually solitary. Herein, we report the first documented use by
A. humeralis of poisonwood, Metopium toxiferum
(L.) Krug and Urban (Sapindales: Anacardiaceae),
as a host plant. We also compare the ability of larvae to feed on T. radicans and M. toxiferum.
MATERIALS AND METHODS
New Host Plant Record
In May 2009, an infestation of A. humeralis
was discovered on M. toxiferum in the Ludlam
Pineland Preserve (Miami-Dade County, Florida)
530
Florida Entomologist 95(2)
by D. Powell and J. Possley. The preserve is a 4 ha
remnant of unaltered pine rockland forest comprised of Pinus elliottii Engelm. var. densa (Pinales: Pinaceae), some hardwoods, and a shrub layer of saw palmetto, Serenoa repens (Bartram) J.
K. Small (Arecales: Arecaceae). Due to prescribed
burns, T. radicans is present at very low densities
in the preserve. Metopium toxiferum is very common, but the sawfly infestation was concentrated
in the western 10% of the preserve.
A cohort of 18 larvae from this infestation was
sent to University of Florida, Gainesville on 22 Jun
2009 and reared in the laboratory on potted M. toxiferum in clear acrylic cylinders (20 cm diam × 60
cm high). The 8 males and 5 females that survived
to adulthood served as the founder population of a
laboratory colony. Females laid a total of 203 eggs,
79.8% of which hatched, beginning on 24 Jul 2009.
As before, these larvae were placed on potted M.
toxiferum. Plants and insects were maintained under a 15:9 h L:D photoperiod at 24.8 ± 1.0 °C and
62.4 ± 5.4% RH. Larvae that had pupated in the
soil at the base of the plant were collected and individually placed into 29.6 ml plastic cups with 15 ml
of moistened vermiculite until adult emergence.
Comparison of Larval Host Plants
Upon emergence, adults were provided potted
T. radicans or M. toxiferum in clear acrylic cylinders (20 cm diam × 60 cm high) for oviposition. Insects were provided Gatorade® (0.46 mg Na, 0.13
mg P, 0.04 g sucrose, and 0.02 g fructose per mL
water) (Perrone et al. 2005) in a 15 ml vial with a
cotton wick. When eggs were about to eclose, the
leaves containing eggs were removed and kept in
Petri dishes (1.5 cm diam × 9 cm high) on moistened filter paper.
Two cohorts of neonates (N = 30 per cohort)
were placed individually in Petri dishes with 2.5
cm leaf discs of T. radicans or M. toxiferum on
moistened filter paper. Leaf discs were replaced
when desiccated or consumed. Mature larvae
were transferred to 29.6 ml plastic cups with 15
ml of moistened vermiculite for pupation. The trial was conducted under a 14:10 L:D photoperiod
at 26.1 ± 0.0 °C and 70.0 ± 0.2% RH. The stadium
for each instar, age at pupation, and survival
were recorded for each larva. An analysis of variance was conducted to detect differences between
host plants in the stadium of each instar, larval
development time, and pupal stadium (SAS Institute 2010) after verifying variance homogeneity
and normality.
June 2012
D. Smith of the Department of Entomology, National Museum of Natural History, Smithsonian
Institution, Washington, DC. The mesonota of the
specimens were red with black spots on the lateral lobes. The coloration was on the paler end of
the range of variation for the species, as is typical
of Florida specimens (Smith 1989). Voucher specimens were deposited in the National Museum of
Natural History in Washington, DC and in the
Florida State Collection of Arthropods in Gainesville, Florida.
Comparison of Larval Host Plants
Females readily oviposited on both T. radicans
and M. toxiferum, and larval development was
complete in 17.9 ± 0.6 d (N = 26) and 17.8 ± 0.6
d (N = 22), respectively (Fig. 1, F1, 46 = 0.19985, P
= 0.6580). Most mortality occurred during pupation.
Larvae on T. radicans had a shorter 1st and
4th stadium (F1, 51 = 7.6768, P = 0.0078 and F1, 46
= 10.3022, P = 0.0024, respectively) than larvae
on M. toxiferum (Fig. 2). The other stadia were of
similar duration (P > 0.1).
The variances of the larval stadia, larval development time, and pupal stadium were homogeneous (Brown-Forsythe test, P > 0.08). Histograms
showed that the data sets are normally distributed, with one exception. The pupal stadium is
bimodal, so the data was stratified to reflect the
two distinct groups: a group that emerged within
18-77 d and another that emerged in 120-222 d.
Each group was analyzed by ANOVA. The insects
in the first group remained in the pupal stage
for 45.0 ± 7.2 d and 48.0 ± 13.7 d on T. radicans
and M. toxiferum, respectively (F1, 15 = 3.2154, P =
0.0931). The insects in the second group remained
in the pupal stage for 148.7 ± 7.8 d and 173.4 ±
10.1 d on T. radicans and M. toxiferum, respectively (F1, 8 = 0.0455, P = 0.8365).
RESULTS
New Host Plant Record
The sawflies collected in 2009 (N = 2) and
2011 (N = 7) were identified as A. humeralis by
Fig. 1. Percent survival of each life stage of Arge humeralis on 2 plant species (N = 30 larvae per plant species). The 6th instar is represented by females only, as
male A. humeralis pupate after 5 instars (Smith 1989).
Scientific Notes
Fig. 2. The mean stadium (± SE) for each instar of
Arge humeralis on 2 plant species (N = 30 larvae per
plant species). Only durations of completed instars are
included in the calculation of means, i.e., if a larva died
before molting to the next instar, that last duration was
not included. Bars with different letters above them are
significantly different within an instar, using analysis
of variance. The 6th instar is represented by females
only, as male A. humeralis pupate after 5 instars (Smith
1989)
CONCLUSIONS
Our results suggest that both T. radicans and
M. toxiferum are equally suitable food sources.
Total larval durations, stadia, and numbers of
emerged adults were similar on the two plant species, with the exception of the 1st and 4th stadia.
Additional fitness measures such as adult weight
and fecundity would be helpful to compare host
suitability of these 2 plant species. Research on
the mating and oviposition behavior of A. humeralis is also needed to understand how these behaviors might influence host selection.
The development time to pupation on T. radicans is consistent with that reported by RegasWilliams and Habeck (1979). The pattern observed of increasing stadia durations with each
successive instar on T. radicans is also consistent
with their report, although their values were
slightly larger. In contrast, pupal duration was
much shorter in the older study (10.3-10.6 d on T.
radicans), suggesting the insects entered a seasonal or laboratory-induced diapause in the current study. The insects in the 1979 study hatched
2 mos earlier in the year than most of ours.
Our discovery of A. humeralis feeding on M.
toxiferum in both 2009 and 2011 represents a
new host record and not an aberrant case. This
new record indicates a wider range of hosts and of
ecosystems than originally known for this insect.
Both T. radicans and M. toxiferum are found in
hammocks and other woodlands, but M. toxiferum
is also found as a shrub in coastal sand dunes
(Small 1933) and generally in drier habitats than
T. radicans. It is unknown how long M. toxiferum
has been used a host, but the native ranges of A.
humeralis (Taeger 2010) and both plant species
(Wunderlin 1998; Tomlinson 1980) all overlap in
531
southern Florida. Use of M. toxiferum as a host
expands the possible native range of A. humeralis to the Caribbean, where this sawfly was once
considered for release as a biocontrol agent of the
introduced T. radicans (Regas-Williams and Habeck 1979).
Both T. radicans and M. toxiferum are in the
Anacardioideae subfamily of the Anacardiaceae
(Mitchell 2006). Both contain oily organic compounds called urushiols (also known as pentadecylcatechols) which can cause contact dermatitis
in humans (Pell 2011). Further investigation into
the chemistry of these plants and other anacards
may help explain the hosts used by A. humeralis,
and predict other potential or unnoticed hosts.
ACKNOWLEDGMENTS
We thank David R. Smith for identification assistance, and Devin Donohue, Judy Gillmore, and Corey
Brutman for laboratory assistance. We also thank the
Miami-Dade County’s Natural Areas Management Division, Environmentally Endangered Lands Program,
and Ludlam Pineland Preserve for their cooperation
and permission to collect insect and plant material used
in the study.
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