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145
Feeding preferences of the willow sawfl y Nematus
oligospilus (Hymenoptera: Tenthredinidae) for
commercial Salix clones
Recibido: 27-I-2012; aceptado: 10-V-2012
ISSN 0373-5680 (impresa), ISSN 1851-7471 (en línea) Rev. Soc. Entomol. Argent. 71 (1-2): 145-150, 2012
LOETTI, Verónica, Alejandra RUBIO y María Isabel BELLOCQ
Departamento de Ecología, Genética y Evolución, Facultad de Ciencias Exactas y Naturales,
Universidad de Buenos Aires – Ciudad Universitaria, Pabellón II, 4to. piso, Ciudad
Autónoma de Buenos Aires (C1428EHA), Argentina; e-mail: vloetti@ege.fcen.uba.ar
Preferencias alimentarias de la avispa sierra de los sauces
Nematus oligospilus (Hymenoptera: Tenthredinidae) por
clones de Salix comerciales
RESUMEN. Nematus oligospilus Förster (Hymenoptera:
Tenthredinidae), o la avispa sierra de los sauces, es nativa del Hemisferio
Norte y se ha convertido en un serio defoliador en plantaciones de sauces
(Salix spp.) del Hemisferio Sur, después de su introducción a principios de
1980. Los estudios sobre las preferencias de hospedador aportan información
útil para el desarrollo de estrategias, donde la avispa sierra puede producir
daño a los árboles y pérdidas económicas. Se evaluó la preferencia alimentaria
de las larvas de N. oligospilus, mediante ensayos de laboratorio; se ofrecieron
en forma simultánea hojas de cuatro clones de sauce, usados comúnmente
en plantaciones comerciales en Argentina (Salix babylonica var sacramenta
Hortus, Salix nigra Marsch., S. babylonica L. x Salix alba L. 131-27 and
Salix matsudana Koidz. x S. alba L. 13-44). Las larvas de N. oligospilus se
alimentaron de las hojas de los cuatro clones. Sin embargo, consumieron
una proporción significativamente mayor de las hojas de S. babylonica var
sacramenta. Estos resultados indican que todos los clones utilizados en los
ensayos fueron palatables para el insecto y que S. babylonica var sacramenta
es el hospedador preferido para la herbivoría de las larvas.
PALABRAS CLAVE. Defoliación. Herbivoría. Salicaceae. Avispa sierra.
ABSTRACT. Nematus oligospilus Förster (Hymenoptera:
Tenthredinidae) is a willow sawfly native to the Northern Hemisphere
which became a serious defoliator in willow plantations (Salix spp.) of the
Southern Hemisphere after being introduced in the early 1980´s. Studies
on host preferences provide useful information for the development of pest
management strategies where the willow sawfly may produce tree damage and
economic loss. Feeding preferences of N. oligospilus larvae were evaluated in
laboratory trials by simultaneously offering leaves from four willow tree clones
commonly used in commercial plantations in Argentina (Salix babylonica var
sacramenta Hortus, Salix nigra Marsch., S. babylonica L. x Salix alba L. 131-
27 and Salix matsudana Koidz. x S. alba L. 13-44). Larvae of N. oligospilus
fed on leaves from the four clones. However, insects consumed a significantly
higher proportion of S. babylonica var sacramenta leaves than of leaves
Rev. Soc. Entomol. Argent. 71 (1-2):145-150, 2012
146
from the other clones. Results indicate that all clones used in the trials were
palatable to the insect, and that S. babylonica var sacramenta is the preferred
host for larval herbivory.
KEY WORDS. Defoliation. Herbivory. Salicaceae. Willow sawfly.
INTRODUCTION
Nematus oligospilus is a willow sawfly
native to the Northern Hemisphere, and it
was introduced into South America in the
early 1980s (Koch & Smith, 2000). It was
first detected in Argentina in 1981 (Smith,
1983) and then spread into Chile where
it was reported in 1986 (González et al.,
1986). Later, it was found in South Africa
(Urban & Earldley, 1995), New Zealand
(Berry, 1997) and more recently in Australia
(Bruzzese & McFadyen, 2006). The larva of
Nematus oligospilus Förster (Hymenoptera:
Tenthredinidae) feeds on leaves of several
species of the Salicaceae family, especially
on willows (Salix spp.) used for commercial
plantations.
Nematus oligospilus is a common willow-
feeding sawfly in the Holarctic Region
(Koch & Smith, 2000). It has low impact on
commercial plantations within its original
distributional range, where it occurs in
relatively low population density (Carr et al.,
1998). In the Southern Hemisphere, however,
outbreaks of N. oligospilus can completely
defoliate (and even kill) trees (Dapoto &
Giganti, 1994; Urban & Earldley, 1995; Ede,
2009). The high reproductive and dispersal
capability of this species has been partially
attributed to thelytokous parthenogenetic
reproduction, favorable environmental
conditions, and the occurrence of extensive
monoculture plantations of exotic Salicaceae
(Koch & Smith, 2000).
Poplars (Populus spp.) and willows are
important resources for pulp production and
non-wood products such as fodder and fuel.
Furthermore, they are used for environmental
purposes such as soil and water protection,
windbreaks, riparian buffers, and
phytoremediation (e.g. Dimitriou & Aronsson,
2005; Marchand & Masse, 2007). The global
area of planted Salicaceae is approximately
7 million hectares, and China, India, France,
Turkey, Italy and Argentine hold the most
extensive areas of planted forests (Ball et al.,
2005). In Argentina, most willow and poplar
plantations are located in the wetlands of
the Lower Delta of the Paraná River (FAO
2001), covering approximately 58000 ha
(Bodorowski, 2006). Approximately 90% of
the planted willows are used for paper and
chipboards. Nematus oligospilus was first
detected in the region in the mid-1980’s
(Toscani et al., 1992) where it caused up to
60% loss in timber production (Cabarcos,
1995), and it is currently considered a forest
pest accounting for significant economic loss
(Giménez, 2006; Dapoto et al., 2011). The
reproductive strategy (Koch & Smith, 2000),
wide host range, favorable environmental
conditions and the presence of extensive
monocultures of Salicaceae, likely influence
the success of N. oligospilus populations in
the Lower Delta of the Paraná River.
Salicaceae species may show different
susceptibility to N. oligospilus infestation.
Previous reports indicate that poplars were
less infested than willows (Dapoto & Giganti,
1994; Urban & Earldley, 1995; Ede, 2009),
and that willow shrubs were less infested
than willow trees (Charles et al., 1998: Finlay
& Adair, 2006). Although some studies have
pointed out the relevance of determining N.
oligospilus preference for oviposition (e.g.
Charles et al., 1998) or feeding (e.g. Urban
& Eardley, 1995), no study has explored
experimentally feeding preferences by
willow species or hybrids. Such information
may contribute to a better understanding
of plant-insect interaction, given that the
existence of food preference could provide
information on the sawfly´s ability to cause
damage to commercial willow plantations,
and help to decide the most appropriate
LOETTI, V. et al. Feeding preferences of Nematus oligospilus 147
clone to grow. Thus, using N. oligospilus
larvae we conducted food preference trials
by simultaneously offering leaves from
four willow tree clones commonly used in
commercial plantations in Argentina.
MATERIAL AND METHODS
We conducted food preference trials
involving N. oligospilus larvae and leaves
from four willow clones. Larvae and leaves
used in the trial were collected from
commercial willow plantations, located in
the Lower Delta of the Paraná River, Buenos
Aires province, Argentina (33º 55’S – 58º
59’W). The area has temperate and subhumid
climate; mean annual temperature range
16.7 °C – 18 °C and annual precipitation is
1000 mm (Malvárez, 1999). The Lower Delta
of the Paraná River is a freshwater wetland
showing high environmental heterogeneity
and biological diversity (Kandus et al., 2006);
part of the region is a UNESCO Biosphere
Reserve since 2000. All willows and poplars
species, varieties and hybrids cultivated
in commercial plantations are reproduced
asexually (clonally) by ramets.
To collect larvae for the experiment,
branches from infested trees were clipped,
placed in glass jars containing a piece of wet
cotton, and brought to the laboratory. Third-
and fourth-stage larvae of N. oligospilus
showing adequate activity after six hours of
starvation were selected for the trial. Larval
activity was considered adequate when it
responded immediately to stimulation with
a fine brush. Leaves for the experiment
were also collected by branch clipping in
commercial plantations from four of the most
commonly used clones in the study area:
Salix babylonica var sacramenta Hortus,
Salix nigra Marsch., S. babylonica L. x Salix
alba L. 131-27 and Salix matsudana Koidz.
x S. alba L. 13-44 (Cerrillo, 2009). Branches
from each clone were labeled and wrapped
in plastic bags with wet cotton to avoid
dehydration. Leaves selected for the trial
were of similar size, with intact leaf laminae
and without visual signs of dehydration.
Traditional cafeteria trials were conducted
in the laboratory under ambient temperature
and photoperiod (approximately 24 ± 4 ºC and
14L: 10D). A trial consisted of simultaneously
offering one leaf of each clone to six larvae
over a 48-hour period. To conduct the trial,
leaves and larvae were placed in a 250 cc-
glass jar (6 cm in diameter and 13 cm in
height) containing a piece of wet cotton to
avoid dehydration. Thus, the jar contained
four leaves of similar size, one from each
clone and six larvae of N. oligospilus. Leaves
were arranged equidistant to each other, and
each larva was used for a single trial and
then discarded. The location of leaves in
each trial was recorded in each jar. The trial
was replicated 12 times.
Leaf margins were drawn prior to the trial
and 48 hours after the trial started, and leaf
areas were estimated using Image-Pro Plus
4.5. The percentage of leaf area consumed
by larvae after 48 hours was calculated by
subtracting the final to the initial leaf area.
Typically, food preferences are quantified
by comparing food consumption and
availability using a variety of possible
indices (see Krebs, 1999). In our study, we
offered a similar amount of each food type
by using similar initial leaf area for all willow
clones. Thus, we estimated preferences by
comparing the mean percentages of leaf
area consumed using a one-way ANOVA
with willow clone as the factor, followed by
Tukey’s post-hoc pair comparisons. By using
ANOVA, we tested for significant differences
in the consumption of willow clones. Prior
to statistical analysis, percentage data were
arcsine-square-root transformed to meet the
ANOVA assumptions.
RESULTS
Prior to the trials, the leaf area offered
to larvae was similar among willow clones
(F3, 44 = 1.44, p = 0.24). During the trials,
larvae of N. oligospilus fed on leaves from
the four clones used (Fig. 1), indicating that
all clones used here were palatable to the
insect. However, mean percentage leaf area
consumed during the trial differed among
clones indicating food preference (F3, 44 =
Rev. Soc. Entomol. Argent. 71 (1-2):145-150, 2012
148
Fig. 1. Mean percentage leaf area (± ES) consumed by larvae of Nematus oligospilus of the four willow
clones included in the food preference trial: S. ba = Salix babylonica var sacramenta, S. ba x S. al = S.
babylonica x Salix alba 131-27, S. mat x S. al = Salix matsudana x S. alba 13-44, and S. ni = Salix nigra.
Different letters indicate significant differences (Tukey’s test, P < 0.05).
5.92, p < 0.05). Post-hoc pair comparisons
showed significantly higher consumption of
S. babylonica var sacramenta leaves than
of leaves from the other clones (p < 0.05).
Larvae fed on similar proportions of Salix
nigra Marsch., S. babylonica L. x Salix alba
L. 131-27 and Salix matsudana Koidz. x S.
alba L. 13-44.
DISCUSSION
Our results are consistent with previous
studies examining N. oligospilus feeding
habits and preferences in other regions
where the species has been introduced. In
our trials, N. oligospilus showed a flexible
use of host willows. Similar results were
found in other laboratory studies where
the hatched larvae successfully developed
to adult by feeding on a variety of willow
species such as Salix lasiolepis in USA (Carr
et al., 1998), S. babylonica, S. fragilis and S.
mucronata in South Africa (Urban & Earldley,
1995), and on 22 willow species/hybrids and
two poplar species (Charles & Allan, 2000;
Charles et al., 2005) commonly planted in
New Zealand. Thus, diet of N. oligospilus
could be categorized as specialist in Salix,
but including a wide range of willow and
some poplar species.
Although N. oligospilus feeds on a
variety of willow species, it did preferred S.
babylonica var sacramenta. Similar to our
findings, Roininen and Tahvanainen (1989)
conducted choice experiments using larvae
of two willow sawflies (Nematus pavidus
Serville and Nematus salicis (Linnaeus))
common in Finland, and found that N.
pavidus fed on all willow species offered but
preferred Salix cv. aquatica and S. viminalis,
whereas N. salicis preferred S. fragilis and
scarcely fed on the other willow species; the
authors speculated that host flexibility would
lead N. pavidus to a more effective use of
those food resources that are only marginally
exploited, potentially favoring the inclusion of
new hosts. According to Charles et al. (2005),
although some host tree species were more
favorable (e.g. S. alba, S. glaucophylloides,
LOETTI, V. et al. Feeding preferences of Nematus oligospilus 149
S. nigra and S. viminalis) than others (e.g.
S. babylonica, S. exigua and S. lasiolepis),
all hosts allowed larvae to complete their
development to the adult stage. Similarly,
different levels of damage in a wide variety of
willow species were observed in plantations
after severe defoliations caused by willow
sawfly outbreaks (Dapoto & Giganti, 1994;
Ede, 2009). In the Argentine Patagonia (Río
Negro and Neuquén provinces), leaves of S.
babylonica L. emerge early in the growing
season and are the first to be consumed
by N. oligospilus; however, other willow
species may become infested as the growing
season progresses, i.e., the sawfly can even
attack poplars when willow foliage is scarce
(Dapoto & Giganti, 1994). Therefore, N.
oligospilus has a flexible use of host, and the
Salicaceae species attacked seemed to be
related to the availability of willow species
in the region and throughout the season.
CONCLUSION
The results of this study showed that
larvae of N. oligospilus preferred feeding
on S. babylonica var sacramenta leaves
over S. babylonica x Salix alba 131-27,
Salix matsudana x S. alba 13-44 and Salix
nigra. This information should be considered
in the development of pest management
strategies in willow plantations, through the
development of resistant Salix phenotypes
or genotypes where the willow sawfly may
produce tree damage and economic loss,
in the context of both tree production and
environmental purposes.
ACKNOWLEDGMENTS
We acknowledge Faplac S.A. and Papel
Prensa S.A. for providing financial and
logistic support.
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