The effect of three species of Eucalyptus on growth and fecundity of the Eucalyptus snout beetle (Gonipterus scutellatus)

Abstract

The Eucalyptus snout beetle, Gonipterus scutellatus , was first detected in NW Spain in 1991, in the area with the largest European eucalypt plantations. Feeding preferences in the field and the effect of three species of Eucalyptus on larval development, survival, and adult fertility were studied. It was estimated that individuals of G. scutellatus consume 1.2–1.7 g of fresh biomass in Eucalyptus cinerea and E. globulus during their development. Diet had a significant effect on larval survival and rate of development, the least suitable tree species being E. obliqua . Nevertheless, females fed with these eucalypt species or with an alternated diet containing all three eucalypts, did not produce significantly different numbers of larvae. In the field, G. scutellatus showed a marked preference for E. globulus, E. longifolia, E. grandis and E. propinqua , and completely avoided other species.

Full text

Introduction
Eucalypt plantations are widespread in temperate
and tropical regions (Anonymous, 1997) because
they grow at a high rate even on poor soils,
making them highly productive. One of the
reasons for the high productivity of eucalypt
plantations in Europe is the absence of significant
herbivore losses, especially by insects. Neverthe-
less in their country of origin, eucalypts are dra-
matically affected by herbivore insects (Morrow,
1977; Fox and Morrow, 1983), and in many parts
of Australia, there is currently a generalized
dieback phenomenon due to repetitive insect
attacks (Lowman and Heatwole, 1992).
In Spain eucalypts are basically cultivated to
produce paper. Two species are widespread: E.
globulus Labill. (325 000 ha) and E. camaldulen-
sis Dehnh. (175 000 ha; Montoya, 1995). In
Galicia (NW Spain), monocultures of E. globulus
are a basic resource in areas where agriculture is
not profitable. There is a growing concern about
The effect of three species of
Eucalyptus
on growth and fecundity
of the
Eucalyptus
snout beetle
(
Gonipterus scutellatus
)
A. CORDERO RIVERA AND S. SANTOLAMAZZA CARBONE
Departmento de Ecoloxía e Bioloxía Animal, Universidade de Vigo, EUET Forestal, Campus Universitario,
36005 Pontevedra, Spain
Summary
The Eucalyptus snout beetle, Gonipterus scutellatus, was first detected in NW Spain in 1991, in the
area with the largest European eucalypt plantations. Feeding preferences in the field and the effect of
three species of Eucalyptus on larval development, survival, and adult fertility were studied. It was
estimated that individuals of G. scutellatus consume 1.2–1.7 g of fresh biomass in Eucalyptus
cinerea and E. globulus during their development. Diet had a significant effect on larval survival and
rate of development, the least suitable tree species being E. obliqua. Nevertheless, females fed with
these eucalypt species or with an alternated diet containing all three eucalypts, did not produce
significantly different numbers of larvae. In the field, G. scutellatus showed a marked preference for
E. globulus, E. longifolia, E. grandis and E. propinqua, and completely avoided other species.
© Institute of Chartered Foresters, 2000 Forestry, Vol. 73, No. 1, 2000
E-mail: acordero@uvigo.es
02 Cordero Rivera (jl/k) 5/1/00 1:31 pm Page 21

the ecological effects of eucalypt plantations
(Anonymous, 1997), mainly due to the losses of
biodiversity because there are almost no herbivore
insects feeding on E. globulus in the Iberian penin-
sula, which means that higher trophic levels are
also scarce (Paiva, 1992). Productivity in these
stands may reach a maximum of 60 m3ha21a21,
but the mean productivity is 20 m3ha21a21(Cal-
vo de Anta, 1992).
In theory, monocultures are intrinsically highly
susceptible to insect outbreaks. Three Australian
insect species have colonized NW Spain, but only
the Eucalyptus snout beetle, Gonipterus scutella-
tus Gyllenhal, has produced significant damage.
Outside Australia, G. scutellatus was first found
in South Africa in 1916 (Tooke, 1955), but it is
now widespread in Africa, America and Europe
(Mansilla and Pérez Otero, 1996). Damage to
eucalypts by G. scutellatus in South Africa is so
intense that its presence is a factor limiting euca-
lypt planting in some localities (Lowman and
Heatwole, 1992). Biological control of this
species by means of the importation of Anaphes
nitens (Hym. Mymaridae), an egg parasitoid, has
been highly successful in many areas (Tooke,
1955; Kidd and Jervis, 1997).
G. scutellatus was first found in 1991 in NW
Spain (Mansilla, 1992). It is now widespread and
causing severe defoliation to eucalypt stands. In
South Africa, susceptible varieties of E. viminalis
Labill. begin to be attacked when the tree is about
2 years old and at 60 months of age the average
height of these varieties is only 3.4 m, when the
resistant varieties have reached 7.12 m (Richard-
son and Meakins, 1986). This is a clear example
of the considerable losses that the attack of G.
scutellatus can cause in susceptible species.
G. scutellatus is a generalist herbivore of Euca-
lyptus spp. In Australia its favorite food plant is E.
viminalis (Tooke, 1955). In spite of this special-
ization, it has been recorded feeding on apple in
New Zealand and Australia (Moutia and Vinson,
1945; Frappa, 1950; Tooke, 1955). Where the bio-
logical control of G. scutellatus is unsuccessful, the
alternative is to use resistant host plant species.
Nevertheless, there are no studies on the effects of
different species of Eucalyptus on success of this
herbivore. The first aim of this study was to
measure the amount of biomass consumption and
the effect of three species of Eucalyptus on growth
and fecundity of G. scutellatus. Specific tests were
carried out to find if a varied diet could improve
female fecundity. If that were the case, a mixture
of different species in the same plot should be
avoided. Tests were also done to assess if this
species of beetle can use apple leaves as food.
Finally, the degree of attack by G. scutellatus was
measured on 20 species of Eucalyptus, to obtain
information on the resistant species that might be
used in highly attacked areas.
Methods
Feeding experiments
Three species of Eucalyptus were used in the
experiments: E. globulus, E. obliqua L’Herit. and
E. cinerea F. Muell. ex Benth., respectively a
frequently attacked species (and dominant in
Galician plantations), one rarely attacked and
one not attacked in South Africa (Tooke, 1935).
To determine if G. scutellatus can feed on apple,
apple leaves were provided to a group of 50 newly
hatched larvae. All experiments were carried out
in a climatic room at 21°C during 14 hours of
light and 19°C during 10 hours of darkness. The
humidity was 80 per cent. The first experiment
was carried out to evaluate the biomass con-
sumed during larval development. Three groups
of 16 newly hatched larvae each were raised on
leaves of the three eucalypt species. Offered leaves
were weighed to the nearest 0.1 mg, by means of
an electrobalance (Denver XE Series Model 50).
During the first week of life, larvae were main-
tained in pairs, in Petri dishes 5 cm in diameter.
The consumption obtained in this way was
divided by two to estimate the consumption per
larva. When larvae reached the second instar, they
were isolated in Petri dishes (9 cm diameter), to
evaluate individual larval consumption. Leaves
were carefully selected, to obtain two groups with
similar weight and surface area. Of these groups,
one leaf was offered as food to the larvae and the
other was used as a control to estimate the loss of
weight of the leaves. Fresh biomass consumption
was estimated by calculating the difference
between the initial weight and the final weight of
the leaf after 24 hours. This weight was corrected
by multiplying this value by the ratio between the
final and initial weights of control leaves.
The second experiment was carried out to
evaluate the effect of eucalypt species on larval
development and survival and adult weight. It
22 FORESTRY
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was carried out with 300 newly hatched larvae.
All the larvae emerged on the same day and were
randomly assigned to three treatments with two
replicates. Each group (50 larvae) received fresh
leaves of E. globulus, E. cinerea or E. obliqua
every 2 days. Larvae were maintained in 1-litre
plastic boxes until pupation. They were weighed
individually every 4 days. For pupation, larvae
were transferred to similar containers with 5 cm
of soil. Adults were weighed and sexed on the day
of their emergence (about 1 month later).
The third experiment was carried out to evalu-
ate the effect of eucalypt species on fecundity and
fertility of the female beetles. Adult G. scutellatus
were collected on 22 February 1997 near Pon-
tevedra (42°279N 8°369W) when they were start-
ing their reproductive activity. Adults were
maintained together and the spontaneously
formed pairs were randomly assigned to four
treatments (10 pairs per treatment). Three treat-
ments received fresh leaves of E. globulus, E.
obliqua or E. cinerea every 2 days. The fourth
treatment received one of the previous species
every other day, in an alternate way. Each pair was
kept in isolation in a tube 6 cm high by 3.5 cm
diameter. Females were maintained with the male
that they spontaneously accepted, since there is
experimental evidence that females of some
species can lay more or fewer eggs depending on
the characteristics of their mate (Eberhard, 1996).
Counts were made of the number of egg masses
laid each day and the number of emerged larvae
for 1 month (or until the death of the female).
Feeding preferences in the field
Feeding preferences were estimated in the eucalypt
plots (established in 1955) at the Center of Forest
Research of Lourizán (Pontevedra), at the end of
the period of reproduction of G. scutellatus in the
spring of 1996. To evaluate the preferences of G.
scutellatus for different species of eucalypts, at
least three trees per species were examined using
binoculars. Damage by G. scutellatus is very
characteristic and is concentrated on the terminal
buds. An arbitrary scale was used, varying from 0
(absence of damage) to 5 (all the terminal branches
severely attacked). If G. scutellatus has any prefer-
ence for certain species it has the opportunity to
choose among them in the same locality.
Results
Experiment 1. Leaf consumption in three species
of Eucalyptus
Figure 1 shows the daily biomass consumed by
the larvae. Results are similar for E. globulus and
EFFECT OF EUCALYPTUS SPECIES ON EUCALYPTUS SNOUT BEETLE 23
Figure 1. Daily biomass consumption by Gonipterus scutellatus larvae on three Eucalyptus species. The
minumum on days 16 and 17 for E. obliqua was due to a temporary unavailability of fresh leaves. Mean
± s.e.
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E. cinerea, with a daily consumption around
0.10–0.15 g for mature larvae. Consumption was
lower towards the end of the feeding period for
E. globulus because some larvae reached the
prepupa and ceased feeding. By contrast, for E.
obliqua, leaf consumption in the first instar larvae
was very low and increased when the surviving
larvae reached the second instar. Daily consump-
tion remained around 0.2 g, with the exception of
days 15 and 16, due to the unavailability of fresh
leaves.
Throughout their development, larvae of G.
scutellatus consumed a quantity of biomass that
varied between 1.0 and 4.6 g (Table 1). Con-
sumption was significantly different between tree
species (ANOVA, F= 209.34 d.f. = 2, P< 0.001),
being specially high E. obliqua (test Scheffé, P<
0.05). This species had low palatability, so that it
was impossible to rear the 16 larvae initially used
for the experiment (see also below). No differ-
ences were detected in the consumption of leaves
of E. globulus and E. cinerea.
Experiment 2. Effect of food on the
development of G. scutellatus
Different feeding treatments had very significant
effects on survival probability (Figure 2). Larvae
fed with E. obliqua suffered a higher mortality
throughout the duration of the experiment. The
groups fed with E. globulus and E. cinerea fol-
lowed a very similar pattern, with the exception
of the emergence of the adults, where one of the
groups fed with E. globulus suffered a very high
mortality (whose cause was unknown) during the
pupal phase.
24 FORESTRY
Table 1: Leaf consumption estimates during larval development of Gonipterus scutellatus in three eucalypt
species
Species Leaf consumption (g) Minimum Maximum
globulus 1.73 ± 0.07 (16) 1.33 2.41
cinerea 1.24 ± 0.03 (16) 1.01 1.55
obliqua 3.49 ± 0.17 (11) 2.65 4.63
Figure 2. The effect of diet on the survival of Gonipterus scutellatus fed on three species of Eucalyptus. Mean
± s.e.
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The growth curve is presented in Figure 3. It is
clear that larvae fed with E. obliqua grew more
slowly than those fed on the other two species.
The effects of eucalypt species on the weight of
larvae was significant from an early stage (3 days
of age) and was manifested clearly in the mature
larvae (ANOVA, P< 0.05 at all the ages). Signifi-
cant differences between replicates were found in
almost all the cases, and there was a food-repli-
cate interaction in some ages of larvae.
Taking into account the effect of sex on adult
weight (females are heavier than males), the
weights of the adults were also affected by the
food species (ANOVA, food, F= 3.502, d.f. = 2,
P= 0.034; replicate, F= 0.787, d.f. = 1, P= 0.377;
sex, F= 34.477, d.f. = 1, P< 0.0001; no inter-
action was significant). In this case the few adults
reared on E. obliqua weighed more than the other
treatments (Figure 3).
Experiment 3 Effect of food on fecundity and
fertility of female beetles
Significant differences were detected among treat-
ments in the number of egg masses laid (F= 4.85,
d.f. = 3, P= 0.006). The average values (± s.e.)
were 31.8 ± 1.8 egg masses (1.1 egg masses per
day) for the females fed with E. obliqua, 18.8 ±
3.3 (0.7 per day) for E. globulus, 22.1 ± 2.2 (0.8
per day) for E. cinerea and 22.8 ± 2.6 (0.8 per
day) for those that received the alternated food.
Nevertheless, the numbers of larvae that hatched
were not significantly different between treat-
ments: 186.1 ± 25.9 for E. obliqua, 112.1 ± 25.6
for E. globulus, 123.9 ± 17.6 for E. cinerea and
122.2 ± 20.8 for alternated food (F= 2.20, P=
0.105).
The fertility of the females tended to diminish
with increasing age (Figure 4; correlation between
age and number of larvae produced: E. globulus
Spearman r= 20.31, P= 0.098; E. cinerea, rs=
20.78, P< 0.001; alternate, rs= 20.58, P=
0.001), except for the group of females fed with
E. obliqua that increased their fertility around 20
days of age (rs= 0.03, P= 0.896).
Feeding preferences
The tests with young apple leaves indicated that
G. scutellatus cannot feed on this species of tree:
all the larvae died in 2–3 days, although they
tasted the offered leaves. Table 2 presents the
attack scores observed in different species of
Eucalyptus in the plots at the Center of Forest
Research of Lourizán (Pontevedra). The results
indicate that besides E. globulus, G. scutellatus
EFFECT OF EUCALYPTUS SPECIES ON EUCALYPTUS SNOUT BEETLE 25
Figure 3. The effect of feeding on three species of Eucalyptus on the growth rate of Gonipterus scutellatus.
Note that E. obliqua was clearly the worst diet. Mean ± s.e.
02 Cordero Rivera (jl/k) 5/1/00 1:31 pm Page 25

attacks preferably E. longifolia Link and Otto, E.
grandis Hill ex Maiden, E. propinqua Deane and
Maiden, E. ovata Labill., E. citriodora Hook. E.
viminalis and E. pauciflora Sieber ex Sprengel.
Samplings carried out in mixed plots agree
with the data presented above. G. scutellatus
clearly prefers to attack E. globulus rather than
E. obliqua in plots located in Lourizán, where
both species grow in mixture (Table 3). In
Folgoso (Forcarei), G. scutellatus showed again a
marked preference for E. globulus (Table 3). In
this locality, the density of G. scutellatus in E.
globulus reached a maximum of 78 egg masses
and more than 600 larvae in a tree of about 4 m
height, but only two attacked trees of E. camald-
ulensis were found among 50 examined. E.
camaldulensis has low palatability for G. scutel-
latus, but surprisingly not for European insect
fauna, since abundant geometrid larvae were
found feeding on this species.
Discussion
In the different countries where it has spread, G.
scutellatus has shown marked preferences for
several species of eucalypt. Tooke (1935, 1955)
presented a list of four groups of species in rela-
tion to the degree of attack observed in South
Africa. On the island of Mauritius E. robusta
Smith, E. tereticornis Smith and E. kirtoniana
Maiden are cited as the most susceptible (Williams
et al., 1951). In Kenya, the attacked species were
E. globulus, E. maidenia F. Muell., E. robusta and
E. smithii R.T. Baker, while E.saligna Smith and
26 FORESTRY
Figure 4. The effect of feeding on single Eucalyptus species or alternating species on fecundity of Gonipterus
scutellatus females. Age refers to days since the start of the experiment.
Table 2: The attack degree by Gonipterus scutellatus
on trees at the eucalypt plots of the Forest Research
Center of Lourizán (NW Spain) in May 1997. The
scale is arbitrary, from 5 (maximum attack) to 0 (no
attack). Mean ± s.e. (n)
Species Attack degree
globulus 5.0 ± 0.0 (3)
longifolia 5.0 ± 0.0 (3)
grandis 4.0 ± 0.0 (3)
propinqua 3.7 ± 0.3 (3)
ovata 2.7 ± 0.3 (3)
citriodora 2.5 ± 0.5 (2)
viminalis 2.0 ± 0.0 (3)
pauciflora 1.7 ± 0.7 (3)
microcorys 1.0 ± 0.8 (3)
rubida 0.8 ± 0.8 (4)
alpina 0.7 ± 0.7 (3)
dives 0.3 ± 0.3 (3)
pilularis 0.3 ± 0.3 (3)
saligna 0.3 ± 0.3 (3)
cornuta 0.0 ± 0.0 (3)
fastigata 0.0 ± 0.0 (3)
ficifolia 0.0 ± 0.0 (2)
niphoploia 0.0 ± 0.0 (3)
obliqua 0.0 ± 0.0 (2)
amygdalina 0.0 ± 0.0 (3)
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E. citriodora were cited as practically immune
(Kevan, 1946). In Madagascar the most suscepti-
ble were E. cornuta, E. viminalis, E. punctata, E.
globulus, E. urnigera and E. rostrata (= E. camal-
dulensis) (Frappa, 1950). In Italy G. scutellatus
showed a clear preference for the leaves of E. glob-
ulus, not attacking the plants of E. cinerea, E.
gunnii Hook. f., E. polyanthemos Schauer, E. stu-
artiana and E. rostrata (Arzone and Meotto,
1978). Finally in Galicia only damages to E. glob-
ulus and E. obliqua have been reported (Mansilla
and Pérez Otero, 1996). Comparing the present
results with the literature it clearly emerges that
the species most cultivated in Galicia, E. globulus,
is one of the favorite species of G. scutellatus.
Nevertheless, the choice of eucalypt species by G.
scutellatus clearly depends on availability, as the
results of Clarke et al. (1998) indicate. They found
that in Tasmania, G. scutellatus avoids E. globu-
lus and E. viminalis, laying most eggs in E. pul-
chella Desf.
Among the species with the highest attacks in
Lourizán, all except E. grandis and E. citriodora
are mentioned by Tooke (1935) as highly suscep-
tible. E. grandis does not appear in Tooke’s list.
The difference in the susceptibility of E. citri-
odora may be due to varietal differences, as was
found for E. viminalis (Richardson and Meakins,
1986). On the other hand, the species least
attacked in Lourizán are also those mentioned by
Tooke (1935) as not very susceptible. The present
evaluation of the damage by G. scutellatus was
made without knowledge of Tooke’s list, and
therefore the results are independent. Clearly the
most susceptible species in South Africa and other
countries are the most susceptible in NW Spain.
Results of this study might therefore be used as a
guide to resistance in other countries.
It seems that G. scutellatus cannot survive on
apple leaves. The literature citations of this
species feeding on apples are probably due to
adults that may be found resting on different
species of plants, but they would not serve as a
substitute food.
Significant differences in the leaf consumption
by G. scutellatus were detected on the three
studied tree species. However, the method used to
estimate consumption is intrinsically subject to
high errors, due to the different morphologies and
densities of the leaves of the different species
(Waller and Jones, 1989). Results indicate that
the larvae of G. scutellatus consume approxi-
mately 11–15 times their final weight during their
development, when they are fed respectively with
E. cinerea or E. globulus, but the value rises to 27
times when the food is E. obliqua (the final weight
of the larvae fed with this species was significantly
smaller). Freitas (1991) fed the larvae of
Gonipterus gibberus with leaves of E. saligna, a
species lightly attacked according to Tooke
(1935). It was estimated that G. gibberus con-
sumed a total of about 12 times their weight,
using the method of Waldbauer (1962), that accu-
rately corrects for differences between the weight
of experimental and control leaves. The value
found in the present experiments with G. scutel-
latus fed with E. globulus and E. cinerea is there-
fore very similar, which suggests that the method
was acceptable. When the density of G. scutella-
tus larvae is very high, they can produce almost
complete defoliation of trees (one tree with 300
larvae can lose more than 500 g of fresh leaves).
Eucalyptus globulus is nevertheless highly resis-
tant to insect damage, because 50 per cent de-
foliation in spring or summer, but not in autumn,
does not significantly reduce initial height growth
EFFECT OF EUCALYPTUS SPECIES ON EUCALYPTUS SNOUT BEETLE 27
Table 3: The mean number of adults, egg masses and larvae of Gonipterus scutellatus, on different eucalypt
species. Trees were 1–4 m high
Species and locality Adults Egg masses Larvae
Lourizán, April 1996
globulus (n= 60) 0.4 ± 0.11 22.0 ± 2.83 1.7 ± 0.31
obliqua (n= 60) 0.05 ± 0.03 8.2 ± 1.76 0.2 ± 0.09
Forcarei, April 1997
globulus (n= 50) 0.4 ± 0.05 22.0 ± 3.11 147.2 ± 20.82
camaldulensis (n= 50) 0.0 ± 0.00 0.6 ± 0.09 2.7 ± 0.38
viminalis (n= 25) 0.1 ± 0.06 4.7 ± 2.96 5.8 ± 3.18
02 Cordero Rivera (jl/k) 5/1/00 1:31 pm Page 27

(Abbott and Wills, 1996). There are no estimates
of productivity losses due to G. scutellatus on E.
globulus, but if it behaves similarly to E. mar-
ginata, more frequent low intensity defoliations
will reduce growth more than less frequent higher
intensity defoliations (Abbott et al., 1993).
The results of experiment 2 indicate that not all
tree species are suitable. E. cinerea that supposedly
was a non-attacked species (Tooke, 1935; Arzone
and Meotto, 1978) was similar to E. globulus in
its palatability. E. obliqua, a species that is used
rarely in the field (Table 3), is not appropriate for
larval development of G. scutellatus. Surprisingly,
the few adults that emerged from larvae fed with
this species were heavier than individuals fed with
E. globulus or E. cinerea. This suggests that even
less palatable species might be used by the insect if
the preferred species are absent.
It is possible that if G. scutellatus has several
eucalypt species in the same plot the fecundity of
the females increases due to a richer diet. The
results of this study suggest that the planting of
several species with different palatabilities does
not increase the fertility of G. scutellatus. Never-
theless, only three species were tested, and it is
still possible that some combinations of species
could produce the mentioned effect.
The surprising result obtained with E. obliqua
and adult fecundity contrasts with the results
mentioned for larval feeding. This might be due
to females fed with E. obliqua being younger,
because they did not reduce their fertility with age
(Figure 4). The low fecundity of females fed with
E. globulus could be due to the fact that they were
fed from buds collected from trees showing little
attack, and for that reason the least palatable
trees of this species could have been selected
unconsciously. This suggests that there is vari-
ability to the attack by G. scutellatus within E.
globulus stands.
Acknowledgements
Funding was provided by the Galician Government
(Xunta de Galicia research project XUGA37101A95).
We thank Rozimar de Campos Pereira and Jose A.
Andrés Abad for their help in laboratory work.
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Received 20 October 1998
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