ARTHROPOD BIOLOGY
Head Capsule Width and Instar Determination for Larvae of
Streblote panda (Lepidoptera: Lasiocampidae)
D. CALVO1,2
AND
J. MA. MOLINA2
Ann. Entomol. Soc. Am. 101(5): 881Ð886 (2008)
ABSTRACT The moth Streblote panda Hübner (1820) (Lepidoptera: Lasiocampidae) is an ornamental and fruit plant pest in southwestern Andalusia. Analysis of head capsule width of 387 larvae
reared on different host plants (Pistacea lentisco L., Arbutus unedo L., Citrus reticulata Blanco, Retama
sphaerocarpa (L.) Boiss, Spartium junceum L., Tamarix gallica L., and Vaccinium spp.) indicates that
growth rate between instars does not Þt well to DyarÕs rule. The number of instars varied from 5 to
8. Head capsule width could be used directly to determine developmental stage of individual in almost
all host plants used. Only for larvae fed on S. junceum, R. sphaerocarpa, Vaccinium corymbosum L.
ÔSharpblueÕ, and Vaccinium ashei Reade ÔBonitaÕ, beyond third instar, did the range of measurements
overlap. These data were analyzed using a combination of statistical methods. A nonlinear leastsquares approach was used to determine width means and ranges for each instar, and misclassiÞcation
probabilities were calculated for each overlapping instar using Z statistics for all head capsule widths.
Various factors are discussed to explain the results.
KEY WORDS Streblote panda, DyarÕs rule, instars, developmental stage, southwestern Andalusia
Determination of instar distribution can provide important information for pest management (Daly
1985). Spray applications usually are done during a
particular stage of larval development to be effective.
For example, Bacillus thuringiensis, a generalized biocide against lepidopteran pests, generally is applied to
coincide with populations at the peak frequencies of
Þrst and second instars, to maximize treatment effects
(Martin and Bonneaux 2006). Accurate determination
of population age and phenology not only provides a
tool for timing spray applications but also for explaining the reasons for treatment failures (McClellan and
Logan 1994). Measurements of head capsule width
give basic information for the development of morphometric and ecological studies addressing pest management (Chatterjee 1939, Williams and McDonald
1982, Fischbacher 1996).
Head capsule width has been used frequently for
instar determination in species of Lepidoptera (Daly
1985, Beaver and Sanderson 1989, McClellan and Logan 1994, Hammack et al. 2003). Dyar (1890) proposed
the Þrst mathematical classiÞcation criteria to determine the numbers of insect instars. Later, the method
was modiÞed (Taylor 1931), because it was demonstrated that it was not applicable to all insect species.
Several methods of statistical analysis can be used to
group individuals into instar classes when measure1 Corresponding author: Instituto de Ciencias Agrarias. Centro Ciencias Medioambientales, CSIC. C/Serrano 115 dpdo, 28006 Madrid, Spain
(e-mail: [email protected]).
2 Crop Protection Area, IFAPA Centro “Las Torres-Tomejil,” Apartado OÞcial, 41200 Alcalá del Rṍo, Sevilla, Spain.
ments overlap between instars (Fox et al. 1972, Caltagirone et al. 1983, Beaver and Sanderson 1989, McClellan and Logan 1994).
The moth Streblote panda Hübner (1820) has been
considered a pest of several ornamental and fruit
plants in the Mediterranean area (Balachowsky 1966,
Zhang 1994, Molina 1998); however, information
about this insect is scarce. There are no published data
on either instar numbers, or head capsule width as a
function of instar. Brief anatomical descriptions of
larval stages are available for this species (Huertas
Dionisio 1980, Bogner 1999, Gómez de Aizpúrua 2002,
Calvo 2004), but no morphometric studies have been
found.
S. panda is distributed throughout the southwestern
Mediterranean Basin, from the southeastern Iberian
Peninsula, through the Sahara and from Morocco to
Egypt. The larvae are polyphagous. The length of S.
panda larval stages, encompassing ⬎50% of the biological cycle, making instar determination a key for
better knowledge of its biology as well as being a
valuable tool for pest management.
Size and variation of head capsule were examined in
S. panda larvae reared on different host plants under
laboratory conditions. The study was carried out to
determine whether head capsule width measurements
provide a reliable estimate of the age of the larvae, and
to characterize and develop criteria to determine instar number of this species. We report the frequency
distribution of each instar and assign ranges to individual instars based on head capsule width Þtted to
normal curves for all raw data.
0013-8746/08/0881Ð0886$04.00/0 䉷 2008 Entomological Society of America
882
ANNALS OF THE ENTOMOLOGICAL SOCIETY OF AMERICA
Vol. 101, no. 5
Number of larvae
300
200
100
50
0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
6.0
6.5
7.0
Head capsule width (mm)
Fig. 1. Frequency distribution of observed head capsule width of S. panda, and Þtted functions to head capsule data for
each instar by using NLLS Þtting to Gaussian curves.
deviations, and growth ratios, were calculated for each
instar and host plant.
Our Þrst approach was to determine the possibility
of DyarÕs rule use (Dyar 1890) to assign specimens to
instars. Linear regression analysis was used to establish
relationships between growth ratio (head capsule
width [millimeters]) at instar i ⫹1/head capsule
width [millimeters] at instar i) and molt number for
larvae of each population (Jobin et al. 1992).
Head capsule data were graphically represented at
0,05-mm size intervals to use Þtted Gaussian distributions to delineate instars. An initial estimate of the
mean and the variance (s2) was computed. These
estimates were used as initial values for nonlinear
least-square Þtting (NLLS) of normal (Gaussian)
curves to each instar as well as for Þnal parameter
estimates that accounted for potential overlap between instar distributions (McClellan and Logan 1994,
Godin et al. 2002).
Once means and variances associated with each
instar subpopulation were estimated using NLLS Þtting, determination of overlapping points to classify
1.8
5 instars
6 instars
7 instars
8 instars
1.7
Growth ratio
Materials and Methods
Larvae used in this study were from an experimental
colony maintained in our laboratory (IFAPA. Centro
ÔLas Torres-TomejilÕ, Seville, Spain), on strawberry
tree (Arbutus unedo L., Ericaceae). The colony was
established in June 2000, but from time to time wild
specimens collected in the Þeld as larvae and reared
on their own host plants were used to avoid inbreeding. Larvae were reared from eggs at 25 ⫾ 1⬚C, 70 ⫾
5% RH, and a photoperiod of 16:8 (L:D) h. From 2001
to 2003, several rearings were done on the following
host plants: mastic (Pistacea lentisco L., Anacardiaceae), strawberry tree (Arbutus unedo, Ericaceae),
blueberry (Vaccinium spp., Ericaceae), tangerine
(Citrus reticulata Blanco, Rutaceae), broom (Retama
sphaerocarpa (L.) Boiss, Fabaceae), Spanish broom
(Spartium junceum L., Fabaceae), and French tamarisk (Tamarix gallica L., Tamaricaceae).
After egg hatch, cohorts of larvae were reared in
groups until the third instar. Larvae were individualized when they molted to fourth instar. Boxes were
examined daily, and the shed head capsules, if present,
were collected and saved in vials with head molt date,
host plant and instar number, which corresponded to
each rearing. Later the capsules were set on glass slides
with gum arabic, ensuring that the facial area of the
head stayed parallel to the surface of the slide to make
sure that the planes of length and width were perpendicular to the axis vision of the microscope. Measurements were made across the greatest width of the
head (genae) in a stereomicroscope with an ocular
micrometer. All the head capsules obtained from 387
larvae were included in the analysis. Head capsule
widths of last instars were not measured because we
allowed complete development in our rearings and
the capsules split after the last ecdysis.
Data were grouped according to host plant, larval
sex, and Þnal number of molts. Means and standard
1.6
1.5
1.4
1.3
1.2
1.1
1
2
3
4
5
Molt number
6
7
Fig. 2. Growth ratio and molt number relationship for all
host plants used in rearings of S. panda. The number of larvae
used on each stadium is speciÞed on Table 1.
5.26 ⫾ 0.00
2
5.04 ⫾ 0.19
5.04 ⫾ 0.15
4.47
4.45 ⫾ 0.21
3
3
5
1
6.05
6.29 ⫾ 0.10
5.39
5.02 ⫾ 0.10
5.81 ⫾ 0.10
5.76 ⫾ 1.74
5.17 ⫾ 0.12
5.39
4.12 ⫾ 0.30
11
15
10
9
1
5
1
3
1
5.13
a
Means within a column followed with different letters are signiÞcantly different (P ⬍ 0.05; NewmanÐKeuls multiple range test).
* P ⬍ 0.05; ** P ⬍ 0.01; and *** P ⬍ 0.001.
b
Data not included in the analysis.
12
4.49 ⫾ 0.13ab
15
4.26 ⫾ 0.08bc
14
3.92 ⫾ 0.09c
18
4.58 ⫾ 0.09a
10
4.66 ⫾ 0.16a
11
4.05 ⫾ 0.11c
11
4.50 ⫾ 0.05ab
6
3.28 ⫾ 0.15d
7
4.29 ⫾ 0.09abc
14
4.11 ⫾ 0.06bc
14
4.13 ⫾ 0.07bc
2
3.98 ⫾ 0.30
F10,121 ⫽ 12.32***
23
3.50 ⫾ 0.05a
24
3.23 ⫾ 0.05b
14
2.83 ⫾ 0.06d
22
3.43 ⫾ 0.05a
10
3.65 ⫾ 0.19a
14
3.20 ⫾ 0.04bc
15
3.48 ⫾ 0.03a
6
2.50 ⫾ 0.06e
15
3.46 ⫾ 0.10a
15
3.07 ⫾ 0.03bc
15
3.00 ⫾ 0.05cd
2
2.70 ⫾ 0.41
F10,163 ⫽ 19.25***
23
2.33 ⫾ 0.03a
24
2.20 ⫾ 0.03b
14
2.07 ⫾ 0.04b
40
2.35 ⫾ 0.02a
10
2.34 ⫾ 0.06a
15
2.34 ⫾ 0.02a
26
2.30 ⫾ 0.02a
6
1.73 ⫾ 0.02c
34
2.18 ⫾ 0.02b
16
2.20 ⫾ 0.02b
30
2.13 ⫾ 0.02b
2
1.84 ⫾ 0.00
F10,225 ⫽ 22.60***
23
1.56 ⫾ 0.01a
24
1.51 ⫾ 0.01ab
27
1.41 ⫾ 0.02c
40
1.57 ⫾ 0.25a
10
1.58 ⫾ 0.02a
15
1.52 ⫾ 0.00ab
32
1.55 ⫾ 0.01a
6
1.51 ⫾ 0.07ab
34
1.47 ⫾ 0.02b
36
1.50 ⫾ 0.01ab
30
1.48 ⫾ 0.02b
2
1.41 ⫾ 0.03
F10,270 ⫽ 12.47***
23
1.05 ⫾ 0.00ab
24
1.03 ⫾ 0.01b
27
1.06 ⫾ 0.01a
70
1.04 ⫾ 0.00ab
10
1.05 ⫾ 0.00ab
78
1.06 ⫾ 0.00a
32
1.05 ⫾ 0.00ab
12
1.02 ⫾ 0.01b
34
1.03 ⫾ 0.01b
36
1.02 ⫾ 0.01b
30
1.03 ⫾ 0.01b
11
1.05 ⫾ 0.01
F10,356 ⫽ 7.33***
Width
(mm)a
Host plant
n
Width
(mm)a
n
Width
(mm)a
n
Width
(mm)a
n
L4
L3
L2
L1
Head capsule width (mean ⴞ SE, millimeters) per instar in each host plant of lappet moth S. panda
Table 1.
Head capsule widths ranged from 0.92 to 6.45 mm,
for all raw data. Peaks are indicated at ⬇1.05, 1.50, 2.05,
3.40, 4.10, and 6.0 mm. Examination of frequency distributions of raw data showed various overlapping
degrees from third instar onward (Fig. 1). Only the
distribution of the Þrst instar was discrete; distributions for the rest of instars exhibited different degrees
of overlap which increased with instar.
Head capsule width of S. panda, in relation to the
total number of instars recorded, only followed the
DyarÕs rule when showed Þve instars. The increment
of instar number produced a decrease of growth ratio,
so did not follow the rule (Fig. 2).
Larvae of S. panda fed on different host plants
showed signiÞcantly differences among instars head
capsule width (Table 1). The data formed a series of
separate distributions, and they did not show overlapping along the development between instars. Except for larvae eared on R. sphaerocarpa, S. junceum,
V. corymbosumÔSharpblueÕ, and V. asheiÔBonitaÕ that
showed various overlapping degrees in the last instars,
generally from Þfth to seventh (Table 2). Growth ratio
was 1.5 from Þrst to third instar, and it decreased as the
number of molts increased.
When the raw data were separated in relation to sex
(Table 3), only larvae with Þve instars showed a constant growth ratio of 1.5. Nonoverlapping distributions
were detected for males with Þve or six instars and
females with Þve instars. From third instar onward,
head capsule widths for females were greater than for
males, and for both sexes the growth ratio decreased
and showed overlapping distributions when larvae
presented seven or eight instars (Table 3).
Table 4 show the values of the mean, range of
variation, overlap points, and the probabilities of misclassiÞcation for each instar, based on the whole data
set after NLLS Þtting of normal curves. As head capsule width increased, so did the probability of misclassiÞcation. MisclassiÞcation probabilities of instar i
as instar i ⫺ 1 was always greater than the probability
of misclassiÞcation of instar i as instar i ⫹ 1.
n
Results
P. lentiscus
A. unedo
C. reticulata
R. shaerocarpa
S. junceum
T. gallica
V. corymbosum ÔOÕNealÕ
V. corymbosum Sharpblue
V. corymbosum ÔMistyÕ
V. ashei ÔWindyÕ
V. ashei Bonita
V. asheib ‘Climax’
L5
n
where X is a normal variable with the mean ␮ and the
standard deviation ␴; Xo is the overlap point, and Z is
the standard normal. All calculations and statistics
were done using GraphPad Prism version 4 for Windows (GraphPad Software Inc., San Diego, CA).
883
1
n
册
Width
(mm)
Xo ⫺ ␮
␴
L6b
冋
P关X ⬎ Xo兴 ⬵ Z ⬎
L7b
individual observations into the appropriate instar was
accomplished by graphical determination. MisclassiÞcation probabilities for overlapping curves were then
computed using the method of Rodrṍguez-Quiroz et
al. (2000). Estimated mean and standard deviation
were selected as the values of the parameters ␮ and ␴,
respectively, using the following equation:
Width
(mm)
CALVO AND MOLINA: INSTAR DETERMINATION OF S. panda
Width (mm)a
September 2008
884
ANNALS OF THE ENTOMOLOGICAL SOCIETY OF AMERICA
Table 2.
Vol. 101, no. 5
Ranges of head capsule widths per instar in each for S. panda larvae reared on different host plants
Host plant
L1
L2
L3
L4
L5
P. lentiscus
A. unedo
C. reticulata
R. shaerocarpa
S. junceum
T. gallica
V. corymbosum OÕNeal
V. corymbosum Sharpblue
V. corymbosum Misty
V. ashei Windy
V. ashei Bonita
0.98Ð1.09
0.92Ð1.09
0.99Ð1.12
0.98Ð1.12
1.05Ð1.05
1.05Ð1.12
0.99Ð1.05
0.99Ð1.05
0.98Ð1.05
0.99Ð1.05
0.99Ð1.05
1.38Ð1.71
1.31Ð1.58
1.32Ð1.58
1.38Ð1.64
1.45Ð1.71
1.51Ð1.58
1.32Ð1.64
1.45Ð1.58
1.31Ð1.58
1.38Ð1.58
1.32Ð1.58
2.04Ð2.63
1.91Ð2.41
1.84Ð2.14
1.84Ð2.63
1.84Ð2.43
2.17Ð2.43
2.11Ð2.41
1.71Ð1.84
1.91Ð2.37
2.11Ð2.43
1.84Ð2.37
3.03Ð3.95
2.89Ð3.81
2.5Ð3.36
2.76Ð3.75
2.37Ð4.67
3.03Ð3.42
3.29Ð3.68
2.3Ð2.63
2.89Ð4.21
2.76Ð3.22
2.7Ð3.29
3.95Ð5.3
3.95Ð5.0
3.42Ð4.51
3.68Ð5.06
3.42Ð5.0
3.42Ð4.47
4.21Ð4.8
2.89Ð3.75
3.95Ð4.74
3.75Ð4.54
3.68Ð4.67
Discussion
There are no literature data on S. panda head capsule size or instar numbers with which to compare our
results. Calculated growth ratios for Þrst to third instars of S. panda seem to Þt DyarÕs rule, and they were
similar to the ones reported for Malacosoma spp.
(Fitzgerald 1995, Verdinelli and Sannia-Passino 2003).
However, Þfth, sixth, seventh, and eighth instars
showed smaller growth ratios than expected. Thus, it
was concluded that, as recorded for other lepidopteran species, S. panda development was not in full
agreement with DyarÕs rule (Gaines and Campbell
1935, Goettel and Philogéne 1979).
This study found variable number of molts for S.
panda larvae. A variable number of instars is well
recorded in the literature for many insect species, and
it largely depends on the nutritive value of the food on
which they fed, which is affected by environmental
factors (Gaines and Campbell 1935, Fogal and Kwain
1972, Schmidt et al. 1977, Goettel and Philogéne 1979,
Jobin et al. 1992, ShaÞei et al. 2001, Esperk et al. 2007).
Temperature was held constant in this study; thus,
the main source of variation for both head capsule
width and instar number is likely to be food type
(namely, plant species) and quality. Some nutritional
factors, such as leaf toughness related to plant phenology, inßuences larval development of this species
Table 3.
Males
Instar
I
II
III
IV
V
VI
VII
Females
L6
4.74Ð5.79
5.13Ð6.31
4.25Ð6.05
4.67Ð5.78
3.36Ð4.87
L7
6.12Ð6.29
4.21Ð4.87
4.74Ð5.39
4.47Ð5.26
on closely related plant species (Calvo 2004, Calvo and
Molina 2004). An increment of instars number mainly
due to adverse developmental conditions can be
found in other lepidopteran species, such as Malacosoma disstria Hübner (Esperk et al. 2007).
Developmental time (measured as number of instars) seems to increase head capsule width overlaps
and misclassiÞcation probabilities, and it must be related with the feeding strategies adopted by this species along its life cycle. S. panda closely follows a
“capital breeding” reproductive pattern (Tammaru
and Haukioja 1996) in which Þnal instar weight is a
more important parameter for survivorship and reproduction than the developmental time (Nijhout
1975, Slansky 1993, Santos and Shields 1998). Instar
numbers, head capsule width, and larval size are
linked to host plant use to reach a minimum pupal
weight (Tammaru and Haukioja 1996, Jönnson 1997).
S. panda show aggregated larvae in the three Þrst
instars. Like in other species, gregarious behavior of S.
panda perhaps could be related with the increment of
foraging efÞciency of caterpillars by facilitating the
establishment of feeding sites, providing thermal beneÞts that improve feeding and assimilation, or improving feeding efÞciency through antipredator defense
(Joos et al. 1988, Stamp and Bowers 1988, Hochuli
2001; see reviews by Stamp and Casey 1993, Costa
Head capsule width (millimeters) per instar of S. panda in relation to the sex of the larvae reared
5 instars (n ⫽ 36)
Mean ⫾ S.E.
1.05 ⫾ 0.01
1.53 ⫾ 0.02
2.35 ⫾ 0.03
3.47 ⫾ 0.03
Mean ⫾ S.E.
}1.5
}1.5
}1.5
1.03 ⫾ 0.01
1.50 ⫾ 0.01
2.18 ⫾ 0.02
3.16 ⫾ 0.05
4.20 ⫾ 0.03
5 instars (n ⫽ 17)
Instar
Mean ⫾ S.E.
I
II
III
IV
V
VI
VII
1.04 ⫾ 0.01
1.59 ⫾ 0.06
2.36 ⫾ 0.04
3.57 ⫾ 0.07
6 instars (n ⫽ 34)
Growth ratio
Mean ⫾ S.E.
}1.5
}1.5
}1.5
}1.3
1.05 ⫾ 0.01
1.32 ⫾ 0.05
2.20 ⫾ 0.05
3.12 ⫾ 0.09
4.11 ⫾ 0.10
5.10 ⫾ 0.20
6 instars (n ⫽ 38)
Growth ratio
Mean ⫾ S.E.
}1.5
}1.5
}1.5
1.03 ⫾ 0.01
1.52 ⫾ 0.01
2.25 ⫾ 0.02
3.34 ⫾ 0.04
4.47 ⫾ 0.05
7 instars (n ⫽ 16)
Growth ratio
Mean ⫾ S.E.
Growth ratio
}1.3
}1.7
}1.4
}1.3
}1.2
1.05
1.45
1.84
2.37
3.42
4.25
5.39
}1.4
}1.3
}1.3
}1.4
}1.2
}1.3
7 instars (n ⫽ 20)
Growth ratio
Mean ⫾ S.E.
}1.5
}1.5
}1.5
}1.3
1.05 ⫾ 0.01
1.48 ⫾ 0.02
2.24 ⫾ 0.05
3.20 ⫾ 0.11
4.36 ⫾ 0.11
5.47 ⫾ 0.11
8 instars (n ⫽ 1)
Growth ratio
8 instars (n ⫽ 6)
Growth ratio
Mean ⫾ S.E.
Growth ratio
}1.4
}1.5
}1.4
}1.4
}1.2
1.05 ⫾ 0.00
1.25 ⫾ 0.09
2.18 ⫾ 0.07
3.11 ⫾ 0.09
4.23 ⫾ 0.12
5.08 ⫾ 0.15
5.91 ⫾ 0.21
}1.2
}1.7
}1.4
}1.4
}1.2
}1.2
September 2008
CALVO AND MOLINA: INSTAR DETERMINATION OF S. panda
Table 4. Head capsule width means (millimeters), range and
probabilities of misclassifying instar of S. panda after NLLS fitting
to normal curves
Instar
(i)
n
Mean ⫾ SD
Range
Overlap
point
(mm)
I
II
III
IV
V
VI
VII
387
279
240
175
134
61
10
1.04 ⫾ 0.02
1.56 ⫾ 0.09
2.29 ⫾ 0.19
3.34 ⫾ 0.30
4.27 ⫾ 0.41
5.44 ⫾ 0.77
5.59 ⫾ 1.12
0.95Ð1.14
1.28Ð1.85
1.66Ð2.90
2.43Ð4.25
3.09Ð5.42
3.80Ð6.86
4.42Ð6.73
1.798
2.727
3.783
4.948
6.907
Probability of
misclassifying
Instar i
as i-1
0.0042
0.0217
0.1182
0.2614
0.1196
Instar i
as i ⫹1
0.0032
0.0117
0.0712
0.0515
0.0291
2006). Later, from fourth instar onward, the species
show solitary larvae. Wider variation ranges in head
capsules widths were recorded for these solitary late
instars between host plants. Larger larvae also have
larger heads and stronger mandibular muscles, which
are capable of producing greater force, inßuencing the
effectiveness of feeding strategies (Bernays 1986).
Variation in head capsule widths and associated molts
among host plants would be then related to variability
in nutritional quality as season progresses, and connected with the particular phenology of each host
plant, and the larvae capabilities to Þnd an optimal
feeding site.
Given the variability in the number of instars of our
data, one would expect a different frequency distribution of head capsule widths among host plants when
we change the developmental conditions (i.e., geographically, variability in abiotic conditions between
years, soils types, fertilization scenarios, and so on).
These results should serve as a caveat to other workers.
The use of DyarÕs Rule without taking into account the
particular scenario conditions could produce important mistakes in the assignment of larvae to each instar,
mainly due to the increased of error probabilities.
Acknowledgments
Funds for this research were provided by D.G. de Investigación y Formación Agraria, Consejerṍa de Agricultura y
Pesca, Junta de Andalucṍa, Projects PIA 1301.02; 03-025, and
Instituto Nacional Investigaciones Agrarias, MAPA, Project
RTA 03-092.
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observed head capsule widths. J. Econ. Entomol. 82: 716 Ð
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Received 12 April 2007; accepted 20 March 2008.