# Blow Flies Visiting Decaying Alligators: Is Succession Synchronous or Asynchronous?

Authors: Mark P. Nelder; John W. McCreadie; Clinton S. Major
Journal: Psyche
 (2009)

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## Abstract

Succession patterns of adult blow flies (Diptera: Calliphoridae) on decaying alligators were investigated in Mobile (Ala, USA) during August 2002. The most abundant blow fly species visiting the carcasses wereChrysomya rufifacies (Macquart), Cochliomyia macellaria (Fabricus), Chrysomya megacephala (Fabricus), Phormia regina (Meigen), and Lucilia coeruleiviridis (Macquart). Lucilia coeruleiviridis was collected more often during the early stages of decomposition, followed by Chrysomya spp., Cochliomyia macellaria, and Phormia regina in the later stages. Lucilia coeruleiviridis was the only synchronous blow fly on the three carcasses; other blow fly species exhibited only site-specific synchrony. Using dichotomous correlations and analyses of variance, we demonstrated that blow fly-community succession was asynchronous among three alligators; however, Monte Carlo simulations indicate that there was some degree of synchrony between the carcasses.


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## Body

## 1. Introduction

Blow flies (Diptera;
Calliphoridae) are ubiquitous insects during the early stages of animal decay
and their  larvae are important in estimating the time since death or the postmortem
interval (PMI) of a carcass [1]. Larval age of the earliest carrion-arriving blow fly species can be
estimated based on data developed from controlled carrion studies [2, 3]. Faunal composition (succession data) of carrion
can be predicted for a given area under specific conditions and the composition
compared to baseline data obtained from an animal model [4–8].Insect succession on carrion has
been examined in detail in the southeastern United States. Studies have been
performed on decaying dogs,Canis lupus L., in Tennessee [9];
pigs, Sus scrofa L., in South Carolina [10] and Florida [11];
humans, Homo sapiens L., in Tennessee [12]; rats, Rattus rattus L., in South Carolina [13]; and a variety of
vertebrate species in North Carolina [14], Mississippi [15], and Louisiana [16]. There have been few published
reports on the subject from Alabama and Georgia, however, are needed for better
understanding of blow fly ecology associated with carrion.Variation associated with blow
fly succession on carcasses placed in the same habitat at the same time has not
been tested. Hence, this raises the question of whether carcasses placed
simultaneously in the same habitat decompose in the same manner and whether all
carcasses experience the same blow fly-succession pattern. To test these
hypotheses, we simultaneously placed three carcasses in the field and compared
the succession of blow flies on each carcass. Specifically, we tested if the
pattern or synchrony of calliphorid succession varied among carcasses.


## 2. Materials and Methods

### 2.1. Study Area

Sites were located in an
evergreen woodlot on the campus of the University of South Alabama,
inside the city limits of Mobile,
Alabama. The woodlot was
dominated by a mixture of pines (loblolly,Pinus taeda L., and longleaf, Pinus palustris Miller) and hardwoods typical of forests occurring on upland areas
associated with sandy loam soils. The canopy was mostly closed with two of
three sites in this forest having greater than 75% cover; site C had less than
50% canopy cover. Oaks present included sand, Quercus geminata Small; southern
red, Quercus falcata Michaux; turkey, Quercus laevis Walter; and water, Quercus nigra L. Other hardwoods included American witch-hazel, Hamamelis virginiana L.; flowering dogwood, Cornus florida
L.; red maple, Acer
rubrum L.; and southern magnolia, Magnolia grandiflora L. The magnolia and maple extended up to the bottom of the pine canopy
and form a patchy subcanopy. Shrubs in the woodlot included American holly, Ilex opaca Aiton; several azalea species, Rhododendron spp.; devilwood, Osmanthus americana (L.); swamp titi,
Cyrilla racemiflora L.; large gallberry, Ilex coriacea (Pursh); and sparkleberry, Vaccinium arboreum Marshall. The floor
was sparse with occasional patches of herbaceous growth occurring in small
openings. Herbaceous species were dominated by downy danthonia, Danthonia sericea Nuttall; fourleaf yam, Dioscorea villosa L.; flowering
spurge, Euphorbia
corollata L.; greater tickseed, Coreopsis major Walter; longleaf woodoats, Chasmanthium sessiliflorum  (Poiret); a sedge, undetermined;
St. Andrew’s cross, Hypericum
hypericoides (L.); strawberry bush, Euonymus americanus L.; and wood spurge, Euphorbia commutatus Engelmann. Vines were
abundant at each site and included Carolina jasmine, Gelsemium sempervirens (L.); catbrier, Smilax
bona-nox L.; cat greenbrier, Smilax glauca Walter; crossvine,
Bignonia capreolata (L.); and muscadine, Vitis rotundifolia Michaux.  This woodlot is typical of forests that have
established on former mesic to dry-mesic longleaf pine sites following removal
of longleaf pine.


### 2.2. Sampling Protocol

The American alligator,Alligator mississippiensis Daudin, was chosen as the model carcass because they were readily
available as fresh-frozen (frozen since May 2002) specimens and relatively little
is known about blow fly succession on these animals [16, 17]. Specimens used were accidentally trapped during turtle surveys in the
Mobile-Tensaw delta in southwestern Alabama. Dead alligators were sealed in
black garbage bags at collection time and frozen at −20°C until needed.
Approximately 24 hours before the beginning of the study, the carcasses were
placed in a walk in refrigerator at 4°C to thaw slowly. Alligator sizes were as
follows: site A: 1.65 m, 17.9 kg; site B: 1.68 m, 20.0 kg; and site C: 1.78 m, 24.3 kg. Each alligator was placed in a stainless-steel wire cage (1.8 × 0.35 × 0.25 m; mesh size = 2.5 × 2.5 cm) to prevent carcass disturbance by vertebrate scavengers
and cage placed in the woodlot 5 August 2002 at 1000 hours (i.e., day zero). For
purposes of this study, calliphorid collection was ceased on 15 August 2002. Cages were
arranged along a single transect, 50 m apart.The decomposition of each
alligator was divided into stages following those of Reed [9] and Johnson
[18]. The beginning and end of these stages were difficult to discern and we
only report approximate time intervals of the stages. It is important to note
that these stages are part of a continuum and not categorical; they are used as
reference points to compare the physical decomposition of the carcasses and are
considered arbitrary in terms of blow fly succession [10, 19, 20].Sticky fly-paper was used to
collect adult blow flies arriving at the carcasses. Two strips of sticky fly
paper (120 cm × 4 cm) were placed on top of each cage at 1000 hours daily.
After 24 hours, the fly paper was removed and placed in a labeled container
with 95% ethanol and new fly paper replaced on cage. Blow flies were later
removed from the sticky paper, identified, and returned to alcohol-labeled
vials. To supplement sticky-paper collections, aerial netting was performed
over the cages (5 minutes per cage). Blow flies collected by aerial netting
were killed in the field using a collecting jar laced with ethyl acetate,
placed in labeled vials, and later pinned in the laboratory. Identifications
were made according to Hall [21], Hall and Townsend [22], Dear [23], and
Whitworth [24].Blow fly larvae were collected
daily from different areas on each carcass and the surrounding ground. Larvae
were placed in plastic containers and transported back to the laboratory. One-half
of the larvae from each carcass were preserved by boiling in water and then
placing them in Kahle’s solution [25]. The remaining larvae
were reared to adults using the following procedure. Larvae (N = 3–5 per
container) were placed on a small piece of raw calf liver (approximately 10 g)
and then wrapped in moist paper towel. A 3 cm layer of vermiculite was added to
150 mL clear-plastic containers; larvae and liver, wrapped in paper towel, were
placed on top of the vermiculite. Pieces of cardboard, furnished with small
holes for air circulation, were used to cover containers. Containers were held
at room temperature (i.e., 22–24°C) with a light: dark regime of 12:12 hours. Containers were
inspected twice daily for the presence of adult blow flies.As the condition of some flies
from the sticky-paper were unsuitable for identification, only adult flies that
contained all relevant taxonomic characters were included in analyses. It was
assumed that damaged specimens would occur in roughly equal proportions among blow
fly species. Reared larvae were used to confirm the identity of sticky-paper
collected adult blow flies.Lucilia cluvia (Walker), Lucilia eximia (Wiedemann), and Lucilia sericata (Meigen) were collected as adults
on the sticky paper and were not reared from larvae found on the carcasses. Voucher
specimens have been deposited in the University of South Alabama’s Arthropod
Depository.


### 2.3. Statistical Analyses

All statistical tests were considered significant atP < .05, and the experiment-wise rate was adjusted for each correlation to maintain a family error rate of P = .05. For each treatment, an experiment-wise adjustment of P-values was made to preserve a family error rate of P = .05. For each species of blow fly, a dichotomous (present/absent) correlation was used to determine the degree of temporal association among each site. Hence, for each species, three correlations were calculated, that is, site A versus site B, site A versus site C, and site B versus site C. As these correlations were special cases of the Pearson-product-moment-correlation coefficient, a z test was used to determine significance [26]. To determine if the relative abundance of each species of blow fly collected on the fly paper differed among sites, an analysis of variance (ANOVA) was used, with number of flies for each species as the response variable, site as the main effect, and day as the block (random variable). For significant main effects, differences among means were determined using the Tukey multiple comparison procedure [27]. All data was normalized before statistical tests.A Monte Carlo approach was used to examine the similarity of community succession among the
three sites used in this study. The intent here was to determine if
combined-species occurrence for all species of blow flies, among all sites,
occurred at a frequency different from that expected by a random model. Combined species co-occurrence
(i.e., the number of times a species occurs on the same day at any pair of
sites, summed for all species in the analyses) at a frequency greater than that
expected by a random model would indicate predictable community succession
among sites [28]. In contrast to correlation or ANOVA analyses, all species were considered
simultaneously in this procedure. Our observed test statistic was the
total number of co-occurrences for all species.
For example, if a
particular species occurred at sites A and B on the same five days, site B and
C on the same four days, and sites A and C on the same six days, then the total
number of observed co-occurrences among sites for that species would be 15. Adding the number of
co-occurrences for all five species of blow flies considered in our study
produces the observed test statistic.The Monte Carlo procedure allows the
probability distribution for the test statistic (in our case, the number of times a species occurs on the
same day at any pair of sites, summed for all species in the analyses)
to be generated while permitting the incorporation of relevant biological
constraints into the model used to generate the test-statistic distribution
[29, 30]. The constraint used in generating our test
statistic distribution was that the frequency of each species’ occurrence, at
each site, was equal to the observed frequency for that species at that site. The test statistic distribution was
generated using 1000 Monte Carlo simulations [29] and the
observed number of total co-occurrences was then compared with the generated
distribution, and if the P-value of the observed co-occurrence was low
(i.e., P < .05), then the observation was judged to be significant.


## 2.1. Study Area

Sites were located in an
evergreen woodlot on the campus of the University of South Alabama,
inside the city limits of Mobile,
Alabama. The woodlot was
dominated by a mixture of pines (loblolly,Pinus taeda L., and longleaf, Pinus palustris Miller) and hardwoods typical of forests occurring on upland areas
associated with sandy loam soils. The canopy was mostly closed with two of
three sites in this forest having greater than 75% cover; site C had less than
50% canopy cover. Oaks present included sand, Quercus geminata Small; southern
red, Quercus falcata Michaux; turkey, Quercus laevis Walter; and water, Quercus nigra L. Other hardwoods included American witch-hazel, Hamamelis virginiana L.; flowering dogwood, Cornus florida
L.; red maple, Acer
rubrum L.; and southern magnolia, Magnolia grandiflora L. The magnolia and maple extended up to the bottom of the pine canopy
and form a patchy subcanopy. Shrubs in the woodlot included American holly, Ilex opaca Aiton; several azalea species, Rhododendron spp.; devilwood, Osmanthus americana (L.); swamp titi,
Cyrilla racemiflora L.; large gallberry, Ilex coriacea (Pursh); and sparkleberry, Vaccinium arboreum Marshall. The floor
was sparse with occasional patches of herbaceous growth occurring in small
openings. Herbaceous species were dominated by downy danthonia, Danthonia sericea Nuttall; fourleaf yam, Dioscorea villosa L.; flowering
spurge, Euphorbia
corollata L.; greater tickseed, Coreopsis major Walter; longleaf woodoats, Chasmanthium sessiliflorum  (Poiret); a sedge, undetermined;
St. Andrew’s cross, Hypericum
hypericoides (L.); strawberry bush, Euonymus americanus L.; and wood spurge, Euphorbia commutatus Engelmann. Vines were
abundant at each site and included Carolina jasmine, Gelsemium sempervirens (L.); catbrier, Smilax
bona-nox L.; cat greenbrier, Smilax glauca Walter; crossvine,
Bignonia capreolata (L.); and muscadine, Vitis rotundifolia Michaux.  This woodlot is typical of forests that have
established on former mesic to dry-mesic longleaf pine sites following removal
of longleaf pine.


## 2.2. Sampling Protocol

The American alligator,Alligator mississippiensis Daudin, was chosen as the model carcass because they were readily
available as fresh-frozen (frozen since May 2002) specimens and relatively little
is known about blow fly succession on these animals [16, 17]. Specimens used were accidentally trapped during turtle surveys in the
Mobile-Tensaw delta in southwestern Alabama. Dead alligators were sealed in
black garbage bags at collection time and frozen at −20°C until needed.
Approximately 24 hours before the beginning of the study, the carcasses were
placed in a walk in refrigerator at 4°C to thaw slowly. Alligator sizes were as
follows: site A: 1.65 m, 17.9 kg; site B: 1.68 m, 20.0 kg; and site C: 1.78 m, 24.3 kg. Each alligator was placed in a stainless-steel wire cage (1.8 × 0.35 × 0.25 m; mesh size = 2.5 × 2.5 cm) to prevent carcass disturbance by vertebrate scavengers
and cage placed in the woodlot 5 August 2002 at 1000 hours (i.e., day zero). For
purposes of this study, calliphorid collection was ceased on 15 August 2002. Cages were
arranged along a single transect, 50 m apart.The decomposition of each
alligator was divided into stages following those of Reed [9] and Johnson
[18]. The beginning and end of these stages were difficult to discern and we
only report approximate time intervals of the stages. It is important to note
that these stages are part of a continuum and not categorical; they are used as
reference points to compare the physical decomposition of the carcasses and are
considered arbitrary in terms of blow fly succession [10, 19, 20].Sticky fly-paper was used to
collect adult blow flies arriving at the carcasses. Two strips of sticky fly
paper (120 cm × 4 cm) were placed on top of each cage at 1000 hours daily.
After 24 hours, the fly paper was removed and placed in a labeled container
with 95% ethanol and new fly paper replaced on cage. Blow flies were later
removed from the sticky paper, identified, and returned to alcohol-labeled
vials. To supplement sticky-paper collections, aerial netting was performed
over the cages (5 minutes per cage). Blow flies collected by aerial netting
were killed in the field using a collecting jar laced with ethyl acetate,
placed in labeled vials, and later pinned in the laboratory. Identifications
were made according to Hall [21], Hall and Townsend [22], Dear [23], and
Whitworth [24].Blow fly larvae were collected
daily from different areas on each carcass and the surrounding ground. Larvae
were placed in plastic containers and transported back to the laboratory. One-half
of the larvae from each carcass were preserved by boiling in water and then
placing them in Kahle’s solution [25]. The remaining larvae
were reared to adults using the following procedure. Larvae (N = 3–5 per
container) were placed on a small piece of raw calf liver (approximately 10 g)
and then wrapped in moist paper towel. A 3 cm layer of vermiculite was added to
150 mL clear-plastic containers; larvae and liver, wrapped in paper towel, were
placed on top of the vermiculite. Pieces of cardboard, furnished with small
holes for air circulation, were used to cover containers. Containers were held
at room temperature (i.e., 22–24°C) with a light: dark regime of 12:12 hours. Containers were
inspected twice daily for the presence of adult blow flies.As the condition of some flies
from the sticky-paper were unsuitable for identification, only adult flies that
contained all relevant taxonomic characters were included in analyses. It was
assumed that damaged specimens would occur in roughly equal proportions among blow
fly species. Reared larvae were used to confirm the identity of sticky-paper
collected adult blow flies.Lucilia cluvia (Walker), Lucilia eximia (Wiedemann), and Lucilia sericata (Meigen) were collected as adults
on the sticky paper and were not reared from larvae found on the carcasses. Voucher
specimens have been deposited in the University of South Alabama’s Arthropod
Depository.


## 2.3. Statistical Analyses

All statistical tests were considered significant atP < .05, and the experiment-wise rate was adjusted for each correlation to maintain a family error rate of P = .05. For each treatment, an experiment-wise adjustment of P-values was made to preserve a family error rate of P = .05. For each species of blow fly, a dichotomous (present/absent) correlation was used to determine the degree of temporal association among each site. Hence, for each species, three correlations were calculated, that is, site A versus site B, site A versus site C, and site B versus site C. As these correlations were special cases of the Pearson-product-moment-correlation coefficient, a z test was used to determine significance [26]. To determine if the relative abundance of each species of blow fly collected on the fly paper differed among sites, an analysis of variance (ANOVA) was used, with number of flies for each species as the response variable, site as the main effect, and day as the block (random variable). For significant main effects, differences among means were determined using the Tukey multiple comparison procedure [27]. All data was normalized before statistical tests.A Monte Carlo approach was used to examine the similarity of community succession among the
three sites used in this study. The intent here was to determine if
combined-species occurrence for all species of blow flies, among all sites,
occurred at a frequency different from that expected by a random model. Combined species co-occurrence
(i.e., the number of times a species occurs on the same day at any pair of
sites, summed for all species in the analyses) at a frequency greater than that
expected by a random model would indicate predictable community succession
among sites [28]. In contrast to correlation or ANOVA analyses, all species were considered
simultaneously in this procedure. Our observed test statistic was the
total number of co-occurrences for all species.
For example, if a
particular species occurred at sites A and B on the same five days, site B and
C on the same four days, and sites A and C on the same six days, then the total
number of observed co-occurrences among sites for that species would be 15. Adding the number of
co-occurrences for all five species of blow flies considered in our study
produces the observed test statistic.The Monte Carlo procedure allows the
probability distribution for the test statistic (in our case, the number of times a species occurs on the
same day at any pair of sites, summed for all species in the analyses)
to be generated while permitting the incorporation of relevant biological
constraints into the model used to generate the test-statistic distribution
[29, 30]. The constraint used in generating our test
statistic distribution was that the frequency of each species’ occurrence, at
each site, was equal to the observed frequency for that species at that site. The test statistic distribution was
generated using 1000 Monte Carlo simulations [29] and the
observed number of total co-occurrences was then compared with the generated
distribution, and if the P-value of the observed co-occurrence was low
(i.e., P < .05), then the observation was judged to be significant.


## 3. Results

Climatological
data was obtained from a weather station located 6.5 km from the study sites.
The mean daily temperature during this study (5–15 August 2002) was26.8±0.5°C
with a mean daily high of 31.2±0.9°C and a mean daily low of 22.8±0.5°C.
Rainfall was limited to a total of 7.5 cm during the study, most (5.2 cm) of
the precipitation occurred during the first 24 hours.Eight species of Calliphoridae
were identified from decaying alligators during this study;Chrysomya rufifacies (Macquart) (N = 253, 31.6% of total 806 blow flies), Cochliomyia macellaria (Fabricius) (N = 216, 27.0%), Chrysomya megacephala (Fabricius) (N = 148, 18.5%), Phormia regina (Meigen) (N = 100,
12.5%), Lucilia
coeruleiviridis (N = 80, 10%), Lucilia cluvia (N = 4, 0.5%), Lucilia
eximia (N = 4, 0.5%), and Lucilia sericata (N = 1, 0.1%). Daily
relative abundances of adult blow flies at each carcass and stage of
decomposition are presented in Tables 1, 2, and 3. The fresh stage began at time zero
and ended approximately at 24 hours. Lucilia coeruleiviridis was the most
prevalent blow fly active about the carcasses in the first 24 hours,
ovipositing in and around the eyes, mouth, and nostrils. Lucilia coeruleiviridis were noted ovipositing on the carcasses within 15 minutes of being
placed in field. At the time of this oviposition, it was not raining.Table 1
Blow fly succession on site A’s decaying alligator, Mobile, Ala, USA (August 2002).
Blow fly speciesDaily abundancea,b (decomposition stage)12345678910FreshBloatActiveChrysomya rufifacies+−+++++++++++++++++Cochliomyia macellaria−−+++++++++++++−Chrysomya megacephala−−+++++++++++++−Phormia regina−−−++++++−+−Lucilia coeruleiviridis++++−−−−−−a“−” =
0 adults collected, “+” 1–5, “++” = 6–15, “+++” = 16–25, “++++” > 26.
bDay 1 represents the first 24 hours of
the study; this 24-hour period began at hour zero (i.e., the time of placement
of the carcass in the field at 1000 hours on 5 August 2002) till 1000 hours the next morning
on 6 August 2002.Table 2
Blow fly succession on site B’s decaying alligator, Mobile, Ala, USA (August 2002).
Blow fly speciesDaily abundancea,b (decomposition stage)12345678910FreshBloatActiveChrysomya rufifacies−−−+++++++++++++Cochliomyia macellaria−−++++++++++−Chrysomya megacephala−+++++−+−+−−Phormia regina−−−++++++−−+Lucilia coeruleiviridis++++−−+−−−a“−” =
0 adults collected, “+” 1–5, “++” = 6–15, “+++” = 16–25, “++++” > 26.
bDay 1 represents the first 24 hours of
the study; this 24-hour period began at hour zero (i.e., the time of placement
of the carcass in the field at 1000 hours on 5 August 2002) till 1000 hours the next morning
on 6 August 2002.Table 3
Blow fly succession on site C’s decaying alligator, Mobile, Ala, USA  (August 2002).
Blow fly speciesDaily abundancea,b (decomposition stage)12345678910FreshBloatActiveChrysomya rufifacies−−−−++++++++++++−−Cochliomyia macellaria−+−++++++++++++−−Chrysomya megacephala−−−++++++++++−−Phormia regina−−−+++++++++++−Lucilia coeruleiviridis++++++++−−−−−−a“−” =
0 adults collected, “+” 1–5, “++” = 6–15, “+++” = 16–25, “++++” > 26.
bDay 1 represents the first 24 hours of
the study; this 24-hour period began at hour zero (i.e., the time of placement
of the carcass in the field at 1000 hours on 5 August 2002) till 1000 hours the next morning
on 6 August 2002.The bloat stage lasted 1–3 days
depending on the site. By the end of this stage, at sites A and B, larval
masses enveloped the head and limb-torso junctions. At site C, decomposition
was slower with maggot masses restricted to the head. The majority of flies
visiting the carcasses during this stage wereLucilia coeruleiviridis (Tables 1–3). Blow flies
encountered in very low numbers during this stage were Lucilia cluvia (N = 4, site B, day 2), Lucilia eximia (N = 3, site B, day 2; N = 1, site
A, day 3), and Lucilia
sericata (N = 1, site B, day 2).The decay stage started
approximately (depending on site) at 72 hours and ended after approximately day
10. Larval masses had spread out from the head and limb-torso junctions and
were consuming decaying flesh in an anterior-to-posterior fashion. The last
part of the alligator to be consumed was the tail; the tongue was never
consumed by larvae and eventually dried up. Large numbers of maggots were noted
leaving the carcasses at sites A and B on day 5 and a day later for site C (day
6). On day 4,Chrysomya
rufifacies and Chrysomya megacephala visited the
carcasses most often. Day 5 was dominated by Chrysomya rufifacies, day 6
by Phormia regina, and the remaining days by Chrysomya rufifacies. Lucilia coeruleiviridis rarely visited the carcasses during this stage.The last stage noted here was the
skeletal remains stage. This stage began approximately on day 10 and continued
until the bones were collected on 15 September 2002 (day 41). The flesh of the
carcasses was largely consumed by the start of this stage. Adult blow flies
rarely visited the carcasses during this stage; therefore, are not depicted in
Tables2–4. Dipterous larvae still present were dominated by the black soldier
fly Hermetia illucens (L.) (Diptera: Stratiomyidae).Table 4
Dichotomous correlations of blow fly adults over a 10-day period among three
sites, each with a single alligator carcass.
SpeciesCorrelation coefficientsSite A versus site BSite A versus site CSite B versus site CChrysomya rufifacies.50.272.534Cochliomyia
macellaria1*.356.356Chrysomya
megacephala.216.802*0Phormia
regina.6.816*.408Lucilia
coeruleiviridis.816*1*.816**significant
at P < .05.Lucilia
coeruleiviridis showed a high degree of temporal synchrony among the three
sites (Table 4). In contrast, Chrysomya
rufifacies appeared to be in complete asynchrony among sites with respect
to its place in the succession on the alligator carcasses. The remaining three
species showed comparison-specific degrees of synchrony/asynchrony among sites.
The analysis of variance (Table 5) indicated that only one species, Cochliomyia
macellaria, showed a significant difference in relative abundance among
sites. This species was collected in greater abundance at site A than sites B
and C. The Monte Carlo analysis (Figure 1)
showed that combined species
co-occurrence occurred at a frequency greater than that expected by a random
model. This would indicate at least some synchrony (predictability) of
community succession among sites. However, as shown by the correlation
analyses, the extent of successional synchrony varied among species and
site.Table 5
Analysis of variance for five species of blow fly adults over a 10-day
period (block) among three sites (main effect), each with a single alligator
carcass.
SpeciesMean number of
blow flies per day ± SEaFPSite ASite BSite CChrysomya
rufifacies12.3±4.010.8±5.59.9±4.30.12.889Cochliomyia
macellaria12.7±3.5a5.5±2.2b8.9±4.0ab4.35.030Chrysomya
megacephala8.4±2.13.1±1.97.0±1.92.63.108Phormia
regina4.9±2.13.0±1.56.4±2.81.28.312Lucilia
coeruleiviridis5.4±2.52.6±0.58.0±0.52.18.175aFor significant ANOVA’s (P < .05) means with different letters
are significantly different at a family error rate of P = .5.Figure 1
Results of 1000 Monte Carlo simulations for blow fly data collected from
fly paper at all three alligator-decay sites. The test statistic for this
simulation is the total number of co-occurrences (i.e., the number of times a
species occurs on the same day in any pair of sites, summed for all species in the
analyses). Closed arrows indicate the critical values at the 95.0% level. The
observed total number of co-occurrences is shown with the open arrow.
Probability of co-occurrence is expressed as a percent.


## 4. Discussion

Watson
and Carlton
[16, 17] used alligator carcasses as models to study arthropod succession
on carrion in Louisiana.
Direct comparisons between our findings and of those made in Louisiana
are not possible for several
reasons. First, our study was done in the summer, and those of Watson and Carlton
[16, 17] were
done in the spring, fall, and winter. Secondly, the geographical location and
vegetation of the sites varied between the studies. Thirdly, the faunal composition of arthropods
associated with carrion may be different between the two studies. However, one
generalization may be made; Lucilia coeruleiviridis is the first blow fly to
arrive at alligator carcasses, and even other carcasses in Louisiana  and Alabama. Therefore, this blow fly species
may be very important in determining the PMI associated with alligators and
possibly other carrion. Lucilia
coeruleiviridis is the first blow fly attracted
to decaying dogs in Tennessee [9], white-tailed deer and pigs in
Louisiana [17], pigs in Florida [11], and
on a variety of mammals in northern Mississippi [15].
However, Cochliomyia
macellaria is the first to appear on decaying pigs in Texas
[31] and in South Carolina [10].Reasons
for the variability of calliphorid succession among our carcasses are unclear,
but may be related to subtle differences in the microhabitat in where each
carcass was placed or differences
in the carcasses themselves. Further attention regarding the floral, chemical,
and physical nature of microhabitats needs to be further explored to isolate
potential sources of variability in insect succession on carrion. We suspect
that the nominal increase in sunlight (lower canopy cover) that site C received
may account for the changes in blow fly succession and physical decay observed
at this site.The variability in insect succession
on carrion has been attributed to a multitude of variables. For example, carcass
size [32], seasonality [9, 18], time since initial
exposure of carrion [33], indoors versus outdoors [34],
sun versus shade [35], burning [36], burying [37], and hanging [38] have all been investigated.
Several studies have evoked the possibility of variation among replicated
carcasses, but none of these investigations confirm this suggestion through
direct observation (e.g., [6, 13, 39, 40]). Implicit to all carrion studies is the
idea that carcasses (of similar physical dimensions) placed in the same habitat
at the same time will exhibit limited differences in the rate of decomposition
or succession of insects. Our results indicate that this variability needs to be
considered in other model carcasses, such as pigs, a model commonly used to
establish baseline forensic data [41]. Although our work
needs to be repeated at different times of the year and in different habitats,
our results suggest that for any particular vertebrate model, replication is
critical.



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*Source: 575362-2008-11-19.xml*