Problems in Estimating the Postmortem Interval in Death Investigations l

E. P. Catts

Department of Entomology Washington State University Pullman, Washington 99164

J. Agric. Entomol. 9(4);245-255 (October 1992)

ABSTRACT Estimation of the postmortem interval (PMl) using faunal diversity, development and succession in human death investigations is based on a number of assumptions. Investigators should not be overly dependent on the few baseline studies currenlly in use to estimate the PM!.

This paper reviews a number of problems to be considered when using inse<:ts to estimate the PMI:

1. Insect activity in inclement weather and at night. 2.1'iming of blow fly oviposition. 3. Mixed fly populations and staggered broods. 4. Seasonal efTects on maggot-generated heat. 5. Effect of maggot-generated heat. 6. Impact of species arriving out of sequence. 7. Availability of gravid blow ny recruits at the scene. 8. Effect of drugs, toxins, and parasitoids on maggot development.

A discussion of these problems is presented along with some approaches to their solution.

KEY WORDS Forensic entomology, postmort.cm interval, COl-pSC, insect fauna, blow nics, Calliphoridae, Diptcra.

The major contribution made by a forensic entomologists in a homicide investigation is an estimate of the duration of the postmortem interval (PMI). Some added contributions include indication of movement of the corpse, and possibly the manner of death. Estimating the PMI involves the setting of the maximal and minimal probable time interval between death and corpse discovery (Fig. 1). The maximal limit is determined by the species of insects present, and the "weather windows" available for activity of these species. The habits and mix of species can be used to give a crude estimate of the earliest time of corpse exposure. 'Weather windows are intervals of conditions, usually warmer tempel'atures and sunshine, when carrion-seeking species are likely to be active. The minimal limit is determined largely by estimating the age of developing immature insects at the time of corpse discovery. The relationship between the age of immatures and the PMI is determined from a few published baseline studies with rates adjusted by interpolation t.o include the influence of climate, season, weather, and location.

I Received for publication 6 March 1991; accepted 14 February 1992.

245

246 J. Agric. Entomol. Vol. 9, No.4 (1992)

Fig. 1. A generalized death and decomposition scenario relating to the setting of an estimated post mortem interval (PM!) by using the analysis of faunal evidence. Note that discovery may occur at. any phase of corpse decompo­sition (modified from Catts and Haskell 1991).

An accurate PMI is of primary importance in any homicide investigation because it may narrow the field of suspects and may aid in identification of the deceased. However, the estimated PMI is based on a number of questionable assumptions, not the least of which is that an insect population is available to exploit the corpse immediately following death. The entomologist's estimate is based on a chain of generally valid assumptions, but variance or error in any one of these assumptions can skew the accuracy of the estimated PMI. This paper will identify some of these assumptions and their weaknesses in an effort to channel future research to improve the accuracy of PMI estimates.

Smith (1986) listed several critical areas needing further research with regard to faunal succession in corpse decomposition:

(1) Effect of premortem eating, drinking, drugs and toxin. (2) Effect of the cause of death. (3) Effect of postmortem treatment of the corpse. (4) Effect of corpse location. (5) Refinement of the effect of heat and humidity. (6) Studies of faunal biology and ecology.

247 CATTS: Estimating the PMI

The emphasis here will be on the early successional arthropods, primarily blow mes (Diptera: Calliphoridae). Blow mes are generally the most dominant, and conspicuous insects in the decomposition process, and their occurrence and biology are used most often in making a PM] estimate.

At present PMI estimates are drawn from a few baseline studies (e.g., Kamal 1958, Greenberg 1991) which require careful interpolation to adjust faunal occurrences to each peculiar death investigation. However, baseline studies are done most often under a controlled laboratory regimen, making interpolation with incomplete field data specious. Studies should be expanded and tested in controlled field situations in order to improve the accuracy of the interpolations. The ultimate goal is to remove reasonable doubt regarding the accuracy of the final PM1 estimate.

Before reviewing the typical PM] scenario some general parameters, listed below, should be considered which apply to the more than 60,000 deliberate deaths (homicides and suicides) in the U.S. each year (Anonymous 1988).

(1) Most homicides occur at night, under cover of darkness, when primary carrion invaders are presumably inactive.

(2) Most homicides are by gunshot (61%), and by stab or slash incisions (14%) (Anonymous 1987).

(3) Most crime investigators only infrequently work on homicide cases. Very few know much about entomology. Large metropolitan areas can assign specialists to such duties. but small urban and rural law enforcement agencies have neither the personnel nor experience for this specialization.

PMI Scenario

Most murders occur under the cover of darkness when first seral wave insects (flies) are not actively seeking carrion. The victim may be whole, muti­lated or shrouded in some way, which can have an effect on time of insect dis­covery and on the rate of subsequent decomposition (Smith 1986). Death may have occurred at the death scene (where the corpse is discovered), or elsewhere, with postmortem transport of the victim to that location. Corpse transport may be a deliberate action by the murderer, the action of secondary scavengers, or of natural phenomena such as landslide, avalanche, or free fioating water trans­port. With the coming of daylight and warming air temperatures insects will discover the corpse. Generally, blow flies are first to arrive and in general their activity requires ambient air temperatures of at least 10°C (Williams 1984).

The first wave of insects, especially certain blow mes and Oesh mes (Sarc­ophagidae), are attracted to the natural moist body openings, open wounds, and/or pooled blood of the victim where they feed and oviposit. An array of arthropods follows in a rather predictable, successional sequence of waves (or seres) to inhabit either the corpse or the seepage area beneath the corpse. These animals feed on the corpse, on corpse seepage, or on other fauna attracted to the remains.

Corpse decomposition passes through a gradation of decompositional phases from fresh to bloat, to decay and to dried or putrid remains (Payne 1965) (Fig. 1).

248 J. Agric. Enlomol. Vol. 9, No.4 (1992)

The weight and size of the corpse decreases rapidly during the active decay phase largely due to the consumption of corpse tissue by developing maggots which crawl away from the body when fully developed, and to loss of Ouids mostly through seepage. This drop in biomass separates active and advanced phases of decomposition (Fig. 1).

After human discovery of the decomposing corpse, death investigators at the crime scene may make representative collections of the more conspicuous inver­tebrate fauna. More often this aspect of death investigation is regarded as repulsive and the collections are left to the coroner during autopsy. Usually these investigators have little knowledge or experience in what to collect, how to collect and how to handle or process insect specimens properly. The rate of maggot development and the identity of fly species which are taken from the remains are generally key evidence for the entomologist in estimating the PMI.

Preserved specimens are identified to species, and to stage and age of develop­ment. Any living immatures collected, are reared to confirm the species detenni­nations made from preserved immature specimens. From these identifications, and from the estimated age of specimens collected from or under the corpse, the forensic entomologist estimates the probable PMJ and submits a formal report justifying this conclusion.

Events Which Influence Accuracy of Estimated PMI

Are there flies ready to start the clock? The clock time of oviposition by the initial wave of carrion invaders is of paramount importance in making the PMI estimate. One assumption usually made is that there is a ready population of first wave insect recruits, especially nies, waiting to begin oviposition on the fresh corpse as soon as environmental conditions allow. This is often the case, especially in mid-summer. However, immediate oviposition may not occur even with those species which prefer carrion in the early fresh phase. Generally female blow nies begin to oviposit from several days to a week or more following emergence and mating (Greenberg 1973). After that they continue to oviposit over a 2- to 3-wk period (Kamal 1958). Thus, even though blow mes may be attracted immediately to fresh remains, they may not O\'iposit, but may undergo a preovipositional pause. They also may feed on pooled blood as a protein sup­plement and then wait for digestion and assimilation of the meal. Even though gravid female nies can retain eggs for 1-2 wk in the absence of a suitable ovipo­sition medium, nies are not uniformly distributed over the landscape. They tend to concentrate in shaded, moist areas with taller canopy vegetation, as may be characterized by an open field/woodland ecotone (Norris 1965). Thus, their dis­tribution tends to be in elongated clumps which follow the local topography and plant community types (Hightower and Alley 1963, Greenberg 1973).

\Vhen are flies active? Another assumption is that first wave Diptera will not actively see carrion 01' oviposit after sundO\\'n, or during inclement weather. However, there is some evidence of nocturnal flight activity and oviposition by some Calliphora species (Green 1951). Recently Greenberg (1990) demonstrated the willingness of Phoenicia sericata (Meigen) to oviposit at night in an urban field setting even though this species is usually diurnally active. This may relate to the closeness of gravid flies to the death scene. Such a situation should be

249 CATTS: Estimating the PMI

anticipated if topography and plant cover indicate proximity of a Oy concentra­tion site with the corpse. On the other hand insects such as ants and beetles ­predators of fly eggs and maggots - show increased foraging activity at carrion during nocturnal periods. Also, brief periods of sunshine interspersed in what might otherwise be considered inclement weather are sufficient to permit adult fly activity at carrion. During cooler seasonal periods sunshine appears to be a more important blow fly activity stimulus than is the ambient air temperature threshold (Greenberg 1973l, probably because of radiant warming to a night threshold above ambient temperatures by direct or reradiated heat. A gravid female of the calliphorid Cynomya cadaverina (Robineau~Desvordy)immobilized on carrion by cold temperatures was observed to resume egg laying when exposed to direct sunlight for only a brief period (personal observation). The influence of meteorological factors on fly activity has been aptly reviewed by Greenberg (1973) but still is an important subject for future forensic research.

What were the weather conditions? Generally, local weather records are used to reconstruct the ambient temperatures experienced by corpse-associated fauna. \Vhether or not conditions recorded at a weather station some distance from the corpse site are valid needs to be considered. Even with a recording sta­tion close by, this should be questioned. Although gross weather conditions may be quite similar, microhabitat conditions may differ considerably and may not be comparable. Vegetational cover, air drainage and slope exposure all influence microhabitat conditions greatly.

Completion of blow fly maggot development is manifested by the rapid loss in corpse body weight when post-feeding maggots leave the remains (Fig. 1). Local site differences were revealed in a study of paired decomposing pigs near Olympia, Washington (E. P. C., unpublished datal. Although both pigs (20-22 kg weight) were located in the same continuous woodland 300 m apart, one carcass was in open sun and one is shade provided by the tree canopy. Air temperatures at the exposed site typically were higher than those for the shaded site, except at night, and the pig in the open was reduced to 17% of its body weight 18 d sooner than the pig in the shade.

Topography, slope exposure and broken cloud cover in a general area produce overriding modifications of area macro-weather. Averaging daily records of tem­perature highs and lows gives only a crude idea of actual site conditions. Conse­quently, calculations of accumulated degree·days (ADD) to estimate the dura­tion of maggot development is often based on weak reference air temperature data. ADD summations using soil temperatures appear to have greater valid.ity. A partial remedy for this problem is to record weather at the death scene for 3-5 d following removal of the corpse to give a basis for some interpolation of consistent weather differences with those of the more distant recording station (Catts and Haskell 1991).

Maggot-mass generated heat. Ambient air temperatures does playa dom­inant role during egg and early larval development, but its effect decreases rapidly. This is because the temperature of'the substrate on which the maggots feed may be warmed to a large degree by maggot and microbial activity, which generates considerable metabolic heat. A large maggot-mass results from an abundance of eggs following "oviposition frenzy" by aggregated gravid female

250 J. At.,,'"ric. Entomol. Vol. 9, No.4 (1992)

blow flies. Eggs develop as the body cools and early maggots develop at a rate which is in synchrony with fluctuations in ambient temperature. In the late sec~

and and entire third iostar however, heat generated by maggot metabolic activi­ties far exceeds ambient temperature and, in effect, produces a stable optimal heat level for accelerated larval development. Lahoratory studies (Kamal 1956) conducted under constant temperature are generally done with maggots too low in numbers and at heat levels too low to simulate this metabolic heating effect. Further, by moving in and out of the mass, individual maggots can somewhat control their own environmental temperatures.

In field studies in Tennessee using swine carcasses and human corpses as surrogate murder victims (N. H. Haskell, Purdue Vniv., personal communica­tion), the maggot-mass heated regions of the body stayed well above that of the air temperature (except at night) so that the skin surface of a carcass in active decay felt warm. Payne (1965) reported a similar situation in his study using thawed fetal pigs. In pig studies during the fall season in Washington maggot­mass temperatures exceeded subfreezing ambient low temperatures by as much as 35"C, even though carcass anal temperatures were only gOC above the low (unpublished data). The highest maggot-mass temperature recorded by this author was 50°C, a level considerably above laboratory baseline studies used to establish maggot development rates. Regardless of season, maggot-mass tem­peratures ranged as much as 35-45"C higher than ambient low temperatures and about 20"C above ambient high temperatures (Fig. 2A, B).

The lower threshold of maggot density needed to generate heat sufficient to override ambient fluctuations is not known, and probably differs among species and perhaps among geographic races within species. Marchenko (1988) reported that accelerated development of maggots began when maggot density was equal in ratio with the weight of the medium (i.e., one maggot/g of medium). Goodbrod and Goff (1990) reported internal maggot-mass feeding temperatures above ambient for different population densities in the laboratory, with maximal tem~

peratures occurring at densities 'of 20 and 40 larvae/g of liver for two blow ny species. Conversely, puparial weights, lengths and adult weights at emergence decreased with increasing population density. Although the population densi­ties in their study appear to be high, they do regularly occur within a corpse in the zone of concentrated maggot-mass activity. Apparently, temperatures in the 40"C range are not detrimental to developing maggots for a brief duration. As maggots move from the carcass in the prepupal or postfeeding period (Figure 1, maggot exodus) there is an exaggerated loss of corpse biomass and the heat of the mass transfers to the ground underlying the remains so that subcarcass temperatures also may exceed ambient temperatures (Fig. 2). As maggots from the mass scatter and pupariate, this ground heat also dissipates.

The heat associated with the maggot-mass serves to heat-load more or less of the carcass, depending on the season (Fig. 2). The highest internal temperatmes are recorded during active decay, that period when the maggots and the mass are more developed. In mild seasons heat-loading may include most of the remains. In contrast, during cool seasons this loading effect is masked because the generated heat dissipates more rapidly due to lower ambient temperatures. The mass then appears to be more compact and the generated heat more localized (Fig. 2B).

251 CATI'S: Estimating the PMI

MAGGoT ~

MASST.--; ........ /

® MID-SUMMER45 ,/

/ . / I \. - CARCA'E>5 CORE T., .

/SUBC.ARCA'i>'5 !35 , T.-/

/ . HIGH AIR T. ! ,(.......

..i. I

i

LOWAIRT• /.. .........._--'/

././

............. 15 '"

'

5 FRE.EZING T.

O+-:...,..cc:;=.=-.::-,--~~~~-..,---.-,-~~~ o 5 \0

DA'<'E> PO'E>TMOR"rE!"\

45

...... .•..

3S

.... ......

..................... /HIGH AIR T. h _' •

.;-....;: "":--_............-.­15 '''''''''''

" -.......­...... ..... ......-'-._._-­..... .•..•..S

.... o FREEZING T. '"

" . .......... -5

•••• __ h _ ~.~.~~ A~R ..~..

-15

\0 15o 5 DAYS POSTMORTEM

Fig. 2. Mid-summer (A) and late fall (B) air temperatures (T), of carcass core, subcarcass and maggot mass recorded at pig carrion, 10 mi SW, Pull­man (Whitman Co.), Washington.

252 J. Agric. Entomol. Vol. 9, No.4 (1992)

Another important consequence of maggot-mass heat is that maggot develop­ment may not be arrested when a corpse is held in a refrigerator prior to autop­sy (N. H. Haskell, Purdue Univ., personal communication). Thus the collection of maggot specimens for preservation at the death scene is preferred if an accu~

rate estimate of PMI is to be made, because maggots collected from a mass may have continued to grow while the body and maggot-mass were in cold storage before autopsy.

The effect of maggot-mass warming may also differ seasonally. In temperate regions during the spring season with gradual warming trends, the rate of egg and early immature maggot development is governed largely by changes in ambient temperatures. This is reversed in the fall season, and the seasonal cool­ing trends probably have much less eITect on maggot development rates because of maggot-mass generated heat.

An abundance of foraging predators, such as ants or yellowjacket wasps, also can have an impact on the rate of decomposition (Early and Goff 1986) and could affect the rale of maggot development. The removal of eggs and smaller maggots may reduce the size of the maggot-mass or disperse the mass sufficiently to reduce the metabolic heat produced. In a cool climate the rate of maggot develop­ment may then be depressed. Further study is needed on 1) the minimal size of the maggot-mass sufficient to modifY substrate temperatures, and 2) the level of predation necessary to deplete the maggot-mass so that generated heat is too rapidly dissipated to accelerate maggot grov.rth.

Numerous workers have noted that carrion in nature may be overloaded with a far greater number of eggs than there is food for maggot development (Norris 1965). This usually results in the production of adult mes of reduced size and fecundity (Greenberg 1973). Generally this is attributed to the adverse effects of competition (Smith 1986). However, heat-loading of the corpse by the developing maggot-mass appears to be one unrecognized benefit of an overabun­dance of blow fly eggs by allowing a more rapid development of the vulnerable maggot stage. Another benefit is an enhanced capacity for wider dispersal of progeny by a greater number of smaller female flies. Perhaps those maggots which hatch early have the advantage of an unlimited food supply over later arrivals, but the benefits of maggot-mass generated heat may offset this advan­tage for maggots developing from eggs laid during the oviposition frenzy.

Generally, studies of maggot development rates have concentrated on single species populations of uniform age. In those studies whel'e mixed-species popu­lations were studied, the outcome of interspecific competition was the focus, not the influence of the interaction on developmental rates of species involved. There also has been little study of interactions involving mixed age classes of single or mixed species of maggot populations. [n reality, the maggot-mass in the decomposing remains is a complex of species in mixed-age classes. The prob­able impact of the maggot-mass heat generation phenomenon on development rates, and ultimately on determination of the minimal PMI, demands much more study.

It is no longer common practice in homicide investigations to record the tem­perature of the corpse at the crime scene. But such information can be very valuable in estimating the PM] from maggot development. Routine death scene

253 CATI'S: Estimating the PMI

investigation procedures should include a record of temperatures for the body surface (shaded>, beneath-corpse substrate, and the maggot-mass whenever possible, prior to movement of the remains (Catts and Haskell 1991).

Movement of the corpse. Disturbance of the decomposing remains inter­rupts the successional process. Even in controlled field studies the infrequent and brief lifting of a decomposing animal carcass on a rigid wire grid was noted to prolong decomposition over that of undisturbed adjacent carcasses (M. L. GoIT, U, of H3\\'aii, N. H. Haskell, Purdue Univ., personal communications). In areas where large carrion scavengers occur (e.g., bears, coyotes, feral dogs, eagles and vultures), moving of a decomposing corpse probably has a similar effect. Road-killed deer carcasses may be moved 20-30 m during the night, pre­sumably by coyotes (personal observation). Haglund et al. (1989) reported that 30 of 46 corpses were disturbed by canid scavengers. Some were scattered or moved as much as 160 m. During warmer seasons this disturbance happens early during decomposition; during cooler seasons it may occur over a much longer period. Perhaps this disturbance of a corpse may delay the normal devel­opment of associated fauna enough to influence the PM] estimate.

Premortem condition of the deceased. The effect of substances ingested, imbibed or otherwise taken internally prior to death was identified by Smith (1986) as an area needing research in forensic entomology. Nuortev8 and Nuorteva (1982) showed that a lethal toxin such as mercury has an adverse effect on maggots feeding on contaminated tissues. In laboratory experiments Goff et al. (1989) demonstrated that serial levels of cocaine and its by-products in tissues proportionately accelerated the rate of development of a sarcophagid fly. Further, in one homicide case study, developing maggots of P. seniata showed a similar accelerated growth response to cocaine (Catts and Haskell 1991). One late-instal" maggot from the nasal passages of' the deceased was nearly double the size of all others from the same sample. The deceased in this case had been seen alive 1 wk earlier and yet the larger maggot indicated a PMI of at least 10 d. Further investigation established that the deceased had "snort­ed" (inhaled) cocaine 6 h prior to death. This suggests that the Jarger maggot had fed on cocaine·enriched tissue which had accelerated its growth. Because larger maggots generally require a longer development time, they are often used as key indicators of minimum PMI. The effect of drugs and toxins on devel­oping maggots is another area deserving much more research and careful con­sideration in estimating the PMI.

A Need For Expanded Research

Numerous case studies have demonstrated that forensic entomology is a valuable adjunct to death investigations (Nuorteva et al. 1974, Erzinclioglu 1983, Greenberg 1985, Goff and Odum 1987). However, our knowledge of the biology and ecology of fauna associated with the decomposing corpse must be refined. The problems discussed in this paper relate to estimating the PMI, a key element in any death investigation. They are all areas in need of immediate research efforts. Studies should include experimental field as well as laboratory research. They should be expanded to include buried, emersed, burned or enclosed carrion. [n this paper the discussion has focused on blow fly ecology

254 J. Agric. EntomoL Vol. 9, No.4 (1992)

because more is already known about these insects. The array of other inverte­brates associated with corpse decomposition is extensive and complex, and much more knowledge of them is needed.

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255 CA'ITS: Estimating the PMI

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