*3.1.1. Extending the PMI*

In many ways, parasitoids offer similar evidence as necrophagous flies, only with regard to a different period of time. For instance, many species of Hymenoptera parasitize older larvae, prepupae, or puparial stages of fly hosts, and do not emerge from the host until a few to several

weeks after unparasitized necrophagous flies have eclosed. Thus, if the developmental parameters of a given parasitic wasp are fully understood, such species can extend the PMI window from the time flies initiate dispersal behavior until the corpse is discovered. This window of time can potentially represent 2–4 weeks or longer depending on the environmental conditions and season. The best example is with *N*. *vitripennis*, a wasp that parasitizes the puparial stages of flies predominantly in the families Calliphoridae and Sarcophagidae, although muscids are readily utilized if discovered by foraging females. Fly hosts cannot be parasitized prior to hardening of the puparium; meaning after pupation is complete, but prior to the onset of eclosion behavior. Thus, a precise window exists into the minimum length of host development on a corpse prior to parasitism by *N*. *vitripennis*. A similar relationship exists for other pupal parasitoids in the genera *Trichomalopsis*, *Muscidifurax*, *Pachycrepoideus*, and *Spalangia*, but they rarely are associated with carrion, and even then, parasitism is typically restricted to muscids [35, 45]. Larval parasitoids from the families Braconidae, Diapriidae, Encyrtidae, and Ichneumonidae show less host specificity than pupal parasitoids, preferring post-feeding larvae as hosts, but also ovipositing in younger larvae and pre-pupae [26, 46, 47] (**Table 1**). Their use in PMI estimations would yield broader time ranges than those based on pupal parasitoids development on fly hosts. A similar trend is true for members of the family Figitidae that deposit eggs in young fly larvae [48], although their host age preferences are narrower than more common larval parasitoids found on carrion. The reality is that PMI estimations based on parasitoids is markedly more complex than those derived from flesheating flies. It is also not as simple as the suggestion by Frederickx et al. [37] that the develop‐ mental time of a given parasitoid is just added to the duration of host development. Why not? Parasitoid progeny development is influenced by multiple factors beyond just ambient temperatures, including host age, size, physiological state, species, whether the fly has been previously parasitized or not, and the size of larval feeding aggregations experienced by the host [31, 49–52]. Developmental data for a particular species of parasitic wasp must also be derived from each relevant host species for use in PMI or period of insect activity calculations [29]. This thought must be extended to also include a wide range of developmental parameters that influence parasitoid growth for *each host utilized by each parasitic wasp* discovered at a crime scene. Such data are available for very few parasitic wasp species.

length) and tendency to arrive during later stages of decay, when the early fly colonizers have already dispersed or are nearing the wandering stage associated with post-feeding. In practice, early colonizers are favored as ecological evidence for all the reasons given for necrophagous Diptera. More significant than size or period of activity on carrion is that the life history characteristics of most parasitoids, other than the pteromalid *Nasonia vitripennis* (Walker), that frequent carrion have not been examined or only limited aspects of the biology and behavior of a few species are known [26, 27]. For example, developmental thresholds and temperatureinfluenced developmental data have been worked out for only two parasitoids, *N*. *vitripen‐ nis* and *Tachinaephagus zealandicus* Ashmead (Hymenoptera: Encyrtidae) [28–31]. Even less is known about seasonal occurrences of parasitoids, with the most extensive work being conducted on *N*. *vitripennis* and to a much lesser extent with *T*. *zealandicus* and *Alysia mandu‐ cator* (Panzer) (Hymenoptera: Braconidae) [29, 31–34], and the parasitoid fauna of most biogeographical regions has never been examined [35–37]. The data available for most species relates to their potential as biological control agents of filth flies, namely muscids, which generally do not translate to carrion communities, or the parasitoids of such flies are not encountered on animal remains [29, 38]. Despite these limitations, several parasitoids have been collected from forensically important flies in Australia, Europe, South America, and United States, and thereby are purported to be potential forensic indicator species [25, 29, 36, 37, 39–41]. In Section 3.1, an examination of whether such potential truly exists for parasitic Hymenoptera is discussed, as will the areas of parasitoid biology in need of further investi‐

Many species of parasitic wasps do show promise as alternative forensic indicator species, especially pupal parasitoids. The question that must be asked is if parasitic Hymenoptera were not overlooked at crime scenes (and, of course, the needed life history data were available), what information could they reveal about death? There are at least four pieces of information that can be derived from parasitic wasps in forensic investigations: (1) Parasitic wasps can extend the PMI window to include the period of time after necrophagous flies cease feeding to when a corpse is discovered, (2) wasp host preferences and seasonal occurrences can reveal if a body was moved from another location prior to discovery, (3) artifacts of past wasp activity remain at the scene for many years after they have dispersed permitting interpretation of period of activity and seasonality, and (4) foraging behavior of adults can be used to locate concealed bodies [1, 23, 42, 43]. In some instance, parasitic wasps have already been useful in case studies (i.e., PMI estimations; [23, 25, 40, 44]), and in yet others, the full potential of fly parasitoids has not been realized because key aspects of their biology remains poorly under‐

In many ways, parasitoids offer similar evidence as necrophagous flies, only with regard to a different period of time. For instance, many species of Hymenoptera parasitize older larvae, prepupae, or puparial stages of fly hosts, and do not emerge from the host until a few to several

gation to put them in line as alternative forensic indicator species.

**3.1. The case for forensic relevance**

72 Forensic Analysis - From Death to Justice

stood.

*3.1.1. Extending the PMI*


**Table 1.** Common families of parasitic Hymenoptera collected from human remains

Further complicating the host-parasite relationship in terms of predicting wasp development times is the physiological state of the parasitoids and conditions of parasitism. Female age directly influences efficiency of foraging behavior for hosts, the length of time needed to parasitize a host, which can be especially long for parasitoids using concealed hosts, and the quality of eggs deposited on a host. Eggs from older females may fail to hatch or larvae may spend more time feeding than is typical of progeny produced by younger adults [53, 54]. A similar effect is associated with larval development on flies that have been previously parasi‐ tized by conspecifics or allospecifics [55–57]. Failure to take into account each of these influences can lead to inaccurate calculations of developmental thresholds and estimations of wasp development times, as in most instances unfavorable host conditions increases the duration of parasitoid development [51, 58, 59]. The complex interactions between parasitoids and their hosts underscore the need for standard protocols in collecting wasp development data for use in forensic entomology [31].
