Heat production in a feeding matrix formed on carrion by communally breeding beetles

Revisiting the ecology and evolution of burying beetle behavior (Staphylinidae: Silphinae)

Investigating fundamental processes in biology requires the ability to ground broad questions in species-specific natural history. This is particularly true in the study of behavior because an organism's experience of the environment will influence the expression of behavior and the opportunity for selection. Here, we provide a review of the natural history and behavior of burying beetles of the genus Nicrophorus to provide the groundwork for comparative work that showcases their remarkable behavioral and ecological diversity. Burying beetles have long fascinated scientists because of their well-developed parenting behavior, exhibiting extended post-hatching care of offspring that varies extensively within and across taxa. Despite the burgeoning success of burying beetles as a model system for the study of behavioral evolution, there has not been a review of their behavior, ecology, and evolution in over 25 years. To address this gap, we leverage a developing community of researchers who have contributed to a detailed knowledge of burying beetles to highlight the utility of Nicrophorus for investigating the causes and consequences of social and behavioral evolution.

Field validation of post-mortem interval estimation based on insect development. Part 1: Accuracy gains from the laboratory rearing of insect evidence

There is a consensus that forensic methods must be valid. The high quality of the method may be fully demonstrated only through validation. Unfortunately, there are very few experimental or casework validations of entomological methods for estimating post-mortem interval (PMI). Here, we present the first part of the results from the field validation of minimum PMI (PMImin) estimates based on insect development. From eight pig carcasses (24-46.4 kg) exposed in a forest habitat of Western Poland, one every one or two months, we collected insect evidence with standard entomological techniques. Using weather station temperatures and the thermal summation method, PMImin was estimated based on insect life stages that were reared under controlled laboratory conditions. Through rearing an insect until the next developmental landmark (or until eclosion), its age at the time of collection (and eventually PMImin) may be estimated by subtracting thermal accumulation in the laboratory from the corresponding thermal constant. We hypothesized that rearing insect evidence significantly improves the accuracy of PMImin compared to the estimation based on non-reared insect evidence. The results clearly supported this hypothesis. However, the accuracy significantly increased only in the case of these insects that developed normally during rearing. When their development in the laboratory was prolonged, PMImin was significantly less accurate. For the normal development sample the accuracy improved in all species and life stages. The largest accuracy gains from rearing were recorded for Calliphora vomitoria (Diptera: Calliphoridae) and Stearibia nigriceps (Diptera: Piophilidae). Moreover, when puparia or third instar larvae were reared, gains were larger than in the case of earlier life stages. In conclusion, this study demonstrated that the method of rearing insect evidence to improve the accuracy of PMImin is valid. However, it needs to be used with caution, since substantial part of the evidence may die or slow down their development in the laboratory, which violates assumptions of the method.

Calibrating insect age at eclosion by size in a gregarious carrion beetle Thanatophilus sinuatus (Staphylinidae: Silphinae)

Recent discoveries have shown that the physiological age at eclosion of forensically useful beetles differs between males and females and between beetles of various sizes. Accordingly, it was postulated that the size and sex of the beetles at eclosion may be used to calibrate their age, which may improve the accuracy of age (and post-mortem interval) estimates in forensic entomology. In this study, we derived thermal summation models for the eclosion for the Central European population of carrion beetles Thanatophilus sinuatus (Fabricius, 1775), (Staphylinidae: Silphinae), and tested the usefulness of sex and size for the calibration of beetle age at eclosion. Although in previous developmental studies, the beetles were reared individually, we reared them in larval aggregations, since in natural conditions T. sinuatus beetles are gregarious. Weak (r2 between 5% and 13%) negative correlations were observed between the size and age of T. sinuatus males or females at eclosion, demonstrating that calibration of age by beetle size and sex may bring only minimal benefits regarding the accuracy of age estimation in this species. However, it may still be worthwhile in the case of extremely large or small beetles. Moreover, the total development times recorded in this study were much shorter than in the previous T. sinuatus study, at 14°C by about 15 days and at 26°C by about 2 days. These differences emphasise the importance of gregariousness for the development of carrion beetles, and at the same time highlight the need for the ecologically-relevant protocols of development studies in forensic entomology.

Black soldier fly (Diptera: Stratiomyidae) larval heat generation and management

Mass production of black soldier fly, Hermetia illucens (L.) (Diptera: Stratiomyidae), larvae results in massive heat generation, which impacts facility management, waste conversion, and larval production. We tested daily substrate temperatures with different population densities (i.e., 0, 500, 1000, 5000, and 10 000 larvae/pan), different population sizes (i.e., 166, 1000, and 10 000 larvae at a fixed feed ratio) and air temperatures (i.e., 20 and 30 °C) on various production parameters. Impacts of shifting larvae from 30 to 20 °C on either day 9 or 11 were also determined. Larval activity increased substrate temperatures significantly (i.e., at least 10 °C above air temperatures). Low air temperature favored growth with the higher population sizes while high temperature favored growth with low population sizes. The greatest average individual larval weights (e.g., 0.126 and 0.124 g) and feed conversion ratios (e.g., 1.92 and 2.08 g/g) were recorded for either 10 000 larvae reared at 20 °C or 100 larvae reared at 30 °C. Shifting temperatures from high (30 °C) to low (20 °C) in between (∼10-d-old larvae) impacted larval production weights (16% increases) and feed conversion ratios (increased 14%). Facilities should consider the impact of larval density, population size, and air temperature during black soldier fly mass production as these factors impact overall larval production.

The Impact of Diet and Photoperiodism on the Life History of Thanatophilus sinuatus (Coleoptera: Silphidae)

The forensically important genus Thanatophilus Leach, 1815 (Coleoptera: Silphidae) is a widespread group of carrion beetles that occupies the Holarctic and Afrotropical realms. It has recently received more recognition, as its species are frequently detected on large carrion, including humans, and can be useful in estimating the time of colonization (TOC) and the post-mortem interval (PMI). The immature stages of Thanatophilus sinuatus (Fabricius, 1775) were reared on three different meat diets (pork liver, pork muscle, and chicken muscle). Diet influence on some of the essential life history traits of the species was examined for different populations. Our findings indicated an accelerated development on pork muscle followed closely by pork liver and a prolonged development on chicken muscle. The diet significantly affected the size, and the body condition (weight/ size) as both pork muscle and liver produced individuals in better physical condition (larger and heavier). The impact of two cyclic photoperiod regimes (16 h Light (L): 8 h Dark (D) and 12 h L: 12 h D) was further evaluated and indicated no significant variability.

When the red-lined carrion beetle disrupts successional dynamics on large vertebrate carcasses

In a field study of insect colonization on domestic pig carcasses, the typical colonization sequence took a different turn midway through decomposition when Necrodes surinamensis (Fabricius) (Coleoptera: Silphidae) dramatically increased in abundance on some carcasses. This provided an opportunity to test the prediction that N. surinamensis would aggregate at clumped carcasses but not as much at dispersed carcasses, thus leaving enough resources in less heavily colonized habitat patches to support weaker competitors, as predicted by the aggregation model of coexistence. Larger aggregations of this silphine beetle resulted in the collapse of blow fly (Diptera: Calliphoridae) maggot populations due to intraguild predation, and a decline or exclusion of late-successional insect species. This phenomenon has been reported in a European Necrodes species but it is the first time it is documented with N. surinamensis, a widespread species in North America. Substantial increases in adult and larval populations of N. surinamensis were first documented on clumped carcasses and were not as pronounced on dispersed carcasses. The specifics of N. surinamensis aggregations are discussed with reference to ecological and forensic implications, including the fact that feeding by N. surinamensis larvae makes the remains appear to have been altered by scavengers and decomposing for longer than they have.

Integrative taxonomy and species distribution models of the genus Diamesus Hope, 1840 (Coleoptera: Staphylinidae: Silphinae)

Integrative taxonomy of Diamesus Hope, 1840 (Coleoptera: Silphinae) is presented. Adults of D. bimaculatus Portevin, 1914 (endemic to Taiwan) and D. osculans (Vigors, 1825) (widely distributed from northern India to Australia) are redescribed, keyed and figured, including characters of the male and female genitalia of both species. Variation in elytral maculation in D. osculans is discussed and illustrated. The absence of diagnostic differences of D. osculans var. reductus Pic, 1917 from D. osculans is discussed, and the former name is confirmed as a junior subjective synonym of D. osculans. Types of all three names available were studied; a lectotype and paralectotypes are designated for the name D. osculans var. bimaculatus Portevin, 1914. Molecular phylogenetic analysis confirms the genus Diamesus is sister group to the genus Necrodes Leach, 1815, and D. osculans and D. bimaculatus are two, well supported clades. Detailed data on the distribution of D. bimaculatus and D. osculans are presented and mapped. Species distribution models for both species were created and interpreted. Diamesus osculans is reported for the first time from India: Uttarakhand, China: Anhui, Hainan, Hunan, Jiangxi, Shaanxi and Zhejiang Provinces, and Australia: Victoria; it is also recently confirmed from Taiwan, being sympatric in distribution there with D. bimaculatus. Available data on the ecology and seasonality of both species of Diamesus are also discussed.

Initial laboratory validation of temperature development models for Necrodes littoralis L. (Staphylinidae: Silphinae)

Development models of necrophagous insects are applied in forensic entomology for post-mortem interval estimation. Such estimates may be used as scientific evidence in legal investigations. For this reason, it is important that the models are valid and that the expert witness is aware of their limitations. Necrodes littoralis L. (Staphylinidae: Silphinae) is a necrophagous beetle species that frequently colonizes human cadavers. Temperature models of development for the Central European population of these beetles were recently published. In this article, we present results of the laboratory validation study for these models. Errors of beetle age estimation differed significantly between the models. Thermal summation models yielded the most accurate estimates, and the isomegalen diagram least accurate estimates. Errors of the beetle age estimation varied across beetle developmental stages and rearing temperatures. In general, most development models of N. littoralis were satisfactorily accurate in estimating beetle age under laboratory conditions; therefore, the study provides initial evidence to support their validity in forensic cases.

Differences in sibling cooperation in presence and absence of parental care in a genus with interspecific variation in offspring dependence

The widely spread evolutionary strategy of parental care is considered an important driver of social evolution. Although offspring were long thought to primarily interact competitively, recent studies revealed the potential importance of sibling cooperation. Theories suggest that the degree of cooperation in offspring interactions depends on the degree of offspring dependence on parental care: offspring unable to forage on their own should compete more, whereas more independent juveniles may increase the degree of cooperation. In this study, we tested the occurrence of sibling cooperation in the absence of posthatching care in several burying beetle species exhibiting varying degrees of offspring dependence. To this end, we measured larval growth rate and survival in the presence and absence of prehatching care using different brood sizes. We found that sibling cooperation cannot be exclusively explained by offspring dependence on parental care. Although only species with more independent larvae cooperated when receiving prehatching care, larval cooperation occurred across species in the absence of care. The latter result suggests that sibling cooperation was already present in an early ancestor of the genus Nicrophorus. Overall, these findings give important insights into the transition from facultative to obligate family life.

Competition of insect decomposers over large vertebrate carrion: Necrodes beetles (Silphidae) vs. blow flies (Calliphoridae)

Large carrion is inhabited by highly variable and interactive communities of insects. Positive interactions in carrion insect communities have been recently the focus in carrion ecology. In contrast, competition between carrion insects is rather undervalued. Here we provide evidence that blow flies (Calliphoridae) and Necrodes beetles (Silphidae), dominant decomposers of large carcasses in terrestrial habitats, compete over carrion. By reanalyzing the results from 90 pig carcasses, we demonstrated that the contribution of the flies and the beetles to the decay was negatively related. The greater part of the large carrion pool was monopolized by blow flies, whereas Necrodes beetles abundantly colonized carcasses, on which blow flies were less effective as decomposers. In behavioral assays, we found that adult beetles killed 4 times more frequently feeding than postfeeding third instar larvae of the flies, with the large decrease in the killing frequency after the larvae reached the age of early third instar. Therefore, adult Necrodes beetles preferentially killed the larvae that were before or in their peak feeding. The study provides evidence that the interaction between blow flies and Necrodes beetles is a combination of indirect exploitative effects of the flies and direct interference effects of the beetles (the mixed competition).

Insect rearing protocols in forensic entomology: Benefits from collective rearing of larvae in a carrion beetle Necrodes littoralis L. (Silphidae)

Forensic entomologists frequently use a developmental method to estimate a post-mortem interval (PMI). Such estimates are based usually on the blow fly larvae or puparia. Data on their development is obtained by rearing them in colonies. In the case of beetles, which can be also useful for PMI estimation, development data is frequently collected by rearing them individually. However, some carrion beetles are gregarious, for instance, Necrodes littoralis (Linnaeus, 1758) (Silphidae). We compared mortality, rate of development and body size of emerged adult beetles reared individually and in aggregations. Mortality was much higher for beetles reared individually, particularly at low temperatures. The rearing protocol affected the time of immature development and the size of adult insects. Individually reared specimens developed much longer at 16°C, whereas at 20°C and 26°C development times of individually reared beetles were slightly shorter. Significant differences in the body size were observed only at 16°C; beetles that developed in aggregations were larger at this temperature. These findings demonstrate that aggregating is particularly beneficial for larvae of N. littoralis at low temperatures, where it largely reduces mortality and facilitates growth. Moreover, these results indicate that in forensic entomology the protocol of individual rearing is unsuitable for gregarious beetles, as it produces reference developmental data of low quality.

Convergence of Social Strategies in Carrion Breeding Insects

Carrion is a highly ephemeral and nutrient rich resource, characterized by extreme biotic and abiotic stressors. We hypothesized that specific constraints of the carrion ecosystem, and especially its nutrient richness, ephemerality, and competition with microbes, have promoted the evolution of social behaviors in necrophagous insects. We show that group living is prevalent among early succession carrion breeding insects, suggesting that this trait has emerged as an adaptation to facilitate survival in the highly competitive environment of fresh carrion. We then highlight how developmental niche construction allows larvae to compete with microbes, efficiently feed on fresh cadavers, and rapidly reach maturity. We observed that larval societies and parental care are two different strategies responding to similar competitive and environmental constraints. We conclude that intra and interspecific competition on carrion are mitigated by social behavior.

Post-Mortem Interval Estimation Based on Insect Evidence: Current Challenges

During death investigations insects are used mostly to estimate the post-mortem interval (PMI). These estimates are only as good as they are close to the true PMI. Therefore, the major challenge for forensic entomology is to reduce the estimation inaccuracy. Here, I review literature in this field to identify research areas that may contribute to the increase in the accuracy of PMI estimation. I conclude that research on the development and succession of carrion insects, thermogenesis in aggregations of their larvae and error rates of the PMI estimation protocols should be prioritized. Challenges of educational and promotional nature are discussed as well, particularly in relation to the collection of insect evidence.