Water, Feeding Efficiencies, and Development of Dermestes maculatus DeGeer (1774)

Introduction

“Thousands have lived without love, not one without water.” ̶ [1].

Dermestid beetles are unique among decompositional insects because of their consumption of dry tissue. Most insects decomposing animals either feed on soft tissues (e.g., blow flies, flesh flies, filth flies, carrion beetles, cheese skippers, and coffin flies), are predators on decompositional insects (e.g., rove beetles, clown beetles, and wasps), or are drawn to animal carcasses because of an associated change in environmental conditions (often increased humidity) (e.g., springtails) or the need for salts (e.g., butterflies) [2].

The varied species in this family live in multiple habitats, provided there are reliable resources available [3]. Dermestids vary greatly in the length of their life cycles, with some species going from egg to adult in six weeks and others taking as long as a year or more to complete development [4]. Like all holometabolous insects, dermestids have four life stages (egg, larvae, pupa, and adult), with a variable number of larval stages but typically 5-6. While many beetle species have long generation times, after one year or longer, many dermestids can complete a generation in weeks to a couple months, which lets them better use temporary resources. However, like other beetles dermestids are relatively long lived (weeks to months) as compared to many other insects.

For most dermestids larva size ranges between less than 1 millimeter after hatching and up to 12mm pre-pupation. In the 15 to 20 days following hatching, larvae spend most of their time going through rapid growth and feeding. Under ideal circumstances this developmental stage spans between 18 and 20 days. Dermestids have different numbers of molts depending on species, and within species some may have variable numbers of larval molts (e.g., 5-9 based on rearing temperature in Dermestes alter DeGeer) [5], facultative diapause (dormancy) (in Trogoderma granarium Everts), and even a rare variation in metamorphosis called retrogression (also in Trogoderma granarium) arising from limitations in food availability, such as through competition [6]. Retrogression is a form of “reverse molting” in which the individual has reduced weight and size after molting and requires additional molts to return to a normal developmental sequence based on limitations in food availability, competition, and completion of other larvae during development.

Although dermestids have varying ecological roles, most in the Dermestes are generally associated with decomposition of dry animal tissues [7], with Dermestes maculatus DeGeer probably the most ubiquitous in North America. Because these beetles are typically found on dried carrion, it is generally assumed that their feeding preference is exclusively for dry tissue. Indeed, dermestids, including D. maculatus, are regarded as specialists on dry tissues in the context of the array of insects feeding on dead animals [8]. Additionally, Dermestes can survive long periods without food and through periods of near desiccation. However, when used for de-fleshing bones, dermestids feed on a combination of wet and dry tissue, and D. maculatus and related species are sometimes found on human bodies early in decompositional succession. Consequently, whether or not D. maculatus is adapted for feeding solely on dry tissue is unclear.

To address this question, a logical approach is to look for differences in development rates between D. maculatus reared on wet and dry food. The development of D. maculatus was examined by Zanetti, et al. [9] who reported rates by larval stage for 6 temperatures (15, 20, 22, 24, 27, 30). As described, Zanetti, et al. [9] reared larvae on “…beef with or without bone, boiled during 10 min to minimize colonization by fungi, mites, and other pathogens.” which presumably represents a hydrated food source. Thus, in addition to direct experimental comparison of development on hydrate and dry food, comparisons of new data with that of Zanetti et al. offer another avenue for evaluating the influence of dietary water on D. maculatus larval growth and survival.

Development is a dynamic process; many factors beyond food influence the speed and efficiency of development, including temperature, humidity, enzyme regulation, and photoperiod [10]. Further differences may be credited to the fact that oviposition in insects on a specific host is determined by various factors that may determine its suitability as a breeding medium, such as nutritional quality, host abundance [11], morphology, environmental conditions, age and size of individual and competition [12, 13]. Consequently, these factors can confound experiments to measure development, unless properly considered as part of the experimental design. Measurements in development can become difficult in a laboratory setting when these factors are not taken into consideration.

Besides providing a means for looking at differences in food use, consumption and conversion rates have not been previously examined for D. maculatus. Knowing these rates could be of use in forensic science for relating dermestid populations and time to tissue removal on decomposed bodies. So, we examined water content of food and its influence on D. maculatus development and survival.

Materials and Methods

Dermestes maculatus for experiments were obtained from colonies and maintained at the University of Nebraska- Lincoln (established in summer 2015) from a combination of purchased and feral beetles. Colonies were maintained at 16:8 L:D, 25°C, and fed dried pork (pig ears). For experiments, colony adults were placed on a diet consisting of 1 gram of wet liver and moistened sections of paper towel to stimulate oviposition. Eggs were laid on the moist paper towel, were transferred to a separate rearing container and maintained until reaching the third larval stage. For colonies and experiments we used DigiTherm® 38-liter Heating/Cooling Incubators which allowed for precision temperature regulation within 0.1°. These incubators have microprocessor-controlled temperature regulation, internal lighting, and a recirculating air system.

We conducted a series of preliminary experiments to establish appropriate larval densities, weight of food, and other experimental conditions before testing the influence of wet versus dry food. In efforts to both reduce error and address variations within stages of insects sampled, we initially examined consumption in first and second larval stages versus later stages and adults. Because the larvae in the first two stages eat so little food (ca. <0.001g dry weight) and this amount represent less than 5% of total consumption, we decided to conduct future experiments with 3rd stage larvae. This decision also reduced problems with survivorship, in that survival rates in the first two stages are highly variable.

We also tested various food sources. As colonies were maintained on pork, we decided early on to use some form of pork in our experiments. Pork liver has long been a substrate of choice in studies of development of carrion insects, however, in looking at wet vs. dry food, liver was unsuitable because of difficulty in establishing water content differences, problems in recovering larvae that had burrowed in the liver, and difficulties in excluding fungal and bacterial contamination in some instances. Other problems were separating frass and shed peritrophic membranes from remaining food. Fortunately, dried pig ears (used as dog treats) proved to be an ideal substrate for testing. We could easily re-hydrate pig ears to provide a workable substrate existing in wet and dry conditions. One potential problem was that in testing small amounts of substrate (specifically, less than 0.5 g) dried pig ears might absorb sufficient moisture from the air so as to confound our treatment and contribute to high variability in measured consumption. We solved this problem by increasing the number of larvae and amount of pig ear per experimental unit, so that the surface area to mass ratio of the pig ear was reduced and therefore absorbed water minimized.

This testing lead to the following observations relative to establishing our pig ear treats: (1) saturated pig ears, which were those soaked water at 25°C for 2 hours to achieve Food % survival Δ larval wt (g) food consumed (g) % conversion feeding rate/larva/day (mg) mean SE mean SE mean SE mean SE mean SE dry 79.5 2.7 0.051 0.019 -0.218 0.065 93.7 64.8 2.1 1 wet 23.2 5.5 0 0.003 -0.815 0.196 4.2 2.7 30 6 food water content: dry vs wet df 1, 25 1, 21 1,21 1, 14 1,21 F 56.27 6.58 12.34 21.35 30.53 P>F <0.0001 0.018 0.0009 0.0004 <0.0001 maximum absorbance (as evidence by constant weight) and a total weight typically double that of a dry pig ear; (2) wet pig ears, which are soaked at 25°C for ca. 30 min to achieve an increase in weight of about 50%; and (3) dry pig, which were ca. <10% moisture content. Preliminary tests showed that when beetles were fed saturated pig ears, the mortality rate in our testing was routinely 100%. To provide measurable consumption on hydrated tissue, we decided to use wet, rather than saturated pig ears for comparisons with dry ears. Additionally, in a natural setting where carrion starts losing fluids immediately after death, the wet pig ear are probably more representative of tissue beetles might natural encounter. One final consideration is the potential influence of intraspecific competition on the larval consumption and associated values. We decided to look at this explicitly by varying starting food amounts to represent different potential resource limitations for larvae.

The wet vs. dry food experiment was a randomized complete block with factorial treatment arrangement. Main effects were food water content (wet pig ears vs. dry pig ears) and food quantity (starting food weights of 6.0, 4.8, 3.6, 2.4, and 1.2 g) with 4 replications. The experimental unit was a 1.7 L plastic box with 10 new molted 3rd stage larvae on pig ear as specified for individual treatments. Experimental units were blocked inside growth chambers by shelves, and maintained at 25.0°C with a light: dark cycle of 12:12 and relative humidity (65±5 %), which are reported as optimal conditions for development [11]. Individuals were weighed at the beginning of the experiment and every 2 days thereafter. Final larval and consumption rates (used to calculate change in larval weight and food consumption) were based on values from the larval measurement immediately before pupation (Table 1). This procedure avoided errors in larval measurements estimated from initial pupal weights, in that pupation can cause a 10-20% reduction in weight [14].

food quantity: starting pig ear weight (6, 4.8, 3.6, 2.4, and 1.2g) for 10 larvae df 4, 25 4, 21 4, 21 1 ,14 4, 21 F 0.58 1.6 1.24 1.45 2.43

Initial testing at all three levels provided enough detail to determine that saturated levels (generally unseen in nature outside of flood events or similar) were deemed excessive. We therefore excluded these treatments from further study and experiments.

Results and Discussion

We found a pronounced difference between how much D. maculatus (chiefly larvae) feed and their feeding rates on wet versus dry tissues. Our findings illustrate not only a preference for dry tissues, but a dependence on them. Indeed, hydrated tissue presents a potentially lethal challenge to D. maculatus. Adults have approximately 50% the daily consumption rates of larvae (averaged over 3rd to the last larval stage). Finally, conversion rate results indicate that D. maculatus must acquire moisture from non-dietary sources.

Conclusions

Rather than representing a physiological adaptation to occupy a specific biome, the physiological preference D. maculatus has evolved seem to show strong evidence of how they’ve niche specialized for a feeding guild, the carrion insects. When contrasted against virtually all other carrion insects, this adaptation for an exclusively dry diet illustrates how D. maculatus are able to avoid competition which is usually a challenge for other decomposers species. On decomposing soft tissues we see great competition among insects, whereas dermestids seem to capitalize on a resource inaccessible to other insects and thereby avoid conflict [17, 18].