Reference no: EM132787587

Post-feeding larvae starts dispersing away from food resource and often burrow into the ground to pupate. In Brazil, Chrysomya albiceps and Chrysomya megacephala burrow into wood shavings to an average depth of 4.0 cm (Gomes and Von Zuben, 2005). Larval dispersal is an important process in the life cycle of blow flies during which the larvae leave their food substrate and search for a suitable place to pupariate (Levaot et al., 1979; Goody et al., 1996). The process of larval dispersal completes when all larvae have buried, and no larvae remain dispersing on the substrate. Hence, a knowledge about where blow flies pupate is critical for an accurate analysis of insect evidence. Research shows that in Australia, pupation behavior differs between two subfamilies of the Calliphoridae: the Chrysomyinae (Chrysomya) pupate on or near the food source on the ground surface, and the Calliphorinae (Lucilia, Calliphora) disperse from the food source and burrow into the soil before pupation (Norris, 1959). After this initial observation on behavioral differences, many studies have been conducted on the dispersal behavior of post-feeding larvae of blow flies.

In our study we analyzed both ascending and descending vertical dispersal behavior of feeding third instar larvae as well as eclosion success of Chrysomya megacephala. Objective of this research was to scrutinize the impact of soil depth on the dispersal behavior larvae and subsequent eclosion success. More than 75% larvae dispersed ascendingly and pupated not more than 25 cm in all the cases of provided depth. As the soil depth escalate, oxygen availability diminishes, therefore it might be the reason that the larvae preferred to move upward towards the soil surface for more oxygen availability. Eclosion success rate has increased when the larvae move ascendingly. Number of adults emerged successfully was significantly greater in ascending dispersal, which reveals larvae preferred to move upward in clay soil (Compact soil). Laboratory experiments on fruit flies (Diptera: Tephritidae) suggest that soil compaction negatively affects the burrowing ability of larvae. Larvae of the Caribbean fruit fly Anastrepha suspensa (Loew) burrow deeper in less compacted soil. The deepest mean depth of pupation for A. suspensa is 3.3 cm in low compaction soil and the shallowest is 0.7 cm in high compaction soil, with no significant difference in percentage of adult emergence based on soil compaction (Hennessey, 1994). Pupation depth by olive fruit flies, Dacus oleae (Gmelin), in different substrate types is affected by compaction. In compact substrates, larvae of Dacus oleae pupate within 5 cm while larvae pupate up to 8 cm deep in uncompacted substrates (Tsitsipis and Papanicolaou, 1979). Therefore, soil compaction affects the pupation behavior of insects; they pupate deeper in less compact soil than in more compact soil. Pore space of soil is negatively correlated with soil compaction; pore space decreases as compaction increases (Babercheck, 1992). This results in reduction of gas exchange in the soil and thus less oxygen is available for the developing fly (Brady and Weil, 2008).

Several studies have investigated the vertical dispersal of post feeding larvae burrowing into the soil to pupate and to emerge as adult flies several days later (Greenberg, 1990; Cammack et al., 2010). Such behavior further characterizes blow flies as r-strategists. The depth at which puparia are located varies with species: Phormia regina about 2 cm, Cochliomyia macellaria at 4-5 cm and Lucila sericata at depths of 11 cm (Greenberg, 1990; Cammack et al., 2010; Ullyett, 1950). Balme et al. investigated ascending vertical dispersal behavior of Protophormia terraenovae and Cochliomyia macellaria and found no significant differences in the number of emerging adults at depths of 5 and 25 cm. However, significantly fewer adults were collected from immatures buried at the 50-cm depth (Balme et al., 2012).

The evaluation of PMI is one of the most important aspects of legal medicine and it could be underestimated if the older dispersing larvae or those that disperse longer and faster or deeper, are not considered (Smith, 1986). Because of this, it is necessary to investigate the pattern of larval dispersal on the pupation site, as demonstrated in this study with larvae of Chrysomya megacephala. Sufficient literature and data are available about the development of blow flies when bodies are exposed above the ground, but not many studies have been done for larvae development when the body is buried. Thus, efforts have been made to know how their development would be affected if a body is buried after the blow flies had laid their eggs.

What are the reasons behind ascending and descending vertical dispersal behaviour?