Common green bottle fly

Distribution and behavior

Lucia sericaca is common all over the temperate and tropical regions of the planet, mainly the Southern Hemisphere: Africa and Australia. It prefers warm and moist climates and accordingly is especially common in coastal regions, but it also is present in arid areas. The female lays her eggs in meat, fish, animal corpses, infected wounds of humans or animals, and excrement. The larvae feed on decomposing tissue. The insect favours species of the genus Ovis, domestic sheep in particular. This can lead to blow fly strike, causing problems for sheep farmers, though L. sericata is not a major cause of blow fly strike in most regions.

Morphology

The defining characteristic of L. sericata, and most used when identifying the adult fly is the presence of three bristles on the dorsal mesothorax. This body region is located on the middle of the back of the fly. L. sericata is almost identical to its sister species, L. cuprina. Identification between these requires microscopic examination of two main distinguishing characteristics. As opposed to L. cuprina, which has a metallic green femoral joint in the first pair of legs, L. sericata is blue-black. Also, when looking at the occipital setae, L. sericata has one to 9 bristles on each side, while L. cuprina has three or less.

Lifecycle

The lifecycle of Lucilia sericata is typical of flies in the family Calliphoridae in that the egg hatches into a larva that passes through three instars, enters a prepupal and then a pupal stage before emerging into the adult stage or imago. The female lays a mass of eggs in a wound, a carcass or corpse, or in necrotic or decaying tissue. The eggs hatch in about 9 hours in warm, moist weather, but may take as long as three days in cooler weather. In this, they differ from the more opportunistic Sarcophagidae, that lay hatching eggs or completely hatched larvae. A single female L. sericata typically lays 150−200 eggs per clutch and may produce 2,000 to 3,000 eggs in her lifetime. The pale yellow or grayish, conical larvae, like those of most blow flies, have two posterior spiracles through which they respire. These larvae are moderately sized, ranging from 10 to 14 millimeters long.

The larva feeds on dead or necrotic tissue for 3 to 10 days, depending on temperature and the quality of the food. During this period the larva passes through three larval instars. At a temperature of 16°C, the first larval instar lasts about 53 hours, the second about 42 hours and the third about 98 hours. At higher temperatures (27°C) the first larval instar lasts about 31 hours, the second about 12 hours, and the third about 40 hours. Third-instar larvae then drop off the host onto soil, where available, where they enter a pupal stage which usually lasts from 6 to 14 days. However, if the temperature is suitably low, a pupa might overwinter in the soil until the temperature rises. After emerging from the pupa, the adult feeds opportunistically on nectar or other suitable food, such as carrion, while it matures. Adults usually lay eggs about 2 weeks after they emerge. Their total lifecycle typically ranges from 2 to 3 weeks, but this varies with seasonal and other circumstances. L. sericata usually completes three or four generations each year in cold, temperate climates, and more in warmer regions.

Forensic importance

L. sericata is an important species to forensic entomologists. Like most calliphorids, L. sericata has been heavily studied and its lifecycle and habits are well documented. Accordingly, the stage of its development on a corpse is used to calculate a minimum post mortem interval, so that it can be used to aid in determining the time of death of the victim. The presence or absence of L. sericata can provide information about the conditions of the corpse. If the insects seem to be on the path of their normal development, the corpse likely has been undisturbed. If, however, the insect shows signs of a disturbed lifecycle, or is absent from a decaying body, this suggests post mortem tampering with the body. Because L. sericata is one of the first insects to colonize a corpse, it is preferred to many other species in determining an approximate time of colonization. Developmental progress is determined with relative accuracy by measuring the length and weight of larval lifecycles.

Veterinary importance

Many blow flies have an impact in the veterinary sense, and L. sericata is no exception. In places such as the UK and Australia, L. sericata is commonly referred to as the "sheep blow fly" since sheep are its primary host in those regions. Although it affects mainly sheep, L. sericata is not host-specific.

In northern Europe, the fly lays its eggs in sheep wool. The larvae then migrate down the wool where they feed directly on the skin surface. This can cause massive lesions and secondary bacterial infections. In the UK, blow fly strike affects an estimated 1 million sheep, as well as 80% of sheep farms each year. This causes a huge economic impact in these regions. Not only does it cost money to treat infected animals, but measures also must be taken to control L. sericata.

Since this fly tends to lay its eggs in wool, a simple and effective way to reduce the incidence of infection is to shear ewes regularly. Enacting simple sanitary measures can also reduce blow fly strike. Timely and proper disposal of carcasses and proper removal of feces can aid in significantly reducing strike. Moving sheep from warm, humid, and sheltered areas to more open areas can also help to reduce blow fly strike, for this eliminates conditions conducive to fly development. Trapping systems such as sticky paper may also be used to control fly numbers. Treating a flock with chemical agents can be costly, but can aid greatly in maintaining the resistance of the flock to L. sericata. Plunge dipping in diazinon can directly kill the fly on contact. This method works from 3 to 8 weeks in controlling the fly. An alternate chemical method is a pyrethroid pour-on, which lasts 6 to 10 weeks depending on the type of pyrethroid used. Cryomazine and dicylanil, which are insect growth regulators, are also effective and last from 10 to 16 weeks. Although chemical treatment can be very effective, it is costly, tedious, and takes up valuable time.

Medical importance

L. sericata has been of medical importance since 1826, when Meigen removed larvae from the eyes and facial cavities of a human patient. L. sericata has shown promise in three separate clinical approaches. First, larvae have been shown to debride wounds with extremely low probability of myiasis upon clinical application. Larval secretions have been shown to help in tissue regeneration. L. sericata has also been shown to lower bacteremia levels in patients infected with MRSA. Basically, L. sericata larvae can be used as biosurgery agents in cases where antibiotics and surgery are impractical.

Larval secretions in vitro enhance fibroblast migration to the wound site, improving wound closure. Larval therapy of L. sericata is highly recommended for the treatment of wounds infected with Gram-positive bacteria, yet is not as effective for wounds infected with Gram-negative bacteria. Also, bacteria from the genus Vagococcus were resistant to the maggot excreta/secreta. Attempts are currently ongoing to extract or synthesize the chymotrypsins found in larval secretions to destroy MRSA without application of the larvae.

Continuing research

Due to this species' high forensic interest, extensive research on its lifecycle has already been conducted. Medically, however, research is ongoing centered on the secretions produced by L. sericata as an agent against MRSA and VRSA, and the larval applications for maggot therapy. In a new antimicrobial agent was isolated from L. sericata secretions and patented under the name Seraticin. Efforts in the latter are geared toward making medical professionals more familiar to the current techniques. Like many other ectoparasites, L. sericata has a huge economic impact on farmers, so many studies and research projects have been put in place since the late 1980s to help farmers reduce their impact.