5. DIPTERA

5.1 Scientific and common names
5.2 Description and recognition features
5.3 Life-cycle
5.4 Ecology
5.5 Damage caused to cured fish
5.6 Ecological factors in relation to loss reduction

Most flies found on cured fish belong to the subfamilies Calliphorinae and Sarcophaginae in the family Calliphoridae. The commonest of these are various species of Chrysomia, but Calliphora, Lucilia, Sarcophaga and Wohlfartia have also been reported. Other families represented in records of flies infesting cured fish are: Muscidae (Antherigona, Musca and Ophyra); Piophilidae (Piophila); Milichidae (Leptometopa); Phoridae (Megaselia); and Ephydridae (Discomyza). All these flies are rather similar in general form, though they show a variety of size and coloration, and their identification requires specialist knowledge.

Source: Adult and larva -by permission of Ministry of Agriculture, Fisheries and Food. U.K. (Crown Copyright)

Figure 4. Dorsal view of adult of Calliphora vicina (top) and lateral view of larva of Piophila casei (bottom).

5.1 Scientific and common names

Diptera: Flies [En]; Mouche [Fr]; Zweifl�gler [Ge]; Mosca [Sp]. The class Diptera includes the families in Table 2; the genera and species listed in the Table are those recorded from cured fish by Haines (1974. 1981). FAO (1981). and Overseas Development Natural Resources Institute (unpublished records).

Table 2. Families, genera and species of flies found on cured fish.

5.2 Description and recognition features

ADULT- The many different species of fly found on cured fish all have the same general form. as shown in Figure 4 (top). Length 2.5-15.0 mm. depeding on species. Colour of body often black and grey but many of the common species have a metallic green. blue or purple sheen on the upper surface. The eyes are large. The two fore-wings are membranous and transparent. but the hind-wings are modified into small club-like halteres. Identification of the many species requires specialist knowledge.

LARVA -General form usually as shown in Figure 4 (bottom). Length varies greatly. depending on species and stage of growth. Body generally cylindrical. but often tapering towards the head. and sometimes with numerous protuberances that may act as false legs (not present in the species illustrated in Figure 4). Colour usually greyish-white or creamish. Distinguished from all other pests found on cured fish by their lack of jointed legs, their very small head, and the reduced mouthparts (mainly consisting of two hook-like mandibles). Differentiation of the species is very difficult, even with specialist knowledge, and specimens of fly larvae collected for identification should therefore be kept alive until they become adults.

5.3 Life-cycle

Adult female flies lay their eggs (or, in some species, small larvae), usually in batches, on the flesh of fish. Flies in the families Calliphoridae, Ephydridae and Muscidae only infest moist fish in the early stages of the curing process; those in the families Milichidae, Phoridae and Piophilidae can infest partially and fully cured fish. The larvae (usually known as maggots) feed on the surface of the flesh and may also burrow deeply into it. In moist fish, particularly, the burrowing can cause serious fragmentation. The larvae of Calliphoridae and Muscidae often tend to aggregate in discrete areas, where they cause severe damage. There are only three larval instars in the types of fly found on cured fish. Larval development of the major fly pests of moist fish can take as little as three days. At pupation, the last larval skin is retained and modified to form a hard protective puparium. Larvae that infest moist fish usually leave the fish in order to pupate, and often burrow into the soil beneath drying racks or mats. The emerging adults are, of course, strong fliers and quickly disperse to new sources of suitable food. The life-cycle of the main pest species of moist fish (notably Chrysomia spp. and other Calliphoridae) can be completed in about seven days under suitable conditions. The species that infest dried fish develop more slowly: e.g., under ambient conditions in Malawi, Piophila casei completes its life-cycle in about 20 days.

5.4 Ecology

Larvae of Calliphoridae, Ephydridae and Muscidae require a high moisture content for development and cannot infest fully cured fish. The moisture requirements of the other flies listed in Table 2 are not known, though Leptometopalatipes develops more quickly on poorly dried fish, whereas Piophila casei can develop on well dried fish.

Little is known about the effect of temperature on fly adults and larvae, but the larvae seem to be relatively tolerant of hot conditions, compared with beetle and mite pests. For example, although infestation by blowfly larvae is reduced at temperatures in excess of 45�C, complete kill requires 20 h at such temperatures: Piophila casei is particularly tolerant of high temperatures: in experiments, all larvae survived 1 h at 52�C and 24 h at 45�C (Smart, 1935) .

The larvae of several of the species can cause myiasis in man or livestock: i.e., they may infect surface wounds, or they may be ingested and continue to develop as parasites in the intestine. Adult flies of most pest species are attracted to rotting material (such as decomposing fish offal) and dung, where they may feed and breed. They may thus transmit pathogenic bacteria when they lay eggs on the fish.

5.5 Damage caused to cured fish

The feeding of the larvae of Calliphoridae on moist fish causes quantitative losses. These losses can be severe if conditions are optimal for fly development: under such conditions, i.e., if unsalted or poorly salted fish is dried slowly because of rain or high humidity, weight losses of 10-30% can be caused by fly larvae. Fragmentation of the fish by fly attack can cause quality loss and may lead to increased risk of damage by beetles and mites. Substantial weight losses due to fragmentation of fish during processing have been recorded, but the contribution of blowfly damage to this has not been separately assessed. Additional costs are implicated in the role of flies as the agents of myiasis and as carriers of pathogens.

5.6 Ecological factors in relation to loss reduction

Initial infestation is due primarily to flying adults. Fly-screens around and over drying racks may reduce infestation pressure during drying. The risk of cross- infestation can be reduced by treatment of the ground beneath drying racks and mats (where flies often pupate) with a recommended insecticide. Improved hygiene at fish processing sites, especially the rapid disposal of wet offal, will reduce fly infestation problems by removing a secondary food source.

The most important fly pests only infest and damage the fish while it is drying. The length of the drying period is thus a critical factor influencing the extent of losses due to fly attack, and any measures that increase the speed of drying of fish will also reduce fly damage.

Thorough and heavy salting provides complete protection against blowfly larvae. The precise effects of lower salt contents have not been studied, although it is known that poor salting techniques allow blowfly attack. The effects of salt on other types of flies have not been investigated.