1.1 Importance of insects
and mites on cured fish
1.2 Life-cycle and development of insects and mites
1.3 General ecology of insects and mites on cured fish
1.4 Collection and examination of insects and mites
1.5 Preservation of specimens
1.6 Main types of insects and mites found on cured fish
Insects and mites are often found infesting cured fish during and after processing, especially in the tropics and subtropics. The purpose of this field guide is to provide basic information on the appearance and ecology of the main types of insects and mites that cause losses to cured fish.
Cured fish can suffer considerable loss of weight due to feeding damage by insect and mite pests. Under adverse conditions, quantitative losses of up to 30% due to fly damage during processing, and up to 50% due to beetle damage during storage for several months, have been reported. Under better conditions of processing and storage, the weight losses due to pest infestation are often much less than these extreme values, but they usually represent a significant wastage that could be reduced by prevention and control of pest attack. Published and unpublished reports of losses due to insect damage on cured fish have been summarized and discussed by FAO (1981).
Pest damage can also cause fragmentation of the cured fish (FAO, 1981), which Can lead to quantitative loss of the smaller fragments (or downgrading of this fraction for use as animal feed) and loss of value due to quality reduction, since a higher price is often obtained for intact pieces of fish. Contamination by live or dead pests, or by their cast skins and frass (excreta), also causes a change in visual quality that may reduce the value of the fish. Additionally, insect and mite pests often transmit mould spores, and the heat and moisture produced by heavy infestations can create conditions suitable for mould growth on fish that has previously been dried.
During development from egg to adult, insects and mites pass through several distinct stages. At the end of each stage, the old skin or cuticle (which forms a thin external skeleton) is shed and the next stage emerges with a fresh cuticle, which is initially elastic and thus allows growth before it hardens
In many insects, including the beetles and flies found on cured fish, the immature stages are completely different in appearance from the adults; this type of life-cycle is called complete metamorphosis. In such insects, the egg hatches to produce a larva, which may have three pairs of jointed legs (e.g., most beetle larvae) or none (e.g., fly larvae). The larva is predominantly concerned with feeding and growth, and passes through several stages (instars) and moults before reaching full size. The last larval instar then changes into a legless pupa, which has no mouth. During the pupal stage, the body of the insect is reorganized into the adult form. This reorganization may be visible in pupae that have thin cuticles (e.g., beetle pupae) but hidden in others (e.g., those fly pupae in which the last larval cuticle is retained and becomes thickened as a puparium enclosing the pupa). Finally, the pupa moults to release the six-legged adult, which usually has one or two pairs of wings. The adult may feed on the same food as the larva or on a different food, or it may be short-lived and not feed at all; whether or not it feeds, the primary purpose of .the adult stage is to reproduce and lay eggs. Detailed information on the structure and development of insects is given by Richards and Davies (1977, 1977 a).
In all mites and some insects, the immature stages are very similar to the adults in appearance, and also in their feeding habits and general behaviour; this type of life-cycle is called incomplete metamorphosis. In mites, the egg hatches into a six-legged larva, which then moults to produce a nymph with the eight legs that typify mites and other arachnids. Development then proceeds through one, two or three nymphal instars. The nymphs usually closely resemble the adult form, differing mainly in their smaller size and lack of external genital openings. In the species of mites commonly found on cured fish, there are two such normal nymphal stages, the protonymph and tritonymph. However, between these two normal stages a special type of deutonymph may occur -a hypopus, which has reduced mouth parts and does not feed, but which has a number of suckers that allow it to cling to insects for dispersal. The last nymphal stage moults directly to the adult, which feeds in the same way as the normal nymphs but has fully-developed genitalia for reproduction. Further information on the development and biology of mites is given by Hughes (1976) and Krantz (1978).
Once an infestation of an insect or mite pest becomes established, its population tends to increase exponentially: i.e., the total numbers follow a geometric series over equal time intervals. Under ideal conditions for the particular pest species, the rate of increase may be very high. For example, in the common beetle pests of cured fish, the optimum rate of increase is 25-30 times in 4 weeks (Howe, 1965); i.e., at optimum conditions, one fertile female could give rise to 15 625-27 000 beetles in 12 weeks. Mite pests have even higher rates of population growth, with optimum rates of increase reaching many hundreds of times per month. If fish processing and storage conditions favour rapid development of pests it is essential that infestations are detected and controlled at an early stage, before large populations cause unacceptable levels of damage.
The actual rates of increase occurring in practice are, however, affected by many environmental factors. The more important of these factors are temperature, moisture, and the nature of the food (physical form and nutritional quality).
Insect and mite development can only occur within certain ranges of temperature, dependent on the species. Within its range, each pest species has an optimum temperature, usually less than 5°C below its maximum temperature limit, at which its rate of increase reaches a peak. Typically, for pests of cured fish this optimum temperature is somewhere between 25° and 35°C. If exposed to temperature in excess of their maximum, insects and mites are eventually killed, unless they can disperse to cooler conditions. Below their optimum temperature, their rate of increase is progressively reduced until, at their minimum temperature limit, development ceases.
The development of insect and mite pests is similarly affected by moisture content and relative humidity. In particular, low moisture levels are an important limiting factor for most pests of cured fish and other dried food. The flies that infest moist partially-cured fish are especially susceptible to lack of moisture and usually cannot develop on the fully-cured product. Most of the other pests have high rates of increase on cured fish with moisture contents in equilibrium with 70-80% r.h. As the equilibrium r.h. is reduced below 70%, the rates of increase of these pests (especially the flies and mites) are considerably reduced, and well-dried fish is much less susceptible to damage by most pests.
The physical nature of cured fish, especially the extent of any fragmentation, affects the availability of the flesh, and thus the rate of increase of pests feeding on it. This is likely to be especially true for mites. Different genera of cured fish have been found to vary in their susceptibility to infestation by beetles and mites, though the nutritional factors governing this variation have not been identified. The presence of the salt on salted cured fish reduces the rates of increase of most insects and mites, though the pest species vary in their response to different concentrations.
Further information on the effects of temperature, moisture and food type on pests of cured fish is given by FAO (1981).
The main groups of pests of cured fish can usually be recognized with the naked eye or, preferably, by the use of a simple hand-lens, while they are still on the fish sample. However, if confirmation or more detailed identification is required, or if insects or mites other than the main pest types seem to. be present, specimens should be collected for examination in the office or laboratory.
The simplest procedure, which reduces the amount of work in the field and also ensures that species are not over-looked, is to take samples of the infested fish, together with the pests, and place them in polythene bags (or similar sealed containers). The samples can then be examined in detail on a tray in the office or laboratory, and the pests extracted carefully. Collection of such samples may be essential if only the larvae or pupae of flies or beetles are present on the fish, and if detailed identification is required. Such specimens should be kept alive on the sample (in jars cloyed by pieces of cloth secured with rubber bands) until the immatures develop into adults, which are easier to identify.
Active adults of flies and beetles are usually easier to catch by using an entomologist's net, and transferring them to small tubes or jars. A suitable "butterfly" net can be purchased from biological equipment suppliers, or it can be made locally by stitching soft mosquito-netting into the shape of a bag and fitting this onto a circular, ovate or triangular frame of metal or bamboo with a light wooden handle. This method is particularly useful for sampling pests that are flying around fish processing sites.
Insects and mites infesting flaked fish can be separated by sieving; the sieve aperture-size required will be governed by the average size of the fish flakes in relation to the size of the particular pests present.
In order to collect insect specimens from relatively intact fish pieces in the field, or to remove them from fish samples in the office or laboratory, one should use either lightweight forceps (preferably soft ones to reduce the likelihood of damaging the specimens) or a small aspirator. Mites can also be collected with an aspirator. It is possible to use a moistened fine artist's brush to collect small insects and mites, but this requires some practice.
The specimens should be placed in small labelled tubes (25 x 50 mm, or smaller,). Most specimens will not be damaged by being kept in such tubes for up to 24 h but, if it is necessary to keep them for longer, preservative fluid should be added to the tube before sealing it, as described in section 1.5, below: this will kill the specimens and prevent their deterioration.
Samples or specimens should always be labelled, and notes made, at the time of collection in the field. The label attached to, or placed in, the sample bag or tube should give the most essential collection data (i.e., the location, type of cured fish, and date) in abbreviated form, together with a sample number. The full collection data, including notes on precise collection details, quality of fish, level of infestation, etc., should then be entered against the sample number in a notebook. It is not advisable to write only the sample number on the label, because field notebooks are sometimes lost or mislaid.
If a label is to be placed inside a sample tube containing preservative fluid, it must be written clearly with a pencil or with permanent black drawing ink; most other inks, whether water- or spirit-based, are dissolved by preservative fluids, and this includes the inks used in ballpoint pens. Further information on collecting and labelling of pest specimens is given by Oldroyd (1970), British Museum (Natural History) (1974), Hodges (1980) and Dobie et al (1984).
In order to confirm, in the office or laboratory, to which main pest group a specimen belongs, a simple hand-lens or bench-lens may be necessary. In order to identify pests more precisely by using identification keys such as those given by Freeman (1980), Dobie et al. (1984) and Halstead (1986) , it is usually essential to use a microscope. A cheap low-power "dissecting" microscope is normally sufficient. Identification of mites, however, by using keys such as those given by Hughes (1976), requires specialist knowledge and a high-power transmitted-light microscope.
If insect and mite specimens are to be retained for further identification, or as reference specimens, or for any other reason, they must be preserved. If the specimens have already been placed in preservative at the time of collection, it is still usually advisable to transfer them to fresh preservative (and to remove any fish flakes or other debris). Adult flies (and other insects) are commonly preserved dry on entomological pins or stuck to card with a water-soluble glue (British Museum (Natural History), 1974). This technique is, however, unsuitable for long-term storage in warm humid climates, where the specimens may be destroyed by mould, and in many climates the dried insects are susceptible to attack by museum beetles and similar scavengers. It is therefore generally preferable to keep most insect and mite pests of cured fish in preservative fluid (as described below) in small labelled tubes. The main exception is that, if adult flies are to be sent to a museum or a specialist for identification, they should be dried and then packed gently in a small box between layers of tissue, as they lose their colour in preservative.
The best general-purpose preservative is Pampel's Fluid. This is prepared by mixing the following ingredients together (in parts by volume to give the quantity required) in the order shown: 30 parts water (preferably distilled); 15 parts 95% ethyl alcohol; 6 parts 40%(w/v) formaldehyde; and 4 parts glacial acetic acid. (Never start with the acid: always add it slowly after mixing the other ingredients.) If possible, let the mixture stand for a few days before use: the unpleasant smells of the acid and aldehyde will then have disappeared. If glacial acetic acid is not available, or is considered too dangerous to transport or stock, replace the water and concentrated acid in the above list with either 7.5 parts water and 26.5 parts 15% acetic acid solution. or 14 parts water and 20 parts 20% acetic acid solution. If industrial acetic acid is not available. A similar (but weaker) preservative can be made by mixing: 35 parts strong vinegar (preferably white); 15 parts absolute (> 99.5%) ethyl alcohol; and 2.5 parts 40%(w/v) formaldehyde.
An alternative preservative. which is particularly suitable for mites and insect larvae. is Oudeman's Fluid. This is made by mixing together (as for Pampel's Fluid): 87 parts 70% ethyl alcohol; 5 parts glycerol; and 8 parts glacial acetic acid. Because of the high proportion of acid to water in this fluid. it is only possible to make it withough glacial acetic acid by using 25% acid solution and concentrated alcohol: 63 parts 95% ethyl alcohol (or 61 parts absolute alcohol plus 2 parts water); 32 parts 25% acetic acid solution; and 5 parts glycerol.
If Pampel's Fluid or Oudeman's Fluid cannot be prepared. then 70% ethyl alcohol or 10% formaldehyde can be used though these both tend to harden specimens and the latter causes discoloration. In the absence of any of these preservatives. a colourless alcoholic liquor can be used. If available. but it should be noted that at normal strengths of only 40% alcohol this is not such an effective preservative.
As described earlier. the specimen tubes must be labelled and the labels must be written in pencil or black drawing ink. The tubes should be well-filled with preservative to cover and protect the specimens even when they are moved. A small wad of soft absorbent paper can be used to reduce the volume of preservative and restrict the movement of robust specimens. but care must be taken not to trap air-bubbles below the paper; this should not be done with delicate or small specimens (small insect larvae or mites) as they often become entangled in the fibres. The tube must be very well sealed to prevent evaporation or leakage of the preservative: cork stoppers should be sealed with melted wax. if possible, and rubber or plastic closures should be secured with adhesive tape.
If specimens are to be sent by post or courier to a specialist for identification. the tubes must be well protected from damage. A simple method using commonly available materials is to bore a hole in a small block of expanded polystyrene packing. push the specimen tube entirely into the hole. and pack the block in a small cardboard box or a padded bag. Other methods are described by British Museum (Natural History) (1974). Hodges (1980) and Dobie et al (1984).
The pests commonly found on cured fish are beetles (Coleoptera) or flies (Diptera). or mites (Acarina).
The adults of the beetles all have six legs. a large thoracic segment behind the head. and a pair of hard wing-cases (elytra) that cover much or all of the abdomen. These elytra are actually modified fore-wings; the hind-wings are normal but are usually completely hidden under the protective elytra. except in a few species which have no hind-wings. The larvae of the beetles normally have 3 pairs of jointed legs. one on each of the 3 segments behind the head. and sometimes have one or two horn-like protuberances at the end of the segmented abdomen. Some beetle larvae are covered densely with long hairs, but others are almost smooth. The main beetle pests of cured fish are Necrobia rufipes and Dermests species.: these are described in sections 2 and 3 of this guide. Other beetle adults are sometimes found on cured fish. The anobiid Lasioderma serricorne (Fabricius). which is small (2.0-2.5 mm) and reddish-brown with its head partly hidden below the thorax. is a pest of many dried commodities and has been reported from cured fish in South Asia. Various Species of predatoryhisterid beetles. especially species of Saprinus and its relatives. are occasionally found on cured fish. especially in association with infestations of Dermestes larvae on which they prey; these beetles are broadly oval almost hairless. black and shining (sometimes with metallic lustre or pale spots). and the elytra are rather shorter than the abdomen (Hinton. 1945 a).
The adults of the flies all have six legs. large eyes, and one pair of membranous wings. Behind and slightly below the base of each wing is a small club-like structure (haltere) that may be partly concealed by a protruding lobe; the paired halteres are gyroscopic balancing organs and are modifications of the hind-wings. Fly larvae. often called maggots. have no leg8. a very small head. and barely visible mouth parts formed mainly of two hook-like mandibles. The larvae are generally cylindrical. though often tapering towards the head. and sometimes have numerous protuberances. The pupae are protected by hardened puparia. which are cylindrical with rounded ends and vary in colour according to species and age. All flies associated with cured fish are in the advanced suborder Cyclorrhapha. and the majority belong to the family Calliphoridae (the blow-flies, blue bottles, green bottles, and flesh flies). The common types of fly found on cured fish are described in section 5 of this guide.
Mites are very small (always less than 1 mm and usually less than 0.5 mm) and their thin-skinned oval bodies are usually a translucent creamish-white. Because of their small size, mites are often not noticed; if cured fish has a dusty rough-textured appearance, the "dust" may in fact be mites. Mites never have wings, and the segmentation of the body that is clear in most insects is not visible in mites. The larvae have only six legs, but the nymphal stages and adults have eight legs. Most of the species of mites that are pests on stored foods belong to the family Acaridae. On cured fish, the commonest acarids are species of Lardoglyphus, which are described in section 4 of this guide: other types of acarid, or similar pest groups, are rarely found on this commodity. Predatory mites, which are usually distinguishable by their long legs and fast movement, may occasionally be seen, but only in small numbers. Accurate identification of mites requires specialist knowledge.