3.1 Specific and
3.2 Description and recognition features
3.5 Damage caused to cured fish
3.6 Ecological factors in relation to loss reduction
Species of Dermestes belong to the beetle family Dermestidae. Several species have been recorded infesting cured fish (or fishmeal): D. maculatus DeGeer, D. frischii, Kugelann, D. ater DeGeer , D. carnivorus Fabr, D. lardarius, L., D. haemorrhoidalis Küster, and D. peruvianus Laporte de Castelnau.
Source: Adult and larva -from Hinton (1945) by courtesy of the British Museum (Natural History)
Figure 2. Dorsal views of adult (left) and larva (right) of Dermestes maculatus.
Dermestes spp.: Hide beetles [En] ; Dermestes [Fr] ;
D. ater: Black larder beetle [En]; Dermeste noir [Fr].
D. carnivorus: Dermeste carnivore [Fr].
D. frischii: Dermeste destructeur du cuir [Fr].
D. haemorrhoidalis: Black larder beetle [En]; Dermeste africain du lard [Fr].
D. lardarius: Bacon beetle, Larder beetle [En]; Dermeste du lard [Fr]; Speckkäfer [Ge]; Dermeste del tocino [Sp].
D. maculatus: Hide beetle, Leather beetle [En]; Dermeste des peaux [Fr] ; Gorgojo del cuero [Sp].
D Peruvianus Peruvian larder beetle [En]; Dermeste péruvien [Fr].
ADULT -Shape elongate oval, a6 shown in Figure 2 (left). Length 5.5-10.0 mm. Cuticle of upper surface of body black or dark brown, covered with black, whitish-grey, brown or yellowish hairs, which form a distinct pattern in some species. Underside of abdomen with black, whitish, brown or golden hairs, often forming a distinct pattern. Antennae rather short but with an obvious club at the tip. Distinguished from Necrobia spp. by the lack of metallic coloration, and larger size. The adults of the different species can be recognized by the characters listed in Table l, but some of these can only be clearly seen with a low- power microscope; full identification keys are given by Freeman (1980) and Peacock (1975).
Table 1. Recognition characters for adults of the species of Dermestes recorded from cured fish.
|Recognition characters||Dermestes species (see footnote*)|
|Each side of thorax with a broad band of dense whitish hairs||Yes||Yes||Yes||No||No||No||No|
|Extreme tip of each elyton (on the mid-line) with a sharp backwardly-pointing tooth||Yes||No||No||No||No||No||No|
|Underside of abdomen mainly white with black spots at the sides and a larger black patch at the tip of the last segment||Yes||Yes||No||No||No||No||No|
|Underside of abdomen mainly white with black spots at the sides but no black partch at the tip of the last segment||No||No||Yes||No||No||No||No|
|Underside of abdomen entirely dark brown or black||No||No||No||Yes||No||No||No|
|Underside of abdomen golden brown with rows darker brown patches at each side and on both sides of the mid-line||No||No||No||No||Yes||No||No|
|Underside of abdomen reddish-brown with yellowish hairs||No||No||No||No||No||Yes||Yes|
|Elytra evenly dark brown or black (hairs mostly black with some whitish or yellowish, or hairs mostly fine and yellowish)||Yes||Yes||No||No||Yes||Yes||Yes|
|Elytra with front half redish-brown and back half dark brown; hairs mainly black with small patches of white, but with a band or golden hairs near the front of the elytra||No||No||Yes||No||No||No||No|
|Elytra with front half clothed in yellowish hairs, except for a dark patch at each shoulder and three pairs of dark spots across the middle; back half of elytra clothed with black hairs||No||No||No||Yes||No||No||No|
|Hairs on elytra coarse and quite long, projecting over the back edges of the elytra as thick fringe; these hairs mainly dark brown of black with yellowish hairs occurring singly||-||-||-||-||-||Yes||No|
|Hairs on elytra fine and short, not forming a fringe on the back edges of the elytra; these hairs mainly pale yellowish||-||-||-||-||-||No||Yes|
* Dermestes species: ma. = maculatus; fr. = frischii; ca. = carnivorus; la. = lardarius; at. = ater; ha. = haemorrhoidalis; pe. = peruvianus
LARVA -Appearance as in Figure 2 (right). Thoracic segments with three pairs of jointed legs. Body densely covered with hairs of various lengths. Underside of body usually yellowish-brown, but upper surface of body mainly dark brown, often with a central yellowish line. Upper surface of last large abdominal segment (the ninth) with two long pointed horn-like protuberances, which may be partly hidden by surrounding hairs. Identification keys to the species are given by Hinton (1945) and Peacock (1975), but these require specialist knowledge. Dermestes larvae are easily distinguished from all others found on cured fish by their hairiness and dark colour.
The adults feed on dried or drying fish, and the females lay their eggs in cracks in the flesh of the fish. The rate of egg-laying is greatly increased if water is available for the female to drink. The larvae burrow into the flesh as they feed on it. The larvae normally pass through five, six or seven instars, but the number of moults is higher under unfavourable conditions. The cast larval skins are commonly found in infested fish, and may be confused with the larvae. Before pupation, the last instar larvae burrow into solid material: this may be the flesh of the fish, but it is commonly the wood of drying racks or store structures, which may be seriously weakened by the tunnelling. The life-cycle of the major pest species takes about 5-7 weeks or longer depending on food type and physical conditions. Under optimum conditions, the rate of population increase of D. maculatus and D. frischii is about 30 times per month. Dermestes adults can fly and can thus easily disperse to new sources of food.
The two commonest species on cured fish in warm climates, D. maculatus and D. frischii, have an optimum temperature in the range 30-35°C and a minimum of 20°C. They need an equilibrium relative humidity of 30% or above, and their optimum is about 75% r.h. D. ater and D. haemorrhoidalis seem to have a slightly lower optimum temperature in the range 27-30°C, but also have a minimum at about 40°C. They can develop in equilibrium relative humidities of at least 40% r.h. and above, with an optimum of about 75% r.h. D. lardarius and D. peruvianus have an optimum temperature of 25°C or slightly higher, and a minimum of about 15°C. They, also, can develop in humidities of at least 40% r.h., but their optimum seems to near 80% r.h. Little is known about the physical requirements of D. carnivorus, but it is frequently found on cured fish in the-hot humid climate of Indonesia.
D. maculatus and D. frischii are the commonest species that-infest cured fish; D. maculatus is especially associated with freshwater fish and D. frischii is more common on marine fish, but these associations are not exclusive. The third commonest species in warm climates is D. ater, which is well-known as a pest of copra but is also often-a significant pest of dried fish in the tropics. D. Carnivorus seems to be an important pest of cured fish in Indonesia; it has also been recorded infesting dried fish in the Philippines and fishmeal in Pakistan. The remaining three species (D. lardarius, D. haemorrhoidalis and D. peruvianus) have only occasionally been recorded from dried fish or fishmeal, and are not significant pests of cured fish.
Feeding by the larvae and adults of Dermestes spp. causes considerable quantitative loss of dried cured fish, and also leads to fragmentation. Quality loss may also be caused by the presence of insect bodies and cast skins. The extent and value of quantitative losses caused to dried fish by Dermestes spp. have been assessed by Various investigators, and estimates range from negligible up to 50% weight loss, depending on length of storage, salt content, moisture content, climatic conditions, and general hygiene during processing and storage. Weight losses due to fragmentation have also been investigated, but the contribution of Dermestes spp. to this process has not been separately assessed. These loss estimates have been reviewed by FAO (1981). Additional costs may arise from damage caused to wooden drying racks and store structures by the mature larvae.
Initial infestation is often due to invasion by flying adults, which lay their eggs on the partially or fully dried fish. Fly-screens around and over drying racks may reduce beetle infestation pressure during processing. Similarly, during storage and transport, the use of clean good-quality sacks will slow down rates of immigration of Dermestes spp. Osuji (1975) found that cross-infestation by Dermestes spp. was reduced when jute sacks were lined with polythene and thick brown paper. Infestation may also be initiated by crawling larvae and adults present in fish residues, or by adults emerging from pupation chambers in wooden structures. The risk of such infestation can be reduced by improved hygiene and by treatment of the wooden structures with a recommended insecticide.
Development of D. maculatus and D. frischii is not possible at 40°C or above. The upper temperature limit for D. ater also seems to be close to 40°C, but the limits for D. haemorrhoidalis, D. lardarius and D. peruvianus appear to be lower. The limit for D. carnivorus is not known but is probably close to 40°C. Temperatures above 40°C can thus be expected to kill or repel all these beetles. Such temperatures can disinfest fish or delay invasion by Dermestes spp., as long as they are achieved throughout the batch: if some of the fish remains at normal temperatures, the beetles will migrate to this. Very well dried fish reduces the rate of increase of Dermestes populations.
Salting of fish gives protection against D. maculatus, partly because larval development is prolonged but mainly because larval mortality increases with increasing salt content. In experiments at 30°C, larval development took 37 days on fish with 3.5% salt content compared with 21 1/2 days on unsalted fish, and mortality reached 100% when the salt content was increased to 9.2% by brining for 1 1/2 h (Osuji, 1975 a). This susceptibility to salt may explain the less frequent occurrence of D. maculatus in cured marine fish, as salt is more commonly used in marine fish processing. D. ater is also adversely affected by salting. By contrast, D. frischii is relatively tolerant of salt. For example, Amos (1968) showed that, at 30°C and 75% r.h., the total development period of 34 days on unsalted fishmeal increased only to 42 and 53 days when the salt contents were 14% and 25%, respectively, though 60% salt content prevented development. However, in these experiments, the presence of salt even at 14% had a considerable effect on larval mortality and on egg-laying, and thorough salting can thus be used to control D. frischii. There are no reports of the response of other Dermestes species to salting.