2.1 Common names
2.2 Description and recognition features
2.5 Damage caused to cured fish
2.6 Ecological factors in relation to loss reduction
Necrobia rufipes (DeGeer) is a beetle of the family Cleridae and is the commonest species of Necrobia found on cured fish. Two related species, N. ruficollis (Fabricius) and N violacea (Linnaeus), are only rarely found on this commodity.
Sources: Adult -by permission of Ministry of Agriculture. Fisheries and Food. U.K. (Crown Copyright); Larva- by permission of ICI Agrochemicals. U.K.
Figure 1. Dorsal views of adult (left) and larva (right) of Necrobia rufipes.
Copra beetle, Red-legged ham beetle [En]; Necrobie ā pattes rouge [Fr]; Schinkenkäfer Koprakäfer [Ge]; Gorgojo de la copra [Sp].
ADULT- Shape as in Figure 1 (left). Length c. 4.5 mm. Upper surface of body (head, thorax, elytra) entirely shining metallic bluish-green. Underside of abdomen entirely dark blue. Legs bright reddish-brown or orange. Antennae mainly reddish-brown but with a dark brown or black club at the tip. Sides of thorax (especially) and elytra with stiff bristle-like hairs. Distinguished from adults of similar species by coloration described above: N. violacea has black or bluish legs and antennae; and N. ruficollis has reddish-brown thorax and base of elytra.
LARVA- Appearance as in Figure 1 (right). Typical beetle larva with three pairs of jointed legs; moderately hairy. Most of body creamish-grey with mottled violet-grey markings on the upper surface. Head, and upper surfaces of the 1st thoracic segment and the last large abdominal segment (the ninth), with brown hardened plates; 2nd and 3rd thoracic segments also with tiny brownish plates. Plate on last large abdominal segment with two horn-like protuberances which curve strongly upwards. Very difficult to distinguish from closely-related species of Cleridae, but easily distinguished from Dermestes larvae by coloration and normal amount of hairs, and from fly larvae by presence of legs and obvious head.
Adult beetles feed on the surface of dried fish, and they lay their eggs in crevices in the fish. The larvae burrow deeply into the flesh; as well as feeding on the fish, they are predatory on the larvae of some flies, and on the eggs and larvae of Dermestes spp. The larvae pass through three or four instars. The last instar larva spins a cocoon in which pupation occurs: this may be within the fish flesh, or the larva may leave the fish and pupate in any dark crevice. The life-cycle takes about 6 weeks or longer depending on food type and physical conditions. Under optimum conditions, the rate of population increase is about 25 times per month. The adults fly actively and can thus easily disperse to new sources of food.
The optimum temperature for development of N rufipes is in the range 30-34°C, and the minimum temperature is 22°C; the maximum temperature limit is not known, but temperatures above 40-42°C are likely to repel or prevent development of this pest. This beetle needs an equilibrium relative humidity of 50% or above. N rufipes can therefore become a pest in tropical and subtropical climates, especially if ambient conditions are rather humid or the fish is not very well dried. It has a cosmopolitan distribution in warm climates.
N rufipes is usually found in association with infestations of Dermestes spp.: its habitat requirements are similar to those of the tropical species of Dermestes, and it benefits from the availability of Dermestes larvae and eggs as prey.
Feeding by larvae and adults of N rufipes causes quantitative loss of dried cured fish, and also leads to fragmentation and to quality losses due to contamination by insect bodies and cast skins. The extent and value of losses due to N rufipes infesting dried fish have not been assessed either in the laboratory or the field, but they will of course be directly related to the length of storage of the fish. When associated with Dermestes infestations, N rufipes is usually in the minority but its contribution to the total beetle damage may be significant.
Initial infestation is usually due to invasion by flying and crawling adults, which lay their eggs on the partially or fully dried fish. Fly-screens around and over drying racks will reduce beetle infestation pressure during processing. Similarly, during storage and transport, the use of clean good-Quality sacks will slow down rates of r immigration of N rufipes. Osuji (1975) found that cross-infestation by N rufipes was reduced when jute sacks were lined with polythene and thick brown paper.
Although the upper lethal temperature for N rufipes is not known precisely, temperatures above 45°C can be expected to kill or repel these beetles. Such temperatures can disinfest fish or delay invasion by N rufipes, 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 N rufipes populations.
Salting of fish gives considerable protection against N rufipes. because larval development is severely retarded In experiments, larval development took 85 days on fish with 3.5% salt content compared with 42 days on unsalted fish (Osuji. 1975 a).