4.1 Specific and
4.2 Description and recognition features
4.5 Damage caused to cured fish
4.6 Ecological factors in relation to loss reduction
Species of Lardoglyphus belong to the family Acaridae in the mite group Astigmata. Three species have been found infesting cured fish: L. konoi (Sasa and Asanuma); L. zacheri Oudemans; and L. angelinae Olsen. The commonest species is L. konoi.
Source: Adult and hypopus -adapted from Hughes (1976) by permission of the Controller of Her Majesty's Stationery Office. U.K. (Crown Copyright)
Figure 3. Dorsal view of adult female of Lardoglyphus konoi (left) and ventral view of hypopus of L. zacheri (right).
L. konoi has occasionally been called the "Fish mite" [En]. N common names have been used for L. angelinae or L. zacheri.
ADULT -Appearance as in Figure 3 (left): oval-bodied with four pairs of legs. Length of body c. 0.3-0.6 mm; females larger than males. As in most acarid mites, the body is smooth and translucent creamish-white with several pairs of hairs, which are only visible under a good hand lens or a low-power microscope. The females differ from other acarid mites found on stored food in having paired (rather than single) claws, and the males are distinctive because the third legs end in two blunt spines instead of claws: these characteristics of Lardoglyphus spp. are only visible with a high-power microscope. Precise identification of specie requires specialist microscopic techniques and knowledge.
HYPOPUS -Appearance as in Figure 3 (right) when viewed from below. Length of body c. 0.2 mm. Legs rather stouter than those of adult. The body hairs are short, and are sometimes thickened into small spines. There are no functioning mouthparts. The underside of the body has numerous small suckers, especially concentrated on a sucker plate behind the bases of the fourth legs.
OTHER STAGES -The protonymph and tritonymph are similar in appearance to the adult but are smaller. The larva is smaller still, and has only three pairs of legs.
Adults and nymphs of Lardoglyphus feed on drying or dried fish. The adult female lays eggs on the fish; after hatching into a six-legged larva, the mite passes through the eight-legged protonymph and tritonymph stages during its development into an adult. This development can be very rapid. At 23°C and 87% r.h. the life-cycles of L. konoi and L. zacheri take only 9-11 and 10-11 days, respectively; at optimum temperatures, development may be even faster. Potential rates of increase have not been estimated for these mites, but they are probably of the order of thousands of times per month under optimum conditions. In such situations, the mites can increase to vast numbers within a few days and, because moderate populations of Lardoglyphus are easily overlooked, a heavy infestation may seem to occur quite suddenly.
Some protonymphs do not change directly into tritonymphs, but instead turn into hypopi first. The hypopi do not feed but are a dispersal stage. They climb onto the larvae and adults of beetle pests, and use their suckers to attach themselves to the insect's cuticle. On larvae, they are found on the soft membrane between segments, especially near the front of the body; on adults, they are most commonly found on the upper surface of the abdomen under the elytra. They may remain on the insect for several days, and are therefore often carried to new infestation sites before they detach themselves and change into tritonymphs. Although Vijayambika and John (1974) have questioned the importance of environmental factors in controlling the production of hypopi, most investigators (e.g., Hughes, 1956; Matsumoto, 1970, 1973) have shown that the proportions of hypopi in Lardoglyphus populations are influenced by such factors as unfavourable temperature and humidity levels, or shortage of food, or overcrowding.
Optimum conditions for the development of Lardoglyphus
konoi are 30-35°C and 87% r.h. L. konoi cannot
develop at 66% r.h. and below, but populations of this species
increase at humidities of 75% and above (up to 98% r.h.). Little
is known about the temperature and humidity responses of L.
L. angelinae, but they are probably similar to those of L. konoi. Lardoglyphus spp. can therefore become pests in many tropical and subtropical situations, especially where ambient conditions are humid or the fish is not well dried.
Infestations of Lardoglyphus spp. on cured fish are usually associated with those of Dermestes spp. and Necrobia rufipes. This is partly due to the similarities in the physical and nutritional requirements of all these pests, but it is mainly because the beetle pests are the main agents of dispersal of the Lardoglyphus hypopi.
Feeding by the adults and nymphs of Lardoglyphus spp. will cause quantitative loss of dried cured fish. Quality loss may result from the contamination by live and dead bodies, which can be very numerous. The extent and value of losses caused to dried fish by Lardoglyphus spp. have not been assessed in either the laboratory or the field..
Initial infestation is often caused by hypopi that have been carried to the fish by beetle pests. The techniques given in sections 2.6 and 3.6 for reducing initial infestation by beetles will therefore also reduce infestation by Lardoglyphus spp. Fragmentation of the fish flesh, caused by beetle and fly attack and by handling damage, increases the surface area of the flesh and may thus encourage heavier mite infestations.
The minimum humidity threshold for Lardoglyphus konoi is between 66% and 75% r.h. If the fish is thoroughly dried, mite problems will be greatly reduced or eliminated. The temperature limits for development are not known, but temperatures above 40-45°C would be expected to kill these mites.
The response of Lardoglyphus spp. to salt has not been investigated. L. konoi has been found infesting salted fish products as well as unsalted dried fish, but the salt contents of the infested products were not recorded.