The larvae of two species of swift moths, Hepialus humuli (L.) and H. lupulinus (L.), live in the soil and are common under grass in the British Isles, but do not usually have any obvious effect upon the grass. The larvae of H. humuli are polyphagous and when they occur under agricultural or horticultural crops they may cause damage by feeding on the roots. Lettuce, strawberry and chrysanthemum are most frequently damaged.
The adults, which are described, fly at dusk, the female seeking the male. The female lays between 200 and 1,600 eggs, with a mean of about 600, over a period of four days. Sweep-net catches showed the ratio of the sexes to be approximately equal, and mercury-vapour light-trap catches showed that the flight period in southern England is principally in June.
In the laboratory, never less than 80 per cent, of eggs hatched, even in dry air, and the shortest incubation period, between 11 and 24 (mean 18) days, was at 20°C. At 5°C. they did not hatch, but remained viable for at least six months.
The larvae were reared in small cells drilled in blocks of plaster of Paris, which were stood in a tray of water, or in vials containing a layer of moist plaster of Paris, and were fed on pieces of carrot.
The growth of the larval head capsules was geometric and allowed 12 instars to be distinguished at 15°C.; at higher temperatures there may be more.
The growth curve of the larvae was S-shaped with a very distinct fall in weight in each instar for several days before moulting. The daily intake of food was weighed, and the conversion rate shown to be small, ranging from 0·00058 to 0·04, as compared with 0·205 to 0·49 recorded for other Lepidopterous larvae.
At 5°C., larvae did not develop beyond the second instar, nor beyond the fourth at 10°C. The optimum temperature for their development (mean period 224 days) was 15°C., although mortality was high. They developed at 20°C. more rapidly (mean 197 days) but more died. Larvae kept in a container sunk in the soil outdoors developed more slowly than those at 15°C. and had only reached the eighth instar by February (239 days after hatching). This evidence, together with that from an artificially infested plot in the field and general field observations, suggests that the usual life-cycle lasts two years.
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