Phase Variation in the Armyworm, Leucania unipuncta HAWORTH

IV. Phase Difference in the Range of Food Tolerance of the Final Instar Larvae

Abstract

It is well known that under epidemic condition the caterpillar of Leucania unipuncta known as a grass-feeder often feeds on a wide variety of plants owing to the shortage of its suitable food plant.
Experiments were done to analyse whether the larvae found in dense population are more tolerant to unpalatable food than the individuals in low density. The larvae which had been reared in isolation and in crowds on normal diet (i.e. maize or Bromus unioloides) were separated individually into containers at the end of their 5th instar, and were forced to feed on the leaves of unusual food plants such as Sasa albo-marginata (Gramineae), Brassica chinensis (Cruciferae), Achyranthes japonica (Amaranthaceae) or Quercus glauca (Fagaceae), except those reared on grasses. The similar experiments were repeated twice under temperatures of 20∼26°C.
All of the individuals reared on grass pupated within a week from the final moult, regardless of their previous condition of rearing density. The body weight at pupal stage was consistently heavier in low-density type (pale larvae) than in high-density type (dark larvae), but there was no significant difference in the ratios of the pupal weight to the final weight of 5th instar (P/V) between both types.
When the larvae were transferred to Sasa or Brassica, their pupation retarded for 1∼3 days and pupal weight decreased, as compared with those of grass-eating insects. In the latter case these harmful effects were more marked in pale larvae than in dark ones. Some of the larvae reared on Achyranthes failed to reach their maturity. Again, the mortality was much higher in low density type; 67 per cent of pale larvae and 27 per cent of dark ones died in one experiment, and 25 and 0 per cent respectively in another. Survived pale larvae also required longer period than the dark ones to complete the development, and the pupal weights of both types were not different from one another. The larvae fed on Quercus died out within few days and no differnce was found in the mortality curves between both types of larvae.
Mean number of faecal pellets per day decreased as increasing the unsuitability of food. The degree of such reduction was marked in high-density type while the lowering of growth ratio was small in it. This may suggest that dark larvae have better ability to utilize unpalatable food than pale ones.
Some of the numerical data in the 2nd experiment are summarized in the following table:
The results mentioned above apparently indicate that the larvae grown under crowded condition have wider range of food tolerance than the larvae in isolation. It is interesting to understand the ecological meaning of phases observed in this insect.