PREFERENCE FOR SITES WITHIN PLANT BY LARVAE OF THE CABBAGE-WEBWORM, HELLULA-UNDALIS (FABR) (LEP, PYRALIDAE)

A comparative study on the cabbage webworm (CWW), Hellula undalis, wit h respect to larvae feeding either on the shoot or leaf of cabbage was carried out both in the laboratory and in the field. For the first 2 days after eclosion from the egg, a CWW larva would tend to remain wit hin the mine in the leaf where the egg was deposited. The Lloyd patchi ness index (x/x) suggested that the larval distribution was contagiou s, although the degree of contagiousness tended to decrease as the lar vae grew due to their dispersal movement. This was indicated by the in ter-plant larval movement study, in which most of these larvae were fo und in the shoot: 84.6% on the source plant and 40% on the neighbourin g plants by the ninth day of experiment. For the larvae that fed on th e shoot, the duration of the larval and pupal stages were significantl y shorter and adult longevity was significantly longer, although the f emales laid a significantly lower number of eggs than those from larva e that fed on the leaves. However, there were no differences in the si ze of adults reared either on the shoot or the leaves (body lengths we re 7.96 +/- 0.12 and 7.82 +/- 0.08 mm, respectively), and in the survi vorship patterns of the adults. The moths emerging from larvae collect ed from the cabbage shoot in the field also lived slightly longer but laid fewer eggs than those from the leaves, with the intrinsic rate of increase (r(m)) of 0.11 and 0.07, respectively. The results indicated that the CWW larvae preferred the cabbage shoot (to the leaves) which provides a natural refuge and protection for the CWW larva under fiel d conditions, although it is relatively a poorer food type (3.47 +/- 0 .17% nitrogen) than the leaf (4.31 +/- 0.30% nitrogen) (P < 0.05). The higher fertility of CWW bred from the leaves could possibly be relate d to the relatively higher percentage nitrogen in the leaves. After ec losion from the eggs, there was initial grouping of larvae in the shoo t, and then an eventual decrease which could be due to the limited car rying capacity of the shoot to accommodate more than one larva.