THE EFFECT OF SEASON AND GROUP-SIZE ON SURVIVORSHIP AND LARVAL GROWTHIN PLAGIODERA-VERSICOLORA

1. The larvae of the multivoltine imported willow leaf beetle, Plagiod era versicolora (Laicharting), are found in larval aggregations during the first half of the larval stage. An experimental study was conduct ed to test for seasonal (brood) and group size effects on larval fitne ss. For each of the three broods, egg group sizes of 1, 4, 8, 16, 24 a nd 32 were established in the field and monitored for the first 6 days post-hatching. 2. There was no effect of group size on larval weight, but there was a significant effect of group size on survival. Individ uals from group size 16 had the highest survival, which coincides with the mean natural cluster size of 15.3 eggs. 3. There were no signific ant interactions between brood and group size. 4. The effect of larval weight on adult fitness was determined in the laboratory and the fiel d. Larval weight was correlated with egg production in the laboratory but not in the field.