Temporal and inter-tree variations of attack by Hypsipyla robusta Moore (Lepidoptera: Pyralidae) in Australian red cedar (Toona australis (F. Muell.) Harmes).
Variations in attack by Hypsipyla robusta were studied with 13 sets of consecutive sampling data in an Australian red cedar (Toona australis [T. ciliata var. australis]) plantation on a farm in New South Wales, over a 31-month period (April 1992 to November 1994). The temporal pattern of infestation levels was closely correlated with rainfall: the larger the amount of rainfall, the higher the infestation levels. Temperature did not appear to affect the general infestation levels, but low daily minimum temperatures in the winter (<6.5°C) were always associated with low proportions of attacked trees. Analyses of logistic regressions showed that the likelihood of attack was affected positively by tree height, diameter at breast height (DBH), and the number of shoots with fully expanded leaves, and negatively by tree form (tree height relative to DBH). The effects were probably due to the low attack frequencies on small trees (height ≤150 cm). For larger trees, the attack frequencies were similar. The intensities of attack were, however, negatively correlated with tree size. Mechanisms for the attack patterns are discussed.