Within-tree distribution of attack by Hypsipyla robusta Moore (Lepidoptera: Pyralidae) in Australian Red Cedar ( Toona australis (F. Muell.) Harmes)

Abstract

Field and laboratory evaluations were made of the within-tree distribution of attack of Hypsipyla robusta in Australian Red Cedar ( Toona australis). Proportions of shoots attacked by the insect increased with shoot height, relative shoot height, shoot length, shoot basal diameter and shoot slenderness (shoot length/shoot basal diameter). The most significant factor influencing the log-odds of attack of shoots was their relative height. Shoots positioned at above 90% of tree height were attacked twice as often as the lower-positioned shoots. This suggests that terminal shoots would be among the first to be attacked once a tree is attacked. There was a gradual shift of feeding loci with respect to host tissues as the larva aged. Feeding by larvae of the first two instars was found most often in terminal foliage or previously damaged tissues of shoots or tree stems. Pith-feeding (tunnelling) started at the later 2nd instar. On average, a larva initiated feeding in a mean of 5.4 different locations during its life time, with a minimum of three and a maximum of 11. Switching of feeding loci was most frequent during early 1st instar and much of the 3rd and early 4th instars.