Eucalypt-Feeding Insects Bite off More Than They Can Chew: Sabotage of Induced Defences?

Abstract

Plant chemical defences that are induced by insect attack have evolved in a wide variety of species, including deciduous and coniferous trees (Rhoades 1979). The forests and woodlands of Australia are dominated by the genus Eucalyptus, which comprises several hundred species of evergreen trees. As yet, there is no evidence that induced defences occur in eucalypts. However, here we report that several species of eucalypt-feeding insects chew off and discard partially-eaten leaves, and we propose the hypothesis that these insects are sabotaging the transmission of induced defences to neighbouring leaves. We have observed larvae from four families of Lepidoptera and one family of Hymenoptera chewing off partially-eaten eucalypt leaves on which they have been feeding. It has been observed on Eucalyptus melliodora, E. viminalis, E. leucoxylon and E. pauciflora. The behaviour of all insect species involved is remarkably similar and is described in some detail for Anthela varia (Anthelidae) on E. leucoxylon (Fig. 1). After feeding on a leaf for up to an hour or more, the larva moves on to the stem and starts chewing around the petiole at the base of the leaf (Fig. 1A). Chewing results in characteristic girdling of the petiole (Fig. 1B), and continues until the leaf falls from the plant (Fig. 1C). The insect then chews on the petiole stump for a few more minutes (Fig. 1D). The complete behavioural sequence (A-D) is very deliberate and may take 10 min or more. If the insect is disturbed, as in Fig. 1B, it will quickly resume chewing, rather than abandon the task. We have also seen the cup moth Doratifera quadriguttata (Limacodidae) (Fig. 2) and two other species in the same family removing partially-eaten leaves. Additionally, it has been noted in two unidentified species of Geometridae and one species of Lasiocampidae. We have also observed partially-eaten eucalypt leaves being cut off by two species of sawfly, Perga affinis and Perga sp. (Fig. 3). Sawfly larvae often feed gregariously, and when a group moves from a leaf, the last larva to depart performs the petiole-chewing. On many occasions we have seen two or more larvae of P. affinis co-operate in chewing off a leaf on which they and other larvae had been feeding. There are only a few records in the literature of insects, all lepidopteran larvae, chewing off partiallyeaten leaves (Scriber 1975, Heinrich and Collins 1983). Damaged leaves may act as searching cues for birds (Heinrich and Collins 1983) and parasitoids (Faeth 1987), however three pieces of evidence lead us to argue that removal of feeding evidence is not the primary function of the behaviour we have observed on eucalypts. Firstly, obligate leaf-removal would be expected if natural enemies using leaf damage as a searching cue had provided the effective selective pressure for this behaviour. However, petiole-chewing behaviour is not obligate in most of the species we have observed. For instance, D. quadriguttata larvae often move from a partially-eaten leaf without chewing it off, suggesting that leaf removal occurs as a facultative response to an immediate stimulus. Induced defences can be mobilized very rapidly (Carroll and Hoffman 1980), however the strength of the response is likely to vary between occasions. Such variation may determine whether an insect