Role of Volatile and Non-Volatile Plant Secondary Metabolites in Host Tree Selection by Christmas Beetles

Abstract

Individual Eucalyptus trees in south-eastern Australia vary considerably in susceptibility to herbivores. On the one hand, studies with insect herbivores have suggested that variation in the concentrations of foliar monoterpenes is related to variation in susceptibility. On the other, studies with marsupial folivores have suggested that variation in the concentrations of sideroxylonals (a group of formylated phloroglucinol compounds) is responsible for variation in susceptibility. We examined relative importance of sideroxylonals and 1,8-cineole (a dominant monoterpene) in host tree selection by Christmas beetles (Anoplognathus species: Coleoptera: Scarabaeidae) by using no-choice experiments, choice/no-choice experiments, and manipulative experiments in which concentrations of sideroxylonals or 1,8-cineole were altered. We used two species of host Eucalyptus, one species of non-host Eucalyptus, and three species of non-host non-Eucalyptus trees. Leaf consumption by Christmas beetles was negatively correlated with the concentrations of sideroxylonals and 1,8-cineole. Artificial increases in the concentration of sideroxylonals or 1,8-cineole reduced leaf consumption by Christmas beetles. An artificial reduction in foliar monoterpenes had no effect on leaf consumption by the beetles when leaves contained high or very low concentrations of sideroxylonals. However, when the concentration of sideroxylonals was moderate, a reduction in the foliar monoterpenes increased leaf consumption by the beetles. Therefore, monoterpenes such as 1,8-cineole may be used as a negative cue by Christmas beetles. The pattern of food consumption on non-host Eucalyptus species and non-host non-Eucalyptus species suggest that both positive and negative cues may be used by Christmas beetles to select host trees.

The Eucalyptus terpene synthase gene family

“…Nonetheless the focus on terpenes in mature leaf tissue of Eucalyptus has meant that the functional and ecological interpretations of their role have been skewed towards questions such as defence against insect and vertebrate herbivory [57-59]. In contrast, reports of terpenes in other tissues such as roots and wound tissue [6] are sparse.…”

Elevated anthocyanins protect young Eucalyptus leaves from high irradiance but also indicate foliar nutritional quality to visually attuned psyllids

“…It is for this reason that they are preferred by many chewing insect herbivores. Many chewing insect herbivores of Eucalyptus specialise on young leaves because they are soft and higher in nitrogen than their tough and better defended (by constitutive metabolites such as the terpenoids) expanded counterparts (Steinbauer et al, 1998;Nahrung & Allen, 2003;Östrand et al, 2008;Matsuki et al, 2011). Consequently, eucalypts may have needed to protect young leaves against such herbivores for which leaf colour could have evolved as a warning signal.…”

Chemical Defenses in Eucalyptus Species: A Sustainable Strategy Based on Antique Knowledge to Diminish Agrochemical Dependency

“…They have proved to provide protection towards competition by weeds and other plants and to avoid detrimental action of herbivores, fungi, bacteria, and viruses. Insects and mammals feeding on leaves have co-evolved in such a way that some secondary metabolites also play important roles in host selection by herbivores (Matsuki et al 2011). Secondary www.intechopen.com metabolites seem to represent the chemical language in plant-environment interactions, continuously growing in number and changing as co-evolution takes place.…”