Document details

Olfactory Mechanisms Underlying Host-Finding by the Eucalyptus Woodborer, Phoracantha semipunctata Fab. (Coleoptera: cerambycidae)

Author(s): Barata, Eduardo Nuno Picoto Lopes

Date: 1997

Persistent ID: http://hdl.handle.net/10174/11023

Origin: Repositório Científico da Universidade de Évora

Subject(s): Eucalyptus woodborer; Coleoptera: cerambycidae; Olfactory mechanisms; Phoracantha semipunctata


Description

"Sem resumo feito pelo autor";- Ecological relationships of plants and animals are universal, of fundamental importance, and paradoxical. Plants use solar energy to fix carbon, and all animal life ultimately depends on use of energy stored by plants. How can an Earth teeming with herbivores, ranging in size from aphids to elephants, be so green? Animals do not eat all plants. Somehow most plants, most of the time, manage to avoid being eaten. Charles Darwin (1859) provided the key to questions borne of this paradox in the "On the Origin of Species" (Darwin, 1987). He proposed the theory of evolutionary adaptation by natural selection. Darwin argued that plants and animals best fitted to their environments leave more offspring than other members of their species and, consequently, pass their inherited advantages on to future generations. Darwin even provided insight into the nature of plant defences and animal ability to cope with those defences. He noted that sheep of different breeds have different susceptibilities to plant poisons. The fittest herbivores have an inherited ability to cope with those plant defences, and therefore to prosper on plants that their competitors cannot eat. Plants have evolved an enormous array of mechanical and chemical defences against herbivores. Herbivores, on the other hand, evolved an array of adaptations to overcome the plant defences. As pointed out by Ehrlich and Raven (1964), "the plant-herbivore interface may be the major zone of interactions responsible for generating terrestrial organic diversity". The phytophagous insects make up a more than a quarter of all macroscopic organisms, and the plants upon which they feed make up another quarter (Bernays and Chapman, 1994). Every green plant has a set of insect herbivores feeding on its roots, steams, buds, leaves, flowers and fruits. Although there is a continuous spectrum of phytophagous insects species, from those feeding on one plant species to those feeding on a wide range of plants of different families, it is common to classify them as follows: 1) monophagous include species that feed on plants within a single genus; 2) oligophagous includes species that feed on various plants in different genera within one plant family; and 3) polvphagous refers to insects feeding on a relatively large number of plant species of different families (Bernays and Chapman, 1994). Most insects have relatively narrow range of host plant species. Out of approximately 310,000 known phytophagous insect species, about 75% are monophagous or oligophagous (Bernays and Chapman, 1994) and these will be faced with the necessity of selecting the appropriate host plant at some stage of their life cycle. The study of the behavioural mechanisms underlying the selection of a suitable host plant by phytophagous insects is a challenging endeavour, and is central to understand how the present insect-plant associations have evolved, and to know what are the proximate causes of the narrow host ranges of most phytophagous insect species. The evolutionary questions have provided motivation for the study of insect-plant interactions. At the ultimate level, insect-plant associations can be explained in terms of relative fitness of insects on their host plants and, presumably, natural selection reinforces behavioural preference for suitable plants and avoidance of unsuitable ones (Feeny, 1992). Phytophagous insects may have had a role to play in the evolution of plants, by selecting for diverse chemical and physical defences. Many ecologists believe that the current diversity of both plants and insects is, in part, a result of their co-evolution (refs. in Jermy, 1993). Through an evolutionary "arras rate" between plants and insects, the plants have evolved defences to reduce herbivore pressure whilst the insects evolved ways for overcoming some of the defences. This again had consequences in a multitude of animals at higher trophic levels. Others believe that the adaptation and diversity of the insects has tended to follow that of the plants, in this case being less important the influence of insects on the evolution of plants. In other words, it is proposed that the evolution of insect-plant associations results primarily from autonomous evolutionary events, i.e. heritable functional changes within the insects' behavioural and sensorial mechanisms would mediate changes in insect-plant associations, and the ecological factors would play a secondary role by either supporting or preventing the establishment of the new genotype with the novel plant preferences (Jermy, 1993). The study of phytophagous insect behaviour tenda to have been neglected, yet the scientific knowledge of the proximate causes of behaviour and their variation is important to answer the evolutionary questions. The behavioural changes are important adaptations to be selected when an insect shifts or widens its host range. Any morphological or physiological changes, potentially adaptive to exploit a new host plant, are inconsequent if they do not occur together with behavioural changes. Thus, knowledge, about behaviour and its variation is essential to the major evolutionary questions of insect-plant relationships. Another important motivation for studying insect-plant interactions is that specialists as well as generalist phytophagous insect species have become economically important as pests on agricultural crops. Increased concern for the environment, the continued need to minimise possible hazards from crop protection agents, and the rapid development of pesticide resistance have placed tremendous demands on, research to provide new methods of pest control. These can only be successfully implemented on the basis of a detailed knowledge about the behavioural mechanisms by which phytophagous insects select host plants. The host plant selection process consists of a sequence of behavioural responses to an array of stimuli associated with host and non-host plants (Visser, 1986). Plant secondary metabolites are important stimuli in this process. These include a wide variety of organic compounds not directly involved in the primary metabolism of plants (photosynthesis, respiration, biosynthesis of proteins), and some of them have been implicated in regulating chemical mediated interactions with other organisms acting as serniochernicals1. Several plant secondary compounds contribute to defense against various organisms, including fungal pathogens, other plants, and herbivorous animals ranging from insects to man, acting as general toxicants or biocides, or may have a specific toxic action directed at a particular target. They may also act as chemical messengers or signals with purely behavioural effect, and in many cases plant chemical defenses have become signals to herbivores for selection of suitable host plants. In phytophagous insects, volatile secondary compounds have been implicated in the process of host plant selection. These are detected at some distance from the plant, affecting insects' orientation during host-finding behaviour. They act as chemical signals that attract the insect to suitable host plants or induce avoidance reactions to unsuitable plants for feeding and/or oviposition. A largo part of insects' olfactory system is thought to be specifically involved in conveying and processing information about plant volatile compounds (Mustaparta, 1992). However, it is scarce the knowledge concerning which volatile compounds or blends of compounds affect the behaviour of phytophagous insects. There is a need to focus research on what are the plant odours (host and non-host) important in host selection of different insect species with different ranges of host plants. Attraction and avoidance are important characteristics of olfaction, and it is therefore important that chernical, signals from both host and non-host plants are identified and studied, as regard their behavioural effect on insects and how such chemo-sensorial input is detected and processed. Semiochemicals that regulate insect-plant interactions have long been seen to have potential for developing new methods of pest management, e.g. through the exploitation of plant secondary metabolism for modification of insect behaviour, or to develop resistant crop cultivars (Pickett et al., 1991; Hallahan et al., 1992). The resistance of some agricultural crops varieties results in many cases from plant chemical features that have an effect on the insect behaviour, however the resistant plants have often been developed without knowledge about what modifies the behaviour. With a greater knowledge about the mechanisms by which the insects select host plants, many more resistant varieties can be developed. The eucalyptus woodborer, Phoracantha semipunctata Fabricius, is a cerambycid of Australian origin that has a range of host plants restricted to species mainly of Eucalyptus (Myrtaceae). It has been introduced to several regions of the world following the introduction of Eucalyptus for ornamental purposes or economic exploitation of its wood and essential oils. Whereas this insect species is of no economic importance in Australia, it has become a severe pest of eucalyptus plantations in, the introduced regions, including Portugal and several other Mediterranean countries (e.g. Spain, Italy, Marrocco and Tunísia). In Portugal, P. semipunctata was detected for the first time in the península of Setúbal in 1980 (Figo, 1981). Since then, the populations have increased dramatically and have spread throughout nearly all eucalyptus plantations of the country, causing significant economic losses for the growers and the pule paper industry, mainly in the central and southern regions of Portugal. P. semipunctata males and females fly at dusk and during the night in search for mates and oviposition sites. The females oviposit in bark cracks or under loose bark, preferentially on recently felled or weakened trees. Tree damage is caused by the larvae feeding on the bark. Heavy infestations result in the destruction of cambium and phloem tissues along the trunk leading to the death of the tree (Chararas, 1969; Drinkwater, 1975; Scriven et aL, 1986). Current methods of P. semipunctata control exploit the extraordinary ability of females to colonise recently felled trees. Log traps for the eggs have been used by Eucalyptus growers in order to .V gr reduce the beetles' population size of the next generation (e.g. Egea, 1982; Tirado, 1984, 1990). Some authors have suggested a role for the odour of Eucalyptus in host plant selection by P. semipunctata (Powell, 1978; Hanks et al., 1993); however this has never been thoroughly investigated.

Document Type Doctoral thesis
Language English
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