Host-Plant Specialisation and Diurnal Dynamics of the Arthropod Community Within Muhlenbergia robusta (Poaceae)

Abstract

It is well-known that herbivorous insects are very specialised in terms of their food. It has been reported that these insects feed on only one or a few genera of plants, even within a single family (Bernays & Graham, 1988; Schoonhoven et al., 2005). Certain factors have been found to be decisive in determining the range of hosts of herbivorous insects. Among the most important are (1) the secondary compounds in the plants, (2) the presence of predators, and (3) the insects’ mating behaviours. These factors are discussed below. Secondary compounds are one of the most effective strategies that plants use to avoid predation by herbivores, for example, as toxins or in feeding deterrents that kill insects or slow their rates of development (Lill & Marquis, 2001; Schowalter, 2006). The noxious effects of secondary compounds on insects are crucial to the preferences of feeding insects, and therefore, the ranges of the host plants of phytophagous insects (Bernays & Graham, 1988; Cates, 1980). Natural enemies can influence the host ranges of phytophagous specialists. Moreover, it has been proposed that species seek out enemy-free spaces to reduce their mortality (Gilbert & Singer, 1975; Lawton, 1978). In fact, Price et al. (1980) recorded insect herbivores that changed their host plant to a new toxic plant that provided protection against enemies. The literature describes some phytophagous insects that restrict their host range to comply with patterns of mate-finding behaviour. This is true even in plants that do not have a relationship with the food preferences of insects (Labeyrie, 1978). Conversely, arthropod predators are generalised in their food selection (Sabelis, 1992). For this reason, habitat selection by arthropods depends on the services that the habitat provides to increase their chances of survival. It has been reported that the abundance of arthropod predators within plant communities is related to habitats offering (1) abundant prey; (2) refuge from predation, e.g., cannibalism and intraguild predation; (3) easier and more effective spotting and capture of prey; (4) a more favourable microclimate; and (5) access to alternative resources (Langellotto & Denno, 2004). In spite of the knowledge gathered about host-plant specialisations within several arthropod species (Bernays & Graham, 1988; Cates, 1980; Feeny, 1976), there is little information about the level of arthropod specialisation in the use of their habitats at the community level (i.e., species that carry out all of their activities on the host plant). Descriptions of arthropod