The water contents of insect diets range from 2 per cent in seeds and grain to 99.9 per cent in xylem sap (Slansky and Scriber 1985). Dow (1986) categorized insect diets according to mechanical type (solid or liquid) and nutrient composition (plant or animal origin) (Fig. 2.1). The solid and liquid plant feeders selected as examples below have to process large volumes of food because plant material is generally suboptimal in nutrient levels.
Caterpillars typically contain 80-90 per cent water, while leaf water content varies from 50 to over 90 per cent across species: it may also vary by 10 per cent on a diurnal basis and may decline by 20 per cent as leaves mature (Slansky 1993). Phytophagous insects tend to adjust their food consumption to maintain constant dry matter intake (see Chapter 2). In evaluating compensatory feeding, care must be taken in comparing foods of different water content, and artificial diets are sometimes diluted with indigestible cellulose to avoid water stress (Slansky and Wheeler 1991; Slansky 1993). It has long been debated whether water intake can be a primary objective of feeding in insects (Edney 1977; Hadley 1994a). Caterpillars at least do not compensate for a decline in leaf moisture levels by increasing their feeding rates (discussed by Slansky 1993).
The growth performance of leaf-chewing larvae is strongly correlated with leaf water content (Slansky and Scriber 1985; Woods and Harrison 2001), but analysis is complicated by covariation of leaf water and nitrogen contents (both being high in young leaves, for example). White (1974) hypothesized that plant water deficits lead to the mobilization of nitrogen, making plants more nutritious for herbivorous insects and leading to episodic outbreaks. Mexican bean beetles, Epilachna varivestis (Coccinellidae), feeding on soybean plants (Glycine max) were used to test this hypothesis (McQuate and Connor 1990a,b). Both physical and chemical changes in foliage are associated with water deficits in plants, so water-deficient soybean foliage was rehydrated before feeding trials to control for physical differences. Free amino acid concentrations increased markedly in foliage grown under water deficits, even after re-watering, but third instar larvae avoided this foliage. Trials under growth chamber, glasshouse, and field conditions showed a consistent tendency for larval survival and growth rate to be reduced on the treatment foliage. It is apparent that plant water deficits can have complex effects on the behaviour and ecology of herbivorous insects.
The relationships between dietary water, feeding, and growth have been extensively investigated in caterpillars of M. sexta under both laboratory and field conditions. When raised on semi-defined artificial diet in the laboratory, fifth instar caterpillars compensate for food dilution by eating more (Timmins et al. 1988) and maintain water homeostasis by modulating faecal water loss (Reynolds and Bellward 1989). Preformed water intake and faecal water loss are the main components of the water budget, metabolic water production and evaporative losses being relatively small, and about half the dietary water intake is incorporated into tissues during rapid growth (Reynolds et al. 1985; Martin and Van't Hof 1988). Evaporative losses are much higher in 'wild' larvae (Woods and Bernays 2000). However, evaporative losses and faecal water savings are superimposed on comparatively enormous water fluxes through the alimentary canal. Woods and Harrison (2001) recently compared the effects of hydric stress on performance (growth) and on the mechanisms of water conservation. Manduca caterpillars reared on low-water diet grew more slowly and showed both short-term changes in faecal water loss and long-term changes in evaporative loss. The water budgets of caterpillars are now relatively well understood.
Homopterans are exclusively phytophagous. Xylem and phloem tissues of plants represent substantial food resources but have certain limitations. Xylem sap is the most dilute food consumed by any herbivore. Phloem sap has high and variable concentrations of sugars, but large volumes must still be imbibed in order to meet nitrogen requirements. Xylem feeding insects such as Cercopoidea and Cicadoidea tend to be larger than phloem feeders because they must cope with the negative tension of xylem fluid in the plant and the resistance of the feeding apparatus (Novotny and Wilson 1997). Differences in xylem fluid composition determine the feeding preferences of xylem feeders, and feeding rates of leafhoppers (Cicadel-lidae) are adjusted in relation to diurnal changes in xylem chemistry, being highest when amino acid conentrations are highest (Brodbeck et al. 1993). Xylem sap has an osmolality of only 9-26 mOsmol kg-1 (Andersen et al. 1992), and the problem of internal flooding is solved in cicadas by osmotic transfer of water from the anterior midgut to the Malpighian tubules in the filter chamber, and production of extremely dilute excreted fluid (Cheung and Marshall 1973). Dehydrated aphids have been reported feeding on xylem rather than phloem (Spiller et al. 1990). Homopteran guts are notable for their complexity (Goodchild 1966), and simpler filter chambers are present in some phloem feeders, such as cicadelloid leaf-hoppers (Eurymelidae) (Lindsay and Marshall 1981). Osmoregulation in phloem feeders is discussed below (Section 4.3.2).
Fruit flies must deal with dilute food sources, such as juices oozing from fruit. 'Bubbling behaviour' in Rhagoletis pomonella (Diptera, Tephritidae) is a form of excretion of excess dietary water: liquid droplets are repeatedly regurgitated and reingested after evaporation, enabling flies to continue feeding (Hendrichs et al. 1992). The consequences of nectar diets for insect water balance depend on body size and flight activity (Nicolson 1998) and excess dietary water is not usually a problem for adult Diptera or Lepidoptera. Ecological aspects of nectar feeding from floral and extra-floral nectaries have been reviewed by Boggs (1987) and Koptur (1992).
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