The cause of reduced growth of Manduca sexta larvae on a low-water diet: Increased metabolic processing costs or nutrient limitation?

Abstract

Relative growth rates and nitrogen accumulation rates are lower for third-instar ~Uanduca sexta larvae on an artificial diet containing 65% water than on one containing 82% water, due to reduced efficiencies of conversion of digested food and digested nitrogen into larval biomass. Uric acid production is 2.@-2.5fold greater, and non-feeding respiration rates 16.0% higher in the larvae on the low-water diet. Food is the major source of water for the larvae, with metabolic water making only a minor contribution to water input. Faecal excretion is the major avenue of water loss, although a significant amount of water is also lost by transpiration. Larvae from the low-water diet retain and use a higher percentage of the water they gain than larvae from the high-water diet (49.4% vs 41.9%). They produce much drier faeces (48.1% water vs 77.3% water), and, because their tissues are less hydrated (81.3% water vs 88.1% water), they synthesize 70% more new, fully hydrated tissue from a given amount of water than larvae from the high-water diet. We discuss problems involved in the use of determinations of efficiency of conversion of digested food in establishing causal links between diet, growth, and metabolic maintenance costs, and also offer a definition of food processing costs that distinguishes them from metabolic costs attributable to other processes, such as food acquisition, growth, and moulting. We conclude that reduced growth and reduced efficiency of conversion of digested food on low-water diets are due to limitation in the amount of water available for the synthesis of new hydrated tissue, and not to the imposition of higher food processing costs.

The effects of quantity and quality of diet nitrogen on the growth, efficiency of food utilization, nitrogen budget, and metabolic rate of fifth-instar Spodoptera eridania larvae (Lepidoptera: Noctuidae)

“…As appealing and plausible as the idea may seem, it should be viewed with considerable caution, since it is based upon the dubious assumption that growth by insect folivores is energy-limited. In fact, it is generally believed that herbivore growth is not limited by energy, but instead, by nitrogen (Southwood, 1973;Slansky and Feeny, 1977;Schroeder, 1981) or water (Schroeder, 1986;Martin and Van't Hof, 1988). Thus, we are inclined to agree with Slansky and Feeny (1977) that "Energy, present in superabundant supply, is unlikely to affect growth rates, and energy efficiencies are thus likely to be consequence rather than causes of growth performance".…”

“…The nitrogen content of artificial diet, larvae, and frass was determined by Kjeldahl analysis with a Tecator Kjeltec 1030 Auto Analyzer and Digestion System (Martin and Van't Hof, 1988). Two to 4 replicates of each frass and larval sample, and 5 replicates of each diet sample, were digested at 410°C for 1 h. It was necessary to pool 2-4 larvae to provide sufficient material for 2 replicate analyses.…”

“…Uric acid content of larvae and frass was determined by adapting the method of Bhattacharya and Waldbauer (1969), as described by Martin and Van't Hof (1988). Two to 4 samples were pooled to provide sufficient material for 2 replicate analyses.…”

Performance of a generalist grasshopper on a C 3 and a C 4 grass: compensation for the effects of elevated CO 2 on plant nutritional quality

“…Protein was measured as total amino acids in 6 M HCl hydrolysates with ninhydrin reagent (Sigma) (Barbehenn 1995). Uric acid was measured with uricase in the frass of 6-14 insects in each species × CO 2 treatment combination (Martin and Van't Hof 1988). The uric acid concentration was multiplied by 2.4 to account for its color factor in the ninhydrin assay, and mean uric acid concentrations were used to correct fecal protein measurements.…”

Effects of elevated atmospheric CO 2 on the nutritional ecology of C 3 and C 4 grass-feeding caterpillars

“…We measured the effects of elevated CO 2 on the nutritional quality of Lolium multiflorum (C 3 ) and Bouteloua curtipendula (C 4 ) (i.e., protein, sugar, starch, fructan, water, fiber, and toughness), and measured the assimilation efficiency of protein, sugar, starch, and fructan in P. unipuncta and S. frugiperda for each of four plant species × CO 2 treatment combinations. Protein is commonly regarded as the most limiting macronutrient for herbivores (Mattson 1980), although other factors, such as water, toughness, and carbohydrates, can also be important components of plant quality for caterpillars (Slansky and Scriber 1985;Martin and Van't Hof 1988;Goverde et al 2002;Haukioja 2003).…”