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1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3. Nitrogen consumption rate (mg Nday-1)

Figure 2.11 Responses of fifth instar Pieris rapae larvae to variation in nitrogen content of their food plants (mainly Cruciferae). (a) Rate of consumption (mg dry mass per day) as a function of plant nitrogen content (% dry mass). (b) Nitrogen utilization efficiency as a function of the rate of consumption of nitrogen (mg N per day).

Note: Data are means ± SEs, based on two experiments with various food plants and a nitrogen fertilization experiment.

Source: Slansky and Feeny (1977).

compounds, so that insects must compensate by eating more and their development is prolonged. The situation is more complex because allelochem-icals may be affected, more carbon being available for allocation to defensive compounds (Coviella et al. 2002).

Although the focus has been on nitrogen as a limiting nutrient in terrestrial systems, Elser et al. (2000) have recently demonstrated that terrestrial plants are also poor in phosphorus. Stoichiometric analyses provide a more quantitative way of thinking about the differences between trophic levels, and the C : N and C : P ratios of terrestrial herbivores are 5-10-fold lower than those of foliage. Phosphorus content is inversely related to body mass in insects, and more recently derived orders tend to have lower nitrogen and phosphorus contents (Fagan et al. 2002; Woods et al. 2004).

Nitrogen is a general indicator of host plant quality, but because other phytochemicals and water vary simultaneously with nitrogen, causality is difficult to prove (Kyto et al. 1996; Speight et al. 1999; Karley et al. 2002). An exception is found in phloem feeders, whose relatively simple but nutritionally unbalanced food provides an excellent opportunity for testing mechanistic relationships between plant quality and insect performance. Nitrogen quality is also important, and this is measured as the concentrations of individual amino acids in phloem sap. Silverleaf whiteflies Bemisia tabaci (Aleyrodidae) feeding on cotton plants with and without fertilizer treatment differ greatly in free amino acid pools, especially the proportion of the non-essential amino acid glutamine (Crafts-Brandner 2002). Another rapid adjustment was in amino nitrogen excretion (but not honeydew production), which essentially stopped for whiteflies fed on low-nitrogen plants. Aphids are serious pests of potato crops, and Karley et al. (2002) compared several performance parameters of Myzus persicae and Macrosiphum euphorbiae on young and old potato plants, and on artificial diets mimicking their phloem sap. Decreased performance on older plants is due to changes in the amino acid profile of the phloem sap, especially a dramatic decline in glutamine levels. It is interesting that there was no significant correlation between the C: N ratio of plant tissue and the phloem sap sucrose:amino acid ratio (Karley et al. 2002). Incidentally, carbon is even more scarce than nitrogen in a xylem diet, and carbon retention by three species of leafhoppers (Homoptera, Cicadellidae) far exceeds nitrogen retention, excess nitrogen being excreted as ammonia (Brodbeck et al. 1993).

Bernays (1986b) compared the utilization of a wheat diet in a grasshopper and a caterpillar of similar size, reared under identical conditions. Values of AD were similar, but ECD was lower in the grasshoppers, and this was attributed to their large investment in cuticle mass. Cuticle consists mostly of protein and chitin, which are 16 and 7 per cent nitrogen by mass, respectively. Using rain forest beetles in Borneo, Rees (1986) found that adult Chrysomelidae carried significantly less exoskeleton in proportion to body mass than did representatives of several other beetle families, and attributed this to a shortage of nitrogen in their plant diet. However, this conclusion may be biased by phylogenetic relatedness and allometric considerations: the fraction of body mass in the skeleton increases with increasing body size (Schmidt-Nielsen 1984). The importance of recycling cuticular nitrogen was demonstrated recently in cockroaches eating and digesting their own exuviae (Mira 2000). This behaviour was more common in females, in insects reared on a low-protein diet, and in those deprived of their endosymbiotic bacteria. Mira also speculated that acquiring particular amino acids might be important, rather than nitrogen in general: phenyalanine is abundant in cuticle but scarce in plant tissues, and is selected by grasshoppers (Behmer and Joern 1993). Pea aphids (A. pisum) lose in their exuviae about 10 per cent of the total amino acids in the tissues of the adult aphid (Febvay et al. 1999).

Contribution of symbionts to nitrogen balance Many insects possess microbial symbionts which assist with apparently unpromising or deficient diets. Their contributions are diverse and hold much potential in the area of pest management (Douglas 1998). The symbionts may be extracellular, like those in the gut lumen of termites, or intracellular, confined to large cells known as mycetocytes. Mycetocyte symbiosis is best known in Blattaria, Homoptera, Phthiraptera, and Coleoptera living on nutritionally poor diets such as wood, plant sap, or vertebrate blood. Nutritional benefits to the host are often assumed to involve nitrogen, but blood-feeding tsetse flies are provided with missing B vitamins and other insects with sterols (for review see Douglas 1989). Simpson and Raubenheimer (1993a) presented a phylogen-etic analysis of the effect of mycetocyte symbionts on the ratio of protein to digestible carbohydrate required in insect diets, using data from an extensive literature on artificial diets. Insects with the lowest protein requirements in relation to carbohydrate were those with endosymbiotic bacteria which presumably contribute to nitrogen metabolism. Use of a geometric framework to investigate performance of newborn pea aphids, A. pisum (Homoptera, Aphididae), demonstrated an intake target based on an 8: 1 ratio of sucrose to amino acids (Abisgold et al. 1994).

The best direct evidence that mycetocyte sym-bionts are important to nitrogen balance comes from aphids. The amino acid composition of phloem sap is unbalanced, and essential amino acids are synthesized by symbiotic bacteria of the genus Buchnera (Febvay et al. 1999; Douglas et al. 2001). These bacteria live inside the host myceto-cytes (occupying most of the cell volume) and are transmitted vertically from a female to her progeny. The symbionts convert non-essential to essential amino acids, and also use dietary sucrose extensively in the synthesis of essential amino acids (Fig. 2.12), even when the diet resembles aphid tissues (and not phloem sap) in composition (Febvay et al. 1999). Aposymbiotic insects, in which heat or antibiotic treatment is used to eliminate the intracellular microorganisms, have been widely used to investigate interactions between partners. Aphid performance is dramatically reduced after treatment with antibiotics, especially when the insects are reared on diets from which individual amino acids have been omitted (Douglas et al. 2001). It has been suggested that symbionts of the silverleaf whitefly B. tabaci (Aleyrodidae) are responsible for production of trehalulose (Davidson et al. 1994), although oligosaccharide synthesis is unchanged in aposymbiotic pea aphids (Wilkinson et al. 1997). Yeast-like endosymbionts in the brown planthopper Nilaparvata lugens (Delphacidae), a major pest of rice, have high uricase activity and may be recycling nitrogen (Sasaki et al. 1996).

Nitrogen recycling also occurs in cockroaches, but here the microbiology is more complicated because they have both a complex hindgut microflora and bacterial endosymbionts, which mobilize urate deposits in the fat body on low-nitrogen diets and convert them to essential amino acids. Nitrogen is often limiting in the diets of these opportunistic scavengers (Kells et al. 1999). Termites survive on diets with very high C : N ratios, but possess hindgut bacteria which contribute significantly to the nitrogen economy of their hosts by recycling uric acid nitrogen and by fixing atmospheric nitrogen (Breznak 2000). Nardi et al. (2002) have recently drawn attention to the substantial contribution that microbes in the guts of arthropod detritivores may be making to nitrogen fixation in terrestrial ecosystems. Molecular techniques have made it possible to study gut microbes and identify their nitrogenase enzymes without the necessity of culturing difficult organisms, which has hindered such studies in the past (Breznak 2000).

2.4.3 Secondary plant compounds

The molecular structure of secondary plant compounds is far better known than their modes of

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