Ebook Diet affects the immune defence and life-history traits of an Arctiid moth Parasemia plantaginis
One of the most important factors affecting the fitness of insect herbivores is their diet that is, the quality of the plant species they eat. Polyphagous herbivores in particular face a challenge, as eating different host plant species can result in differences in life-history traits, such as growth, development time and fecundity. These differences may be due to them having a limited possibility to co-evolve with all of their potential host plants, which have differing chemical (nutritional value, secondary metabolites) and other traits (e.g. mechanical defence) that affect the herbivores’life-history traits (Gordon, 1961; Erickson and Feeny, 1974; Cates, 1980; Price et al., 1980; Berenbaum and Zangerl, 1999). Thus, it is likely that generalist herbivores are adapted to the most common secondary metabolites and are more sensitive to defensive compounds that only occur in some plant genera (e.g. Levins and McArthur, 1966). However, plant secondary metabolites are not always harmful to herbivores; some of them are used as feeding cues, especially by specialist herbivores, and some can be beneficial to the herbivore. Carotenoids, for example, are important antioxidants and reduce the harmful effects of stress caused by, for example, ultraviolet radiation or infection (Demming-Adams and Adams, 1996; Ouchane et al., 1997).
It has often been demonstrated that generalist herbivores perform differently on different host plant species (e.g. Price et al., 1980; Bernays and Chapman, 1994). In spite of the performance differences, genetic interactions in the performance of herbivores feeding on different host plants have generally not been found (see, for example, Jaenike, 1990 and references therein). Genetic interactions in herbivores’ growth or other general performance measures on different host plant species would suggest that there is a trade-off in the metabolism of allelochemicals between different host plant species. The absence of these interactions has been interpreted to mean that the ‘metabolic load’ of detoxifying capacity (which is expected to be energy limited) has by itself a trivial effect on larvae (Scriber and Feeny, 1979; Appel and Martin, 1992). Looking for energy costs is perhaps not the best way to seek to understand the feeding costs of herbivores on many different host plant species.
Diet can also have indirect effects on the survival of herbivorous insects. It has been well established that insects’ host plant species affects the probability of their being predated and parasitized (Fox et al., 1990; Farrar and Kennedy, 1993; Lill et al., 2002; Mira and Bernays, 2002). Predation pressures on herbivores vary depending on their host plant species. Herbivores may, for example, be more obvious to predators on one plant than on another (Gross and Price, 1988; Ohsaki and Sato, 1994). Moreover, plant-derived compounds in herbivores can make them inedible and provide protection from predators (Rothschild, 1973; Dobler and Rowell-Rahier, 1994; Camara, 1997; Stamp, 2001)and parasitoids (Nieminen et al., 2003; Singer and Stireman, 2003) , especially in species that are specialized to plant species that contain harmful substances. In spite of the numerous studies demonstrating significant differences in parasitoid load among insects feeding on different host plants in nature, the mechanisms behind these differences are unclear. Lill et al.(2002) presented several hypotheses to explain the differences in parasitoid load in generalists feeding on different host plants: plant volatile-related differences in parasitoid attraction and/or retention rates; variation in the conspicuousness of cater-pillars on different host plants; density-dependent foraging by parasitoids; the presence and abundance of other herbivore species on the host plants; and host-plant effects on caterpillar resistance to parasitism. Another factor affecting parasite load, demonstrated by Benrey and Denno (1997), is that ow-quality host plants can slow down the development of the herbivore and thus increase the probability of it being parasitized.
The primary insect defence against hymenopteran and dipteran parasitoids (Nappi, 1975; Godfray, 1994)as well as against nematodes (Stoffolano, 1986) and fungi (Vey and Götz, 1986)is encapsulation. An encapsulation reaction is a general response to foreign intrusions: all small inert particles inside an insect are encapsulated (Nappi, 1975; Lackie, 1988). In encapsulation, reaction cells circulating in the haemocoel recognize an object as foreign and attach to it. A closed, blackened capsule is formed around the object and the intruder dies through lack of oxygen and/or nutrients (Nappi, 1975; Godfray, 1994). There are differences in encapsulation reaction among individuals, since it requires resources and has been found to be more efficient in individuals in good general condition (Kraaijeveld and Godfray, 1997). Thus it is likely that variation in host plants can result in differences in encapsulation ability in herbivorous insects, either through the direct effect of plant chemical composition providing the necessary substances for the herbivore to mount an encapsulation reaction or indirectly via differences in general vigour on different host plants.
Encapsulation reactions create free radicals that can seriously damage the cells of an insect (Nappi et al., 1995; von Schantz et al., 1999). Antioxidants acquired from the diet can prevent damage to the cells (Johnson and Felton, 2001)and thus reduce the cost of immune defence against parasitoids. Different plant species have different amounts of antioxidants and thus the food plant of an herbivore is important, especially if the likelihood of pathogen attack is high. However, not all plant antioxidants are beneficial to herbivores; some can have adverse effects. For example, a common plant phenol and antioxidant chlorogenic acid has been reported to deter some herbivores (Matsuda and Senbo, 1985). Additionally, the same substances that act as antioxidants in some or most situations can be pro-oxidants in different chemical environments. Even though pro-oxidants are generally harmful to the cells of the herbivore, they can also prevent viruses from entering the larva via the gut wall (Hoover et al., 1998).
In this study, we examined whether an insect’s ability to fight parasites and parasitoids is affected by their larval diet. More specifically, we tested if Arctiid moth larvae’s encapsulation ability varies among their host plants and among families. Using artificial implants in a laboratory, we were able to test specifically whether the differences in parasitoid loads among herbivores on different host plants that have been observed in nature might be dependent directly on the host plants’ effects on resistance to parasitism. We chose host plants that we expected to differ in secondary metabolite content, but to be relatively similar in nitrogen content, which has been shown to have an important effect on herbivore growth (Mattson, 1980). To evaluate the effect of plant chemicals to these herbivores, we analysed the chemical constituents of the diets we used in this study. Larvae were fed on a combination of their natural host plant species and an artificial diet. We can expect that parasitoids, and larvae’s ability to fight them by encapsulation, are very important to the survival of the larvae and thus to their fitness. We were particularly interested in whether encapsulation is correlated with general larval performance on different host plant species, or if the host plant independently affects the encapsulation rate of Parasemia plantaginis. Moreover, we studied the costs of encapsulation by testing possible trade-offs in the encapsulation and other fitness measures among diets, which could affect the evolution of diet choice of herbivorous species. Also, we tested whether there are genotype environment interactions in Arctiid moths among host plant species in life-history traits and encapsulation ability, as this is an indicator of different feeding specializations among families.
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