Herbivory

There is evidence that increased [CO2] may modify growth and composition of plants and may change the C/N ratio of leaves (Wong, 1979; Norby et al., 1986; William et al., 1986; Curtis et al., 1989; Kuehny et al., 1991). Plants grown under elevated [CO2] have a lower leaf N concentration, due to the increase in carbohydrate production that 'dilutes' the protein content of the leaf (Lambers, 1993). This increased carbohydrate supply in plants exposed to elevated [CO2] tends to increase the concentration of secondary compounds in leaves. Such compounds play an important role in distinct ecological functions, including allelopathy and the deterrence of herbivores (Baas, 1989; Dicke and Sabelis, 1989; Lambers, 1993). These changes in plant composition alter interactions between plant and herbivory (see Chapter 16, this volume). Prolonged development, increased food consumption, decreased food processing efficiency and general growth reduction are some of the typical responses of insects to reduced leaf N concentration (Roth and Lindroth, 1995). Hence, changes in plant composition in response to rising [CO2] may influence the feeding habits and spread of insect populations. This aspect was investigated in the potato FACE experiment for Colorado potato beetle (Leptinotarsa decemlineata Say), which is considered one of the most important worldwide pests of potato. The experiment addressed questions concerning the intensity of beetle attacks on potato under elevated [CO2] conditions, and the effect of changes in leaf composition on the growth rates of beetle larval populations and on their winter survival (Hare, 1990).

The Colorado potato beetle was originally confined to the semi-desert areas in Colorado, USA, where it fed on wild species of Solanum, especially S. rostratum. It spread rapidly when the country was opened - helped by human transport and by cultivation of potato, which proved to be an excellent host. Colorado potato beetle adults emerge from the soil in late spring and lay eggs on potato leaves. The eggs hatch; the larvae develop, by consuming a considerable amount of green tissues, and then pupate in the soil. Adult emergence may occur in the same season or, more often in temperate climates, in the following season. For instance, in northern Germany and Holland there is usually only one generation per year, but in southern Europe two or three generations per year have been observed.

The Colorado potato beetle's feeding behaviour was investigated in the potato FACE experiment by collecting a large number of larvae younger than 2 days in areas of the field that were at ambient [CO2]. Larvae were subdivided in groups and fed with leaves collected from plants exposed to a range of [CO2]. Larval growth rates (i.e. biomass gained per day) and consumption rates (i.e. food ingested per day) were determined. Results clearly indicated that growth of L. decemlineata larvae was sensitive to changes in leaf composition (N concentration and C/N ratio). Larvae grew faster when feeding on leaves grown in ambient [CO2] than on leaves grown in high [CO2] (Fig. 9.9), but differences between mean daily growth rates of larvae fed on leaves of plants exposed to 460, 560 and 660 |mmol CO2 mol-1 were not appreciable. Larval size at the end of the experiment was affected by the quality of the foliage ingested, with larvae from the ambient treatments having 23.8% larger dry mass than those fed from leaves grown in high [CO2] (ambient = 29.54 ± 3.2 mg larva-1, high [CO2] = 22.51 ± 3.7 mg larva-1). However, the total amount of food ingested by the larvae and the leaf consumption rates were the same for all the treatments. Lower protein intake

Fig. 9.9. Growth rate (mg dry weight day-1) of Colorado potato beetle (L. decemlineata) larvae fed with leaves grown at ambient [CO2] (open bars) and elevated [CO2] (solid bars). (Difference in growth rate significant at P< 0.05 indicated by * or at P < 0.01 by **.)

Days of the experiment

Fig. 9.9. Growth rate (mg dry weight day-1) of Colorado potato beetle (L. decemlineata) larvae fed with leaves grown at ambient [CO2] (open bars) and elevated [CO2] (solid bars). (Difference in growth rate significant at P< 0.05 indicated by * or at P < 0.01 by **.)

as a result of changed leaf composition decreased the growth rates of Colorado beetle larvae feeding on potato leaves and, possibly, on other wild relatives of this species. Reduced growth of the larvae may result in lower larvae reserves at the time of pupation, with possible consequences for the ability of the insect to survive winter conditions while diapausing into the soil.

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