Plant quality or quantity? Host exploitation strategies in three Chrysomelidae species associated with Asteraceae host plants

Phytophagous insects which feed on the leaves of herbaceous host plants have to Aadapt their life histories to the fact that protein nitrogen is usually highest in growing tissues in spring. AWe monitored field populations of larvae and adults of three chrysomelid species {Galeruca tanaceti (L.) (main Ahost Achillea millefolium (L.) Yarrow), Cassida rubiginosa (Mueller) (main host Cirsium arvense (L.) Scop.) Aand Oreina luctuosa (Suffrian) (host Centaurea scabiosa (L.)) together with the amount of protein nitrogen of Atheir food resources and host plant biomass. As expected, the development of host quality, measured as Aconcentration of protein nitrogen, and host plant biomass showed inverse trends during the season. The euryphagous G. Atanaceti attacks Achillea early and profits from high nitrogen concentrations in the leaves. Occasional Aoverexploitations of local populations of Achillea are compensated by the capacity to move to other host species. In C. Arubiginosa, a species with a host range restricted to the Cardueae, the main larval feeding activity is postponed Ato a period when the nitrogen content of the host leaves had dropped to 50% of its initial value, but when host plant Abiomass had increased by 30%. In the monophagous O. luctuosa the larval development is synchronized with a still Alater phase of host phenology, at which the nitrogen content is below 50% but plant biomass has reached its Amaximum. There seem to be selection factors, which oppose the use of high quality food in spring and which force Athe latter two species to postpone their larval development to a later time in the year. This could be caused by Anumerous factors like, for example, mean daytime temperature. Later in the season the larvae have to cope with the low Aquality of their host plants. They have, however, the advantage of large quantities of food available. laboratory study with adults and mature larvae of O. luctuosa shows that this Aspecies can overcome low levels of protein nitrogen either by selecting younger leaves with higher Anitrogen concentrations or by increasing the daily food consumption rate (RCR) on leaves with a low level of nitrogen Aand by a prolongation of the feeding period. In this way the larvae compensate the effect of lower daily growth Arates (RGR) and a lower food conversion index (ECI) on poor food quality: Regardless of the level of protein nitrogen Athere was no statistically significant difference in total gain of weight during the third-instar feeding period and Ain the weight at the end of the third larval stage. The three investigated chrysomelids show that there exists a broad Aspectrum of adaptations to overcome the dilemma of variable food quality.